import { LOG_VGA } from './const.js'
import { h, round_up_to_next_power_of_2, view } from './lib.js'
import { dbg_assert, dbg_log } from './log.js'
import { IO } from './io.js'
import { BusConnector } from './bus.js'
import { PCI } from './pci.js'

// Always 64k
const VGA_BANK_SIZE = 64 * 1024

const MAX_XRES = 2560
const MAX_YRES = 1600
const MAX_BPP = 32

//const VGA_LFB_ADDRESS = 0xFE000000; // set by seabios
const VGA_LFB_ADDRESS = 0xe0000000

// Equals the maximum number of pixels for non svga.
// 8 pixels per byte.
const VGA_PIXEL_BUFFER_SIZE = 8 * VGA_BANK_SIZE

const VGA_MIN_MEMORY_SIZE = 4 * VGA_BANK_SIZE

// Avoid wrapping past VGA_LFB_ADDRESS
const VGA_MAX_MEMORY_SIZE = 256 * 1024 * 1024

// @see http://www.osdever.net/FreeVGA/vga/graphreg.htm#06
const VGA_HOST_MEMORY_SPACE_START = Uint32Array.from([
    0xa0000, 0xa0000, 0xb0000, 0xb8000,
])

// @see http://www.osdever.net/FreeVGA/vga/graphreg.htm#06
const VGA_HOST_MEMORY_SPACE_SIZE = Uint32Array.from([
    0x20000, // 128K
    0x10000, // 64K
    0x8000, // 32K
    0x8000, // 32K
])

// Minimal interface for the CPU fields VGA uses.
interface VGACpu {
    io: IO
    wasm_memory: WebAssembly.Memory
    devices: { pci: PCI }
    read8(addr: number): number
    write8(addr: number, value: number): void
    svga_allocate_memory(size: number): number
    svga_allocate_dest_buffer(size: number): number
    svga_mark_dirty(): void
    svga_fill_pixel_buffer(bpp: number, offset: number): void
    svga_dirty_bitmap_min_offset: Int32Array
    svga_dirty_bitmap_max_offset: Int32Array
}

// Minimal interface for the screen adapter (ScreenAdapter or DummyScreenAdapter).
interface VGAScreenAdapter {
    FLAG_BLINKING: number
    FLAG_FONT_PAGE_B: number
    set_mode(graphical: boolean): void
    set_size_text(cols: number, rows: number): void
    set_size_graphical(
        width: number,
        height: number,
        virtual_width: number,
        virtual_height: number,
    ): void
    put_char(
        row: number,
        col: number,
        chr: number,
        flags: number,
        bg_color: number,
        fg_color: number,
    ): void
    update_cursor(row: number, col: number): void
    update_cursor_scanline(start: number, end: number, visible: boolean): void
    update_buffer(layers: VGALayer[]): void
    clear_screen(): void
    set_font_bitmap(
        height: number,
        width_9px: boolean,
        width_dbl: boolean,
        copy_8th_col: boolean,
        bitmap: Uint8Array,
        bitmap_changed: boolean,
    ): void
    set_font_page(page_a: number, page_b: number): void
}

interface VGALayer {
    image_data: ImageData | null
    screen_x: number
    screen_y: number
    buffer_x: number
    buffer_y: number
    buffer_width: number
    buffer_height: number
}

export class VGAScreen {
    cpu: VGACpu
    bus: BusConnector
    screen: VGAScreenAdapter
    vga_memory_size: number
    cursor_address: number
    cursor_scanline_start: number
    cursor_scanline_end: number

    // Number of columns in text mode
    max_cols: number

    // Number of rows in text mode
    max_rows: number

    // Width in pixels in graphical mode
    screen_width: number

    // Height in pixels in graphical mode
    screen_height: number

    // Logical width in pixels of virtual buffer available for panning
    virtual_width: number

    // Logical height in pixels of virtual buffer available for panning
    virtual_height: number

    // The rectangular fragments of the image buffer, and their destination
    // locations, to be drawn every screen_fill_buffer during VGA modes.
    layers: VGALayer[]

    // video memory start address
    start_address: number

    // Start address - a copy of start_address that only gets updated
    // during VSync, used for panning and page flipping
    start_address_latched: number

    // Unimplemented CRTC registers go here
    crtc: Uint8Array

    crtc_mode: number
    horizontal_display_enable_end: number
    horizontal_blank_start: number
    vertical_display_enable_end: number
    vertical_blank_start: number
    underline_location_register: number
    preset_row_scan: number
    offset_register: number
    line_compare: number
    graphical_mode: boolean

    // VGA palette containing 256 colors for video mode 13, svga 8bpp, etc.
    // Needs to be initialised by the BIOS
    vga256_palette: Int32Array

    // VGA read latches
    latch_dword: number
    svga_version: number
    svga_width: number
    svga_height: number
    svga_enabled: boolean
    svga_bpp: number
    svga_bank_offset: number

    // The video buffer offset created by VBE_DISPI_INDEX_Y_OFFSET (in bytes)
    svga_offset: number
    svga_offset_x: number
    svga_offset_y: number

    pci_space: number[]
    pci_id: number
    pci_bars: { size: number }[]
    pci_rom_size: number
    pci_rom_address: number
    name: string

    index_crtc: number
    dac_color_index_write: number
    dac_color_index_read: number
    dac_state: number
    dac_mask: number
    dac_map: Uint8Array

    attribute_controller_index: number
    palette_source: number
    attribute_mode: number
    color_plane_enable: number
    horizontal_panning: number
    color_select: number

    sequencer_index: number
    plane_write_bm: number
    sequencer_memory_mode: number
    clocking_mode: number
    graphics_index: number
    character_map_select: number

    plane_read: number
    planar_mode: number
    planar_rotate_reg: number
    planar_bitmap: number
    planar_setreset: number
    planar_setreset_enable: number
    miscellaneous_graphics_register: number

    color_compare: number
    color_dont_care: number
    max_scan_line: number
    miscellaneous_output_register: number
    port_3DA_value: number

    font_page_ab_enabled: boolean

    dispi_index: number
    dispi_enable_value: number

    svga_memory: Uint8Array
    diff_addr_min: number
    diff_addr_max: number
    diff_plot_min: number
    diff_plot_max: number

    image_data: ImageData | null
    dest_buffet_offset: number

    vga_memory: Uint8Array
    plane0: Uint8Array
    plane1: Uint8Array
    plane2: Uint8Array
    plane3: Uint8Array
    pixel_buffer: Uint8Array

    constructor(
        cpu: VGACpu,
        bus: BusConnector,
        screen: VGAScreenAdapter,
        vga_memory_size: number,
    ) {
        this.cpu = cpu
        this.bus = bus
        this.screen = screen
        this.vga_memory_size = vga_memory_size

        this.cursor_address = 0
        this.cursor_scanline_start = 0xe
        this.cursor_scanline_end = 0xf
        this.max_cols = 80
        this.max_rows = 25
        this.screen_width = 0
        this.screen_height = 0
        this.virtual_width = 0
        this.virtual_height = 0
        this.layers = []
        this.start_address = 0
        this.start_address_latched = 0
        this.crtc = new Uint8Array(0x19)
        this.crtc_mode = 0
        this.horizontal_display_enable_end = 0
        this.horizontal_blank_start = 0
        this.vertical_display_enable_end = 0
        this.vertical_blank_start = 0
        this.underline_location_register = 0
        this.preset_row_scan = 0
        this.offset_register = 0
        this.line_compare = 0
        this.graphical_mode = false
        this.vga256_palette = new Int32Array(256)
        this.latch_dword = 0
        this.svga_version = 0xb0c5
        this.svga_width = 0
        this.svga_height = 0
        this.svga_enabled = false
        this.svga_bpp = 32
        this.svga_bank_offset = 0
        this.svga_offset = 0
        this.svga_offset_x = 0
        this.svga_offset_y = 0

        if (
            this.vga_memory_size === undefined ||
            this.vga_memory_size < VGA_MIN_MEMORY_SIZE
        ) {
            this.vga_memory_size = VGA_MIN_MEMORY_SIZE
        } else if (this.vga_memory_size > VGA_MAX_MEMORY_SIZE) {
            this.vga_memory_size = VGA_MAX_MEMORY_SIZE
        } else {
            // required for pci code
            this.vga_memory_size = round_up_to_next_power_of_2(
                this.vga_memory_size,
            )
        }
        dbg_log('effective vga memory size: ' + this.vga_memory_size, LOG_VGA)

        const pci_revision = 0 // set to 2 for qemu extended registers

        // Experimental, could probably need some changes
        // 01:00.0 VGA compatible controller: NVIDIA Corporation GT216 [GeForce GT 220] (rev a2)
        this.pci_space = [
            0x34,
            0x12,
            0x11,
            0x11,
            0x03,
            0x01,
            0x00,
            0x00,
            pci_revision,
            0x00,
            0x00,
            0x03,
            0x00,
            0x00,
            0x00,
            0x00,
            0x08,
            VGA_LFB_ADDRESS >>> 8,
            VGA_LFB_ADDRESS >>> 16,
            VGA_LFB_ADDRESS >>> 24,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0xbf,
            0xfe,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0xf4,
            0x1a,
            0x00,
            0x11,
            0x00,
            0x00,
            0xbe,
            0xfe,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
            0x00,
        ]
        this.pci_id = 0x12 << 3
        this.pci_bars = [
            {
                size: this.vga_memory_size,
            },
        ]

        // TODO: Should be matched with vga bios size and mapping address
        // Seabios config for this device:
        // CONFIG_VGA_PCI=y
        // CONFIG_OVERRIDE_PCI_ID=y
        // CONFIG_VGA_VID=0x10de
        // CONFIG_VGA_DID=0x0a20

        this.pci_rom_size = 0x10000
        this.pci_rom_address = 0xfeb00000

        this.name = 'vga'

        this.index_crtc = 0

        // index for setting colors through port 3C9h
        this.dac_color_index_write = 0
        this.dac_color_index_read = 0
        this.dac_state = 0

        this.dac_mask = 0xff

        this.dac_map = new Uint8Array(0x10)

        this.attribute_controller_index = -1
        this.palette_source = 0x20
        this.attribute_mode = 0
        this.color_plane_enable = 0
        this.horizontal_panning = 0
        this.color_select = 0

        this.sequencer_index = -1

        // bitmap of planes 0-3
        this.plane_write_bm = 0xf
        this.sequencer_memory_mode = 0
        this.clocking_mode = 0
        this.graphics_index = -1
        this.character_map_select = 0

        this.plane_read = 0 // value 0-3, which plane to read
        this.planar_mode = 0
        this.planar_rotate_reg = 0
        this.planar_bitmap = 0xff
        this.planar_setreset = 0
        this.planar_setreset_enable = 0
        this.miscellaneous_graphics_register = 0

        this.color_compare = 0
        this.color_dont_care = 0

        this.max_scan_line = 0

        this.miscellaneous_output_register = 0xff
        this.port_3DA_value = 0xff

        this.font_page_ab_enabled = false

        this.dest_buffet_offset = 0

        const io = cpu.io

        io.register_write(0x3c0, this, this.port3C0_write)
        io.register_read(0x3c0, this, this.port3C0_read, this.port3C0_read16)

        io.register_read(0x3c1, this, this.port3C1_read)
        io.register_write(0x3c2, this, this.port3C2_write)

        io.register_write_consecutive(
            0x3c4,
            this,
            this.port3C4_write,
            this.port3C5_write,
        )

        io.register_read(0x3c4, this, this.port3C4_read)
        io.register_read(0x3c5, this, this.port3C5_read)

        io.register_write_consecutive(
            0x3ce,
            this,
            this.port3CE_write,
            this.port3CF_write,
        )

        io.register_read(0x3ce, this, this.port3CE_read)
        io.register_read(0x3cf, this, this.port3CF_read)

        io.register_read(0x3c6, this, this.port3C6_read)
        io.register_write(0x3c6, this, this.port3C6_write)
        io.register_write(0x3c7, this, this.port3C7_write)
        io.register_read(0x3c7, this, this.port3C7_read)
        io.register_write(0x3c8, this, this.port3C8_write)
        io.register_read(0x3c8, this, this.port3C8_read)
        io.register_write(0x3c9, this, this.port3C9_write)
        io.register_read(0x3c9, this, this.port3C9_read)

        io.register_read(0x3cc, this, this.port3CC_read)

        io.register_write(0x3d4, this, this.port3D4_write, this.port3D4_write16)
        io.register_write(0x3d5, this, this.port3D5_write, this.port3D5_write16)

        io.register_read(0x3d4, this, this.port3D4_read)
        io.register_read(0x3d5, this, this.port3D5_read, this.port3D5_read16)

        // use same handlers for monochrome text-mode's alternate port addresses 0x3B4/0x3B5 as for the regular addresses (0x3D4/0x3D5)
        io.register_write(0x3b4, this, this.port3D4_write, this.port3D4_write16)
        io.register_write(0x3b5, this, this.port3D5_write, this.port3D5_write16)

        io.register_read(0x3b4, this, this.port3D4_read)
        io.register_read(0x3b5, this, this.port3D5_read, this.port3D5_read16)

        io.register_read(0x3ca, this, function () {
            dbg_log('3CA read', LOG_VGA)
            return 0
        })

        // use same handler for monochrome text-mode's alternate port address 0x3BA as for its regular address (0x3DA)
        io.register_read(0x3da, this, this.port3DA_read)
        io.register_read(0x3ba, this, this.port3DA_read)

        // Bochs VBE Extensions
        // http://wiki.osdev.org/Bochs_VBE_Extensions
        this.dispi_index = -1
        this.dispi_enable_value = 0

        io.register_write(0x1ce, this, undefined, this.port1CE_write)

        io.register_write(0x1cf, this, undefined, this.port1CF_write)
        io.register_read(0x1cf, this, undefined, this.port1CF_read)

        const vga_offset = cpu.svga_allocate_memory(this.vga_memory_size) >>> 0
        this.svga_memory = view(
            Uint8Array,
            cpu.wasm_memory,
            vga_offset,
            this.vga_memory_size,
        )

        this.diff_addr_min = this.vga_memory_size
        this.diff_addr_max = 0
        this.diff_plot_min = this.vga_memory_size
        this.diff_plot_max = 0

        this.image_data = null

        this.vga_memory = new Uint8Array(4 * VGA_BANK_SIZE)
        this.plane0 = new Uint8Array(
            this.vga_memory.buffer,
            0 * VGA_BANK_SIZE,
            VGA_BANK_SIZE,
        )
        this.plane1 = new Uint8Array(
            this.vga_memory.buffer,
            1 * VGA_BANK_SIZE,
            VGA_BANK_SIZE,
        )
        this.plane2 = new Uint8Array(
            this.vga_memory.buffer,
            2 * VGA_BANK_SIZE,
            VGA_BANK_SIZE,
        )
        this.plane3 = new Uint8Array(
            this.vga_memory.buffer,
            3 * VGA_BANK_SIZE,
            VGA_BANK_SIZE,
        )
        this.pixel_buffer = new Uint8Array(VGA_PIXEL_BUFFER_SIZE)

        io.mmap_register(
            0xa0000,
            0x20000,
            (addr: number) => this.vga_memory_read(addr),
            (addr: number, value: number) => this.vga_memory_write(addr, value),
        )

        cpu.devices.pci.register_device(this)
    }

    get_state(): any[] {
        const state: any[] = []

        state[0] = this.vga_memory_size
        state[1] = this.cursor_address
        state[2] = this.cursor_scanline_start
        state[3] = this.cursor_scanline_end
        state[4] = this.max_cols
        state[5] = this.max_rows
        state[6] = this.vga_memory
        state[7] = this.dac_state
        state[8] = this.start_address
        state[9] = this.graphical_mode
        state[10] = this.vga256_palette
        state[11] = this.latch_dword
        state[12] = this.color_compare
        state[13] = this.color_dont_care
        state[14] = this.miscellaneous_graphics_register
        state[15] = this.svga_width
        state[16] = this.svga_height
        state[17] = this.crtc_mode
        state[18] = this.svga_enabled
        state[19] = this.svga_bpp
        state[20] = this.svga_bank_offset
        state[21] = this.svga_offset
        state[22] = this.index_crtc
        state[23] = this.dac_color_index_write
        state[24] = this.dac_color_index_read
        state[25] = this.dac_map
        state[26] = this.sequencer_index
        state[27] = this.plane_write_bm
        state[28] = this.sequencer_memory_mode
        state[29] = this.graphics_index
        state[30] = this.plane_read
        state[31] = this.planar_mode
        state[32] = this.planar_rotate_reg
        state[33] = this.planar_bitmap
        state[34] = this.max_scan_line
        state[35] = this.miscellaneous_output_register
        state[36] = this.port_3DA_value
        state[37] = this.dispi_index
        state[38] = this.dispi_enable_value
        state[39] = this.svga_memory
        // this.graphical_mode_is_linear
        state[41] = this.attribute_controller_index
        state[42] = this.offset_register
        state[43] = this.planar_setreset
        state[44] = this.planar_setreset_enable
        state[45] = this.start_address_latched
        state[46] = this.crtc
        state[47] = this.horizontal_display_enable_end
        state[48] = this.horizontal_blank_start
        state[49] = this.vertical_display_enable_end
        state[50] = this.vertical_blank_start
        state[51] = this.underline_location_register
        state[52] = this.preset_row_scan
        state[53] = this.offset_register
        state[54] = this.palette_source
        state[55] = this.attribute_mode
        state[56] = this.color_plane_enable
        state[57] = this.horizontal_panning
        state[58] = this.color_select
        state[59] = this.clocking_mode
        state[60] = this.line_compare
        state[61] = this.pixel_buffer
        state[62] = this.dac_mask
        state[63] = this.character_map_select
        state[64] = this.font_page_ab_enabled

        return state
    }

    set_state(state: any[]): void {
        this.vga_memory_size = state[0]
        this.cursor_address = state[1]
        this.cursor_scanline_start = state[2]
        this.cursor_scanline_end = state[3]
        this.max_cols = state[4]
        this.max_rows = state[5]
        if (state[6]) {
            this.vga_memory.set(state[6])
        }
        this.dac_state = state[7]
        this.start_address = state[8]
        this.graphical_mode = state[9]
        this.vga256_palette = state[10]
        this.latch_dword = state[11]
        this.color_compare = state[12]
        this.color_dont_care = state[13]
        this.miscellaneous_graphics_register = state[14]
        this.svga_width = state[15]
        this.svga_height = state[16]
        this.crtc_mode = state[17]
        this.svga_enabled = state[18]
        this.svga_bpp = state[19]
        this.svga_bank_offset = state[20]
        this.svga_offset = state[21]
        this.index_crtc = state[22]
        this.dac_color_index_write = state[23]
        this.dac_color_index_read = state[24]
        this.dac_map = state[25]
        this.sequencer_index = state[26]
        this.plane_write_bm = state[27]
        this.sequencer_memory_mode = state[28]
        this.graphics_index = state[29]
        this.plane_read = state[30]
        this.planar_mode = state[31]
        this.planar_rotate_reg = state[32]
        this.planar_bitmap = state[33]
        this.max_scan_line = state[34]
        this.miscellaneous_output_register = state[35]
        this.port_3DA_value = state[36]
        this.dispi_index = state[37]
        this.dispi_enable_value = state[38]
        this.svga_memory.set(state[39])
        // state[40];
        this.attribute_controller_index = state[41]
        this.offset_register = state[42]
        this.planar_setreset = state[43]
        this.planar_setreset_enable = state[44]
        this.start_address_latched = state[45]
        this.crtc.set(state[46])
        this.horizontal_display_enable_end = state[47]
        this.horizontal_blank_start = state[48]
        this.vertical_display_enable_end = state[49]
        this.vertical_blank_start = state[50]
        this.underline_location_register = state[51]
        this.preset_row_scan = state[52]
        this.offset_register = state[53]
        this.palette_source = state[54]
        this.attribute_mode = state[55]
        this.color_plane_enable = state[56]
        this.horizontal_panning = state[57]
        this.color_select = state[58]
        this.clocking_mode = state[59]
        this.line_compare = state[60]
        if (state[61]) {
            this.pixel_buffer.set(state[61])
        }
        this.dac_mask = state[62] === undefined ? 0xff : state[62]
        this.character_map_select = state[63] === undefined ? 0 : state[63]
        this.font_page_ab_enabled = state[64] === undefined ? false : state[64]

        this.screen.set_mode(this.graphical_mode)

        if (this.graphical_mode) {
            // Ensure set_size_graphical will update
            this.screen_width = 0
            this.screen_height = 0

            if (this.svga_enabled) {
                this.set_size_graphical(
                    this.svga_width,
                    this.svga_height,
                    this.svga_width,
                    this.svga_height,
                    this.svga_bpp,
                )
                this.update_layers()
            } else {
                this.update_vga_size()
                this.update_layers()
                this.complete_replot()
            }
        } else {
            this.set_font_bitmap(true)
            this.set_size_text(this.max_cols, this.max_rows)
            this.set_font_page()
            this.update_cursor_scanline()
            this.update_cursor()
        }
        this.complete_redraw()
    }

    vga_memory_read(addr: number): number {
        if (this.svga_enabled) {
            // vbe banked mode (accessing svga memory through the regular vga memory range)
            return this.cpu.read8(
                (((addr - 0xa0000) | this.svga_bank_offset) + VGA_LFB_ADDRESS) |
                    0,
            )
        }

        const memory_space_select =
            (this.miscellaneous_graphics_register >> 2) & 0x3
        addr -= VGA_HOST_MEMORY_SPACE_START[memory_space_select]

        // VGA chip only decodes addresses within the selected memory space.
        if (
            addr < 0 ||
            addr >= VGA_HOST_MEMORY_SPACE_SIZE[memory_space_select]
        ) {
            dbg_log(
                'vga read outside memory space: addr:' + h(addr >>> 0),
                LOG_VGA,
            )
            return 0
        }

        this.latch_dword = this.plane0[addr]
        this.latch_dword |= this.plane1[addr] << 8
        this.latch_dword |= this.plane2[addr] << 16
        this.latch_dword |= this.plane3[addr] << 24

        if (this.planar_mode & 0x08) {
            // read mode 1
            let reading = 0xff

            if (this.color_dont_care & 0x1) {
                reading &=
                    this.plane0[addr] ^
                    ~(this.color_compare & 0x1 ? 0xff : 0x00)
            }
            if (this.color_dont_care & 0x2) {
                reading &=
                    this.plane1[addr] ^
                    ~(this.color_compare & 0x2 ? 0xff : 0x00)
            }
            if (this.color_dont_care & 0x4) {
                reading &=
                    this.plane2[addr] ^
                    ~(this.color_compare & 0x4 ? 0xff : 0x00)
            }
            if (this.color_dont_care & 0x8) {
                reading &=
                    this.plane3[addr] ^
                    ~(this.color_compare & 0x8 ? 0xff : 0x00)
            }

            return reading
        } else {
            // read mode 0

            let plane = this.plane_read
            if (!this.graphical_mode) {
                // We store all text data linearly and font data in plane 2.
                // TODO: works well for planes 0 and 2, but what about plane 1?
                plane &= 0x3
            } else if (this.sequencer_memory_mode & 0x8) {
                // Chain 4
                plane = addr & 0x3
                addr &= ~0x3
            } else if (this.planar_mode & 0x10) {
                // Odd/Even host read
                plane = addr & 0x1
                addr &= ~0x1
            }
            return this.vga_memory[(plane << 16) | addr]
        }
    }

    vga_memory_write(addr: number, value: number): void {
        if (this.svga_enabled) {
            // vbe banked mode (accessing svga memory through the regular vga memory range)
            this.cpu.write8(
                (((addr - 0xa0000) | this.svga_bank_offset) + VGA_LFB_ADDRESS) |
                    0,
                value,
            )
            return
        }

        const memory_space_select =
            (this.miscellaneous_graphics_register >> 2) & 0x3
        addr -= VGA_HOST_MEMORY_SPACE_START[memory_space_select]

        if (
            addr < 0 ||
            addr >= VGA_HOST_MEMORY_SPACE_SIZE[memory_space_select]
        ) {
            dbg_log(
                'vga write outside memory space: addr:' +
                    h(addr >>> 0) +
                    ', value:' +
                    h(value),
                LOG_VGA,
            )
            return
        }

        if (this.graphical_mode) {
            this.vga_memory_write_graphical(addr, value)
        } else if (!(this.plane_write_bm & 0x3)) {
            if (this.plane_write_bm & 0x4) {
                // write to plane 2 (font-bitmap)
                this.plane2[addr] = value
            }
        } else {
            this.vga_memory_write_text_mode(addr, value)
        }
    }

    vga_memory_write_graphical(addr: number, value: number): void {
        let plane_dword: number = 0
        const write_mode = this.planar_mode & 3
        let bitmask = this.apply_feed(this.planar_bitmap)
        const setreset_dword = this.apply_expand(this.planar_setreset)
        const setreset_enable_dword = this.apply_expand(
            this.planar_setreset_enable,
        )

        // Write modes - see http://www.osdever.net/FreeVGA/vga/graphreg.htm#05
        switch (write_mode) {
            case 0:
                value = this.apply_rotate(value)
                plane_dword = this.apply_feed(value)
                plane_dword = this.apply_setreset(
                    plane_dword,
                    setreset_enable_dword,
                )
                plane_dword = this.apply_logical(plane_dword, this.latch_dword)
                plane_dword = this.apply_bitmask(plane_dword, bitmask)
                break
            case 1:
                plane_dword = this.latch_dword
                break
            case 2:
                plane_dword = this.apply_expand(value)
                plane_dword = this.apply_logical(plane_dword, this.latch_dword)
                plane_dword = this.apply_bitmask(plane_dword, bitmask)
                break
            case 3:
                value = this.apply_rotate(value)
                bitmask &= this.apply_feed(value)
                plane_dword = setreset_dword
                plane_dword = this.apply_bitmask(plane_dword, bitmask)
                break
        }

        let plane_select = 0xf

        switch (this.sequencer_memory_mode & 0xc) {
            // Odd/Even (aka chain 2)
            case 0x0:
                plane_select = 0x5 << (addr & 0x1)
                addr &= ~0x1
                break

            // Chain 4
            // Note: FreeVGA may have mistakenly stated that this bit field is
            // for system read only, yet the IBM Open Source Graphics Programmer's
            // Reference Manual explicitly states "both read and write".
            case 0x8:
            case 0xc:
                plane_select = 1 << (addr & 0x3)
                addr &= ~0x3
                break
        }

        // Plane masks take precedence
        // See: http://www.osdever.net/FreeVGA/vga/seqreg.htm#02
        plane_select &= this.plane_write_bm

        if (plane_select & 0x1) this.plane0[addr] = (plane_dword >> 0) & 0xff
        if (plane_select & 0x2) this.plane1[addr] = (plane_dword >> 8) & 0xff
        if (plane_select & 0x4) this.plane2[addr] = (plane_dword >> 16) & 0xff
        if (plane_select & 0x8) this.plane3[addr] = (plane_dword >> 24) & 0xff

        const pixel_addr = this.vga_addr_to_pixel(addr)
        this.partial_replot(pixel_addr, pixel_addr + 7)
    }

    // Copies data_byte into the four planes, with each plane
    // represented by an 8-bit field inside the dword.
    apply_feed(data_byte: number): number {
        let dword = data_byte
        dword |= data_byte << 8
        dword |= data_byte << 16
        dword |= data_byte << 24
        return dword
    }

    // Expands bits 0 to 3 to ocupy bits 0 to 31. Each
    // bit is expanded to 0xFF if set or 0x00 if clear.
    apply_expand(data_byte: number): number {
        let dword = data_byte & 0x1 ? 0xff : 0x00
        dword |= (data_byte & 0x2 ? 0xff : 0x00) << 8
        dword |= (data_byte & 0x4 ? 0xff : 0x00) << 16
        dword |= (data_byte & 0x8 ? 0xff : 0x00) << 24
        return dword
    }

    // Planar Write - Barrel Shifter
    // @see http://www.phatcode.net/res/224/files/html/ch25/25-01.html#Heading3
    // @see http://www.osdever.net/FreeVGA/vga/graphreg.htm#03
    apply_rotate(data_byte: number): number {
        const wrapped = data_byte | (data_byte << 8)
        const count = this.planar_rotate_reg & 0x7
        const shifted = wrapped >>> count
        return shifted & 0xff
    }

    // Planar Write - Set / Reset Circuitry
    // @see http://www.phatcode.net/res/224/files/html/ch25/25-03.html#Heading5
    // @see http://www.osdever.net/FreeVGA/vga/graphreg.htm#00
    apply_setreset(data_dword: number, enable_dword: number): number {
        const setreset_dword = this.apply_expand(this.planar_setreset)
        data_dword |= enable_dword & setreset_dword
        data_dword &= ~enable_dword | setreset_dword
        return data_dword
    }

    // Planar Write - ALU Unit
    // @see http://www.phatcode.net/res/224/files/html/ch24/24-01.html#Heading3
    // @see http://www.osdever.net/FreeVGA/vga/graphreg.htm#03
    apply_logical(data_dword: number, latch_dword: number): number {
        switch (this.planar_rotate_reg & 0x18) {
            case 0x08:
                return data_dword & latch_dword
            case 0x10:
                return data_dword | latch_dword
            case 0x18:
                return data_dword ^ latch_dword
        }
        return data_dword
    }

    // Planar Write - Bitmask Unit
    // @see http://www.phatcode.net/res/224/files/html/ch25/25-01.html#Heading2
    // @see http://www.osdever.net/FreeVGA/vga/graphreg.htm#08
    apply_bitmask(data_dword: number, bitmask_dword: number): number {
        let plane_dword = bitmask_dword & data_dword
        plane_dword |= ~bitmask_dword & this.latch_dword
        return plane_dword
    }

    text_mode_redraw(): void {
        const split_screen_row = this.scan_line_to_screen_row(this.line_compare)
        const row_offset = Math.max(
            0,
            (this.offset_register * 2 - this.max_cols) * 2,
        )
        const blink_enabled = this.attribute_mode & (1 << 3)
        const fg_color_mask = this.font_page_ab_enabled ? 7 : 0xf
        const bg_color_mask = blink_enabled ? 7 : 0xf
        const FLAG_BLINKING = this.screen.FLAG_BLINKING
        const FLAG_FONT_PAGE_B = this.screen.FLAG_FONT_PAGE_B

        let addr = this.start_address << 1

        for (let row = 0; row < this.max_rows; row++) {
            if (row === split_screen_row) {
                addr = 0
            }

            for (let col = 0; col < this.max_cols; col++) {
                const chr = this.vga_memory[addr]
                const color = this.vga_memory[addr | 1]
                const blinking = blink_enabled && color & (1 << 7)
                const font_page_b =
                    this.font_page_ab_enabled && !(color & (1 << 3))
                const flags =
                    (blinking ? FLAG_BLINKING : 0) |
                    (font_page_b ? FLAG_FONT_PAGE_B : 0)

                this.bus.send('screen-put-char', [row, col, chr])

                this.screen.put_char(
                    row,
                    col,
                    chr,
                    flags,
                    this.vga256_palette[
                        this.dac_mask &
                            this.dac_map[(color >> 4) & bg_color_mask]
                    ],
                    this.vga256_palette[
                        this.dac_mask & this.dac_map[color & fg_color_mask]
                    ],
                )

                addr += 2
            }

            addr += row_offset
        }
    }

    vga_memory_write_text_mode(addr: number, value: number): void {
        this.vga_memory[addr] = value

        const max_cols = Math.max(this.max_cols, this.offset_register * 2)
        let row: number
        let col: number

        if (addr >> 1 >= this.start_address) {
            const memory_start = (addr >> 1) - this.start_address
            row = (memory_start / max_cols) | 0
            col = memory_start % max_cols
        } else {
            const memory_start = addr >> 1
            row =
                ((memory_start / max_cols) | 0) +
                this.scan_line_to_screen_row(this.line_compare)
            col = memory_start % max_cols
        }

        dbg_assert(row >= 0 && col >= 0)

        if (col >= this.max_cols || row >= this.max_rows) {
            return
        }

        let chr: number
        let color: number

        // XXX: Should handle 16 bit write if possible
        if (addr & 1) {
            color = value
            chr = this.vga_memory[addr & ~1]
        } else {
            chr = value
            color = this.vga_memory[addr | 1]
        }
        const blink_enabled = this.attribute_mode & (1 << 3)
        const blinking = blink_enabled && color & (1 << 7)
        const font_page_b = this.font_page_ab_enabled && !(color & (1 << 3))
        const flags =
            (blinking ? this.screen.FLAG_BLINKING : 0) |
            (font_page_b ? this.screen.FLAG_FONT_PAGE_B : 0)
        const fg_color_mask = this.font_page_ab_enabled ? 7 : 0xf
        const bg_color_mask = blink_enabled ? 7 : 0xf

        this.bus.send('screen-put-char', [row, col, chr])

        this.screen.put_char(
            row,
            col,
            chr,
            flags,
            this.vga256_palette[
                this.dac_mask & this.dac_map[(color >> 4) & bg_color_mask]
            ],
            this.vga256_palette[
                this.dac_mask & this.dac_map[color & fg_color_mask]
            ],
        )
    }

    update_cursor(): void {
        const max_cols = Math.max(this.max_cols, this.offset_register * 2)
        let row: number
        let col: number

        if (this.cursor_address >= this.start_address) {
            row = ((this.cursor_address - this.start_address) / max_cols) | 0
            col = (this.cursor_address - this.start_address) % max_cols
        } else {
            row =
                ((this.cursor_address / max_cols) | 0) +
                this.scan_line_to_screen_row(this.line_compare)
            col = this.cursor_address % max_cols
        }

        dbg_assert(row >= 0 && col >= 0)

        // NOTE: is allowed to be out of bounds
        this.screen.update_cursor(row, col)
    }

    complete_redraw(): void {
        dbg_log('complete redraw', LOG_VGA)

        if (this.graphical_mode) {
            if (this.svga_enabled) {
                this.cpu.svga_mark_dirty()
            } else {
                this.diff_addr_min = 0
                this.diff_addr_max = VGA_PIXEL_BUFFER_SIZE
            }
        } else {
            this.text_mode_redraw()
        }
    }

    complete_replot(): void {
        dbg_log('complete replot', LOG_VGA)

        if (!this.graphical_mode || this.svga_enabled) {
            return
        }

        this.diff_plot_min = 0
        this.diff_plot_max = VGA_PIXEL_BUFFER_SIZE

        this.complete_redraw()
    }

    partial_redraw(min: number, max: number): void {
        if (min < this.diff_addr_min) this.diff_addr_min = min
        if (max > this.diff_addr_max) this.diff_addr_max = max
    }

    partial_replot(min: number, max: number): void {
        if (min < this.diff_plot_min) this.diff_plot_min = min
        if (max > this.diff_plot_max) this.diff_plot_max = max

        this.partial_redraw(min, max)
    }

    reset_diffs(): void {
        this.diff_addr_min = this.vga_memory_size
        this.diff_addr_max = 0
        this.diff_plot_min = this.vga_memory_size
        this.diff_plot_max = 0
    }

    destroy(): void {}

    vga_bytes_per_line(): number {
        let bytes_per_line = this.offset_register << 2
        if (this.underline_location_register & 0x40) bytes_per_line <<= 1
        else if (this.crtc_mode & 0x40) bytes_per_line >>>= 1
        return bytes_per_line
    }

    vga_addr_shift_count(): number {
        // Count in multiples of 0x40 for convenience
        // Left shift 2 for word mode - 2 bytes per dot clock
        let shift_count = 0x80

        // Left shift 3 for byte mode - 1 byte per dot clock
        shift_count += ~this.underline_location_register & this.crtc_mode & 0x40

        // Left shift 1 for doubleword mode - 4 bytes per dot clock
        shift_count -= this.underline_location_register & 0x40

        // But shift one less if PEL width mode - 2 dot clocks per pixel
        shift_count -= this.attribute_mode & 0x40

        return shift_count >>> 6
    }

    vga_addr_to_pixel(addr: number): number {
        const shift_count = this.vga_addr_shift_count()

        // Undo effects of substituted bits 13 and 14
        // Assumptions:
        //  - max_scan_line register is set to the values shown below
        //  - Each scan line stays within the offset alignment
        //  - No panning and no page flipping after drawing
        if (~this.crtc_mode & 0x3) {
            let pixel_addr = addr - this.start_address

            // Remove substituted bits
            pixel_addr &= (this.crtc_mode << 13) | ~0x6000

            // Convert to 1 pixel per address
            pixel_addr <<= shift_count

            // Decompose address
            let row = (pixel_addr / this.virtual_width) | 0
            const col = pixel_addr % this.virtual_width

            switch (this.crtc_mode & 0x3) {
                case 0x2:
                    // Alternating rows using bit 13
                    // Assumes max scan line = 1
                    row = (row << 1) | ((addr >> 13) & 0x1)
                    break
                case 0x1:
                    // Alternating rows using bit 14
                    // Assumes max scan line = 3
                    row = (row << 1) | ((addr >> 14) & 0x1)
                    break
                case 0x0:
                    // Cycling through rows using bit 13 and 14
                    // Assumes max scan line = 3
                    row = (row << 2) | ((addr >> 13) & 0x3)
                    break
            }

            // Reassemble address
            return (
                row * this.virtual_width +
                col +
                (this.start_address << shift_count)
            )
        } else {
            // Convert to 1 pixel per address
            return addr << shift_count
        }
    }

    scan_line_to_screen_row(scan_line: number): number {
        // Double scanning. The clock to the row scan counter is halved
        // so it is not affected by the memory address bit substitutions below
        if (this.max_scan_line & 0x80) {
            scan_line >>>= 1
        }

        // Maximum scan line, aka scan lines per character row
        // This is the number of repeats - 1 for graphic modes
        const repeat_factor = 1 + (this.max_scan_line & 0x1f)
        scan_line = Math.ceil(scan_line / repeat_factor)

        // Odd and Even Row Scan Counter
        // Despite repeated address counter values, because bit 13 of the shifted
        // address is substituted with bit 0 of the row scan counter, a different
        // display buffer address is generated instead of repeated
        // Assumes maximum scan line register is set to 2 or 4.
        // Note: can't assert this as register values may not be fully programmed.
        if (!(this.crtc_mode & 0x1)) {
            scan_line <<= 1
        }

        // Undo effects of substituted bit 14
        // Assumes maximum scan line register is set to 2 or 4
        // Note: can't assert this as register values may not be fully programmed.
        // Other maximum scan line register values would result in weird addressing
        // anyway
        if (!(this.crtc_mode & 0x2)) {
            scan_line <<= 1
        }

        return scan_line
    }

    set_size_text(cols_count: number, rows_count: number): void {
        dbg_assert(!this.graphical_mode)
        this.max_cols = cols_count
        this.max_rows = rows_count

        this.screen.set_size_text(cols_count, rows_count)
        this.bus.send('screen-set-size', [cols_count, rows_count, 0])
    }

    set_size_graphical(
        width: number,
        height: number,
        virtual_width: number,
        virtual_height: number,
        bpp: number,
    ): void {
        dbg_assert(this.graphical_mode)

        virtual_width = Math.max(virtual_width, 1)
        virtual_height = Math.max(virtual_height, 1)

        const needs_update =
            this.screen_width !== width ||
            this.screen_height !== height ||
            this.virtual_width !== virtual_width ||
            this.virtual_height !== virtual_height

        if (needs_update) {
            this.screen_width = width
            this.screen_height = height
            this.virtual_width = virtual_width
            this.virtual_height = virtual_height

            if (typeof ImageData !== 'undefined') {
                const size = virtual_width * virtual_height
                const offset = this.cpu.svga_allocate_dest_buffer(size) >>> 0

                this.dest_buffet_offset = offset
                this.image_data = new ImageData(
                    new Uint8ClampedArray(
                        this.cpu.wasm_memory.buffer,
                        offset,
                        4 * size,
                    ),
                    virtual_width,
                    virtual_height,
                )

                this.cpu.svga_mark_dirty()
            } else {
                // TODO: nodejs
            }

            this.screen.set_size_graphical(
                width,
                height,
                virtual_width,
                virtual_height,
            )
            this.bus.send('screen-set-size', [width, height, bpp])
        }
    }

    update_vga_size(): void {
        if (this.svga_enabled) {
            return
        }

        const horizontal_characters = Math.min(
            1 + this.horizontal_display_enable_end,
            this.horizontal_blank_start,
        )
        let vertical_scans = Math.min(
            1 + this.vertical_display_enable_end,
            this.vertical_blank_start,
        )

        if (!horizontal_characters || !vertical_scans) {
            // Don't update if width or height is zero.
            // These happen when registers are not fully configured yet.
            return
        }

        if (this.graphical_mode) {
            let screen_width = horizontal_characters << 3

            // Offset is half the number of bytes/words/dwords (depending on clocking mode)
            // of display memory that each logical line occupies.
            // However, the number of pixels latched, regardless of addressing mode,
            // should always 8 pixels per character clock (except for 8 bit PEL width, in which
            // case 4 pixels).
            let virtual_width = this.offset_register << 4
            let bpp = 4

            // Pixel Width / PEL Width / Clock Select
            if (this.attribute_mode & 0x40) {
                screen_width >>>= 1
                virtual_width >>>= 1
                bpp = 8
            } else if (this.attribute_mode & 0x2) {
                bpp = 1
            }

            const screen_height = this.scan_line_to_screen_row(vertical_scans)

            // The virtual buffer height is however many rows of data that can fit.
            // Previously drawn graphics outside of current memory address space can
            // still be drawn by setting start_address. The address at
            // VGA_HOST_MEMORY_SPACE_START[memory_space_select] is mapped to the first
            // byte of the frame buffer. Verified on some hardware.
            // Depended on by: Windows 98 start screen
            const available_bytes = VGA_HOST_MEMORY_SPACE_SIZE[0]

            const bytes_per_line = this.vga_bytes_per_line()
            const virtual_height = bytes_per_line
                ? Math.ceil(available_bytes / bytes_per_line)
                : screen_height

            this.set_size_graphical(
                screen_width,
                screen_height,
                virtual_width,
                virtual_height,
                bpp,
            )

            this.update_vertical_retrace()
            this.update_layers()
        } else {
            if (this.max_scan_line & 0x80) {
                // Double scanning means that half of those scan lines
                // are just repeats
                vertical_scans >>>= 1
            }

            const height =
                (vertical_scans / (1 + (this.max_scan_line & 0x1f))) | 0

            if (horizontal_characters && height) {
                this.set_size_text(horizontal_characters, height)
            }
        }
    }

    update_layers(): void {
        if (!this.graphical_mode) {
            this.text_mode_redraw()
        }

        if (this.svga_enabled) {
            this.layers = []
            return
        }

        if (!this.virtual_width || !this.screen_width) {
            // Avoid division by zero
            return
        }

        if (!this.palette_source || this.clocking_mode & 0x20) {
            // Palette source and screen disable bits = draw nothing
            // See http://www.phatcode.net/res/224/files/html/ch29/29-05.html#Heading6
            // and http://www.osdever.net/FreeVGA/vga/seqreg.htm#01
            this.layers = []
            this.screen.clear_screen()
            return
        }

        const start_addr = this.start_address_latched

        let pixel_panning = this.horizontal_panning
        if (this.attribute_mode & 0x40) {
            pixel_panning >>>= 1
        }

        const byte_panning = (this.preset_row_scan >> 5) & 0x3
        const pixel_addr_start = this.vga_addr_to_pixel(
            start_addr + byte_panning,
        )

        const start_buffer_row = (pixel_addr_start / this.virtual_width) | 0
        const start_buffer_col =
            (pixel_addr_start % this.virtual_width) + pixel_panning

        let split_screen_row = this.scan_line_to_screen_row(
            1 + this.line_compare,
        )
        split_screen_row = Math.min(split_screen_row, this.screen_height)

        const split_buffer_height = this.screen_height - split_screen_row

        this.layers = []

        for (
            let x = -start_buffer_col, y = 0;
            x < this.screen_width;
            x += this.virtual_width, y++
        ) {
            this.layers.push({
                image_data: this.image_data,
                screen_x: x,
                screen_y: 0,
                buffer_x: 0,
                buffer_y: start_buffer_row + y,
                buffer_width: this.virtual_width,
                buffer_height: split_screen_row,
            })
        }

        let start_split_col = 0
        if (!(this.attribute_mode & 0x20)) {
            // Pixel panning mode. Allow panning for the lower split screen
            start_split_col =
                this.vga_addr_to_pixel(byte_panning) + pixel_panning
        }

        for (
            let x = -start_split_col, y = 0;
            x < this.screen_width;
            x += this.virtual_width, y++
        ) {
            this.layers.push({
                image_data: this.image_data,
                screen_x: x,
                screen_y: split_screen_row,
                buffer_x: 0,
                buffer_y: y,
                buffer_width: this.virtual_width,
                buffer_height: split_buffer_height,
            })
        }
    }

    update_vertical_retrace(): void {
        // Emulate behaviour during VSync/VRetrace
        this.port_3DA_value |= 0x8
        if (this.start_address_latched !== this.start_address) {
            this.start_address_latched = this.start_address
            this.update_layers()
        }
    }

    update_cursor_scanline(): void {
        const disabled = this.cursor_scanline_start & 0x20
        const max = this.max_scan_line & 0x1f
        const start = Math.min(max, this.cursor_scanline_start & 0x1f)
        const end = Math.min(max, this.cursor_scanline_end & 0x1f)
        const visible = !disabled && start < end
        this.screen.update_cursor_scanline(start, end, visible)
    }

    // Attribute controller register / index write
    // @see http://www.osdever.net/FreeVGA/vga/attrreg.htm
    // @see http://www.mcamafia.de/pdf/ibm_vgaxga_trm2.pdf page 89
    // @see https://01.org/sites/default/files/documentation/intel-gfx-prm-osrc-hsw-display_0.pdf page 48
    port3C0_write(value: number): void {
        if (this.attribute_controller_index === -1) {
            dbg_log('attribute controller index register: ' + h(value), LOG_VGA)
            this.attribute_controller_index = value & 0x1f
            dbg_log(
                'attribute actual index: ' + h(this.attribute_controller_index),
                LOG_VGA,
            )

            if (this.palette_source !== (value & 0x20)) {
                // A method of blanking the screen.
                // See http://www.phatcode.net/res/224/files/html/ch29/29-05.html#Heading6
                this.palette_source = value & 0x20
                this.update_layers()
            }
        } else {
            if (this.attribute_controller_index < 0x10) {
                dbg_log(
                    'internal palette: ' +
                        h(this.attribute_controller_index) +
                        ' -> ' +
                        h(value),
                    LOG_VGA,
                )
                this.dac_map[this.attribute_controller_index] = value

                if (!(this.attribute_mode & 0x40)) {
                    this.complete_redraw()
                }
            } else
                switch (this.attribute_controller_index) {
                    case 0x10:
                        dbg_log(
                            '3C0 / attribute mode control: ' + h(value),
                            LOG_VGA,
                        )
                        if (this.attribute_mode !== value) {
                            const previous_mode = this.attribute_mode
                            this.attribute_mode = value

                            const is_graphical = (value & 0x1) !== 0
                            if (
                                !this.svga_enabled &&
                                this.graphical_mode !== is_graphical
                            ) {
                                this.graphical_mode = is_graphical
                                this.screen.set_mode(this.graphical_mode)
                            }

                            if ((previous_mode ^ value) & 0x40) {
                                // PEL width changed. Pixel Buffer now invalidated
                                this.complete_replot()
                            }

                            this.update_vga_size()

                            // Data stored in image buffer are invalidated
                            this.complete_redraw()

                            this.set_font_bitmap(false)
                        }
                        break
                    case 0x12:
                        dbg_log(
                            '3C0 / color plane enable: ' + h(value),
                            LOG_VGA,
                        )
                        if (this.color_plane_enable !== value) {
                            this.color_plane_enable = value

                            // Data stored in image buffer are invalidated
                            this.complete_redraw()
                        }
                        break
                    case 0x13:
                        dbg_log(
                            '3C0 / horizontal panning: ' + h(value),
                            LOG_VGA,
                        )
                        if (this.horizontal_panning !== value) {
                            this.horizontal_panning = value & 0xf
                            this.update_layers()
                        }
                        break
                    case 0x14:
                        dbg_log('3C0 / color select: ' + h(value), LOG_VGA)
                        if (this.color_select !== value) {
                            this.color_select = value

                            // Data stored in image buffer are invalidated
                            this.complete_redraw()
                        }
                        break
                    default:
                        dbg_log(
                            '3C0 / attribute controller write ' +
                                h(this.attribute_controller_index) +
                                ': ' +
                                h(value),
                            LOG_VGA,
                        )
                }

            this.attribute_controller_index = -1
        }
    }

    port3C0_read(): number {
        dbg_log('3C0 read', LOG_VGA)
        return (this.attribute_controller_index | this.palette_source) & 0xff
    }

    port3C0_read16(): number {
        dbg_log('3C0 read16', LOG_VGA)
        return this.port3C0_read() | ((this.port3C1_read() << 8) & 0xff00)
    }

    port3C1_read(): number {
        if (this.attribute_controller_index < 0x10) {
            dbg_log(
                '3C1 / internal palette read: ' +
                    h(this.attribute_controller_index) +
                    ' -> ' +
                    h(this.dac_map[this.attribute_controller_index]),
                LOG_VGA,
            )
            return this.dac_map[this.attribute_controller_index] & 0xff
        }

        switch (this.attribute_controller_index) {
            case 0x10:
                dbg_log(
                    '3C1 / attribute mode read: ' + h(this.attribute_mode),
                    LOG_VGA,
                )
                return this.attribute_mode
            case 0x12:
                dbg_log(
                    '3C1 / color plane enable read: ' +
                        h(this.color_plane_enable),
                    LOG_VGA,
                )
                return this.color_plane_enable
            case 0x13:
                dbg_log(
                    '3C1 / horizontal panning read: ' +
                        h(this.horizontal_panning),
                    LOG_VGA,
                )
                return this.horizontal_panning
            case 0x14:
                dbg_log(
                    '3C1 / color select read: ' + h(this.color_select),
                    LOG_VGA,
                )
                return this.color_select
            default:
                dbg_log(
                    '3C1 / attribute controller read ' +
                        h(this.attribute_controller_index),
                    LOG_VGA,
                )
        }
        return 0xff
    }

    port3C2_write(value: number): void {
        dbg_log('3C2 / miscellaneous output register = ' + h(value), LOG_VGA)
        this.miscellaneous_output_register = value
    }

    port3C4_write(value: number): void {
        this.sequencer_index = value
    }

    port3C4_read(): number {
        return this.sequencer_index
    }

    // Sequencer register writes
    // @see http://www.osdever.net/FreeVGA/vga/seqreg.htm
    // @see http://www.mcamafia.de/pdf/ibm_vgaxga_trm2.pdf page 47
    // @see https://01.org/sites/default/files/documentation/intel-gfx-prm-osrc-hsw-display_0.pdf page 19
    port3C5_write(value: number): void {
        switch (this.sequencer_index) {
            case 0x01: {
                dbg_log('clocking mode: ' + h(value), LOG_VGA)
                const previous_clocking_mode = this.clocking_mode
                this.clocking_mode = value
                if ((previous_clocking_mode ^ value) & 0x20) {
                    // Screen disable bit modified
                    this.update_layers()
                }
                this.set_font_bitmap(false)
                break
            }
            case 0x02: {
                dbg_log('plane write mask: ' + h(value), LOG_VGA)
                const previous_plane_write_bm = this.plane_write_bm
                this.plane_write_bm = value
                if (
                    !this.graphical_mode &&
                    previous_plane_write_bm & 0x4 &&
                    !(this.plane_write_bm & 0x4)
                ) {
                    // End of font plane 2 write access
                    this.set_font_bitmap(true)
                }
                break
            }
            case 0x03: {
                dbg_log('character map select: ' + h(value), LOG_VGA)
                const previous_character_map_select = this.character_map_select
                this.character_map_select = value
                if (
                    !this.graphical_mode &&
                    previous_character_map_select !== value
                ) {
                    this.set_font_page()
                }
                break
            }
            case 0x04:
                dbg_log('sequencer memory mode: ' + h(value), LOG_VGA)
                this.sequencer_memory_mode = value
                break
            default:
                dbg_log(
                    '3C5 / sequencer write ' +
                        h(this.sequencer_index) +
                        ': ' +
                        h(value),
                    LOG_VGA,
                )
        }
    }

    port3C5_read(): number {
        dbg_log('3C5 / sequencer read ' + h(this.sequencer_index), LOG_VGA)

        switch (this.sequencer_index) {
            case 0x01:
                return this.clocking_mode
            case 0x02:
                return this.plane_write_bm
            case 0x03:
                return this.character_map_select
            case 0x04:
                return this.sequencer_memory_mode
            case 0x06:
                return 0x12
            default:
        }
        return 0
    }

    port3C6_write(data: number): void {
        if (this.dac_mask !== data) {
            this.dac_mask = data
            this.complete_redraw()
        }
    }

    port3C6_read(): number {
        return this.dac_mask
    }

    port3C7_write(index: number): void {
        // index for reading the DAC
        dbg_log('3C7 write: ' + h(index), LOG_VGA)
        this.dac_color_index_read = index * 3
        this.dac_state &= 0x0
    }

    port3C7_read(): number {
        // prepared to accept reads or writes
        return this.dac_state
    }

    port3C8_write(index: number): void {
        this.dac_color_index_write = index * 3
        this.dac_state |= 0x3
    }

    port3C8_read(): number {
        return (this.dac_color_index_write / 3) & 0xff
    }

    // DAC color palette register writes
    // @see http://www.osdever.net/FreeVGA/vga/colorreg.htm
    // @see http://www.mcamafia.de/pdf/ibm_vgaxga_trm2.pdf page 104
    // @see https://01.org/sites/default/files/documentation/intel-gfx-prm-osrc-hsw-display_0.pdf page 57
    port3C9_write(color_byte: number): void {
        const index = (this.dac_color_index_write / 3) | 0,
            offset = this.dac_color_index_write % 3
        let color = this.vga256_palette[index]

        if ((this.dispi_enable_value & 0x20) === 0) {
            color_byte &= 0x3f
            const b = color_byte & 1
            color_byte = (color_byte << 2) | (b << 1) | b
        }

        if (offset === 0) {
            color = (color & ~0xff0000) | (color_byte << 16)
        } else if (offset === 1) {
            color = (color & ~0xff00) | (color_byte << 8)
        } else {
            color = (color & ~0xff) | color_byte
            dbg_log(
                'dac set color, index=' + h(index) + ' value=' + h(color),
                LOG_VGA,
            )
        }

        if (this.vga256_palette[index] !== color) {
            this.vga256_palette[index] = color
            this.complete_redraw()
        }
        this.dac_color_index_write++
    }

    port3C9_read(): number {
        dbg_log('3C9 read', LOG_VGA)

        const index = (this.dac_color_index_read / 3) | 0
        const offset = this.dac_color_index_read % 3
        const color = this.vga256_palette[index]
        const color8 = (color >> ((2 - offset) * 8)) & 0xff

        this.dac_color_index_read++

        if (this.dispi_enable_value & 0x20) {
            return color8
        } else {
            return color8 >> 2
        }
    }

    port3CC_read(): number {
        dbg_log('3CC read', LOG_VGA)
        return this.miscellaneous_output_register
    }

    port3CE_write(value: number): void {
        this.graphics_index = value
    }

    port3CE_read(): number {
        return this.graphics_index
    }

    // Graphics controller register writes
    // @see http://www.osdever.net/FreeVGA/vga/graphreg.htm
    // @see http://www.mcamafia.de/pdf/ibm_vgaxga_trm2.pdf page 78
    // @see https://01.org/sites/default/files/documentation/intel-gfx-prm-osrc-hsw-display_0.pdf page 29
    port3CF_write(value: number): void {
        switch (this.graphics_index) {
            case 0:
                this.planar_setreset = value
                dbg_log('plane set/reset: ' + h(value), LOG_VGA)
                break
            case 1:
                this.planar_setreset_enable = value
                dbg_log('plane set/reset enable: ' + h(value), LOG_VGA)
                break
            case 2:
                this.color_compare = value
                dbg_log('color compare: ' + h(value), LOG_VGA)
                break
            case 3:
                this.planar_rotate_reg = value
                dbg_log('plane rotate: ' + h(value), LOG_VGA)
                break
            case 4:
                this.plane_read = value
                dbg_log('plane read: ' + h(value), LOG_VGA)
                break
            case 5: {
                const previous_planar_mode = this.planar_mode
                this.planar_mode = value
                dbg_log('planar mode: ' + h(value), LOG_VGA)
                if ((previous_planar_mode ^ value) & 0x60) {
                    // Shift mode modified. Pixel buffer invalidated
                    this.complete_replot()
                }
                break
            }
            case 6:
                dbg_log('miscellaneous graphics register: ' + h(value), LOG_VGA)
                if (this.miscellaneous_graphics_register !== value) {
                    this.miscellaneous_graphics_register = value
                    this.update_vga_size()
                }
                break
            case 7:
                this.color_dont_care = value
                dbg_log("color don't care: " + h(value), LOG_VGA)
                break
            case 8:
                this.planar_bitmap = value
                dbg_log('planar bitmap: ' + h(value), LOG_VGA)
                break
            default:
                dbg_log(
                    '3CF / graphics write ' +
                        h(this.graphics_index) +
                        ': ' +
                        h(value),
                    LOG_VGA,
                )
        }
    }

    port3CF_read(): number {
        dbg_log('3CF / graphics read ' + h(this.graphics_index), LOG_VGA)

        switch (this.graphics_index) {
            case 0:
                return this.planar_setreset
            case 1:
                return this.planar_setreset_enable
            case 2:
                return this.color_compare
            case 3:
                return this.planar_rotate_reg
            case 4:
                return this.plane_read
            case 5:
                return this.planar_mode
            case 6:
                return this.miscellaneous_graphics_register
            case 7:
                return this.color_dont_care
            case 8:
                return this.planar_bitmap
            default:
        }
        return 0
    }

    port3D4_write(register: number): void {
        dbg_log('3D4 / crtc index: ' + register, LOG_VGA)
        this.index_crtc = register
    }

    port3D4_write16(register: number): void {
        this.port3D4_write(register & 0xff)
        this.port3D5_write((register >> 8) & 0xff)
    }

    port3D4_read(): number {
        dbg_log('3D4 read / crtc index: ' + this.index_crtc, LOG_VGA)
        return this.index_crtc
    }

    // CRT controller register writes
    // @see http://www.osdever.net/FreeVGA/vga/crtcreg.htm
    // @see http://www.mcamafia.de/pdf/ibm_vgaxga_trm2.pdf page 55
    // @see https://01.org/sites/default/files/documentation/intel-gfx-prm-osrc-hsw-display_0.pdf page 63
    port3D5_write(value: number): void {
        switch (this.index_crtc) {
            case 0x1:
                dbg_log('3D5 / hdisp enable end write: ' + h(value), LOG_VGA)
                if (this.horizontal_display_enable_end !== value) {
                    this.horizontal_display_enable_end = value
                    this.update_vga_size()
                }
                break
            case 0x2:
                if (this.horizontal_blank_start !== value) {
                    this.horizontal_blank_start = value
                    this.update_vga_size()
                }
                break
            case 0x7: {
                dbg_log('3D5 / overflow register write: ' + h(value), LOG_VGA)
                const previous_vertical_display_enable_end =
                    this.vertical_display_enable_end
                this.vertical_display_enable_end &= 0xff
                this.vertical_display_enable_end |=
                    ((value << 3) & 0x200) | ((value << 7) & 0x100)
                if (
                    previous_vertical_display_enable_end !==
                    this.vertical_display_enable_end
                ) {
                    this.update_vga_size()
                }
                this.line_compare =
                    (this.line_compare & 0x2ff) | ((value << 4) & 0x100)

                const previous_vertical_blank_start = this.vertical_blank_start
                this.vertical_blank_start =
                    (this.vertical_blank_start & 0x2ff) | ((value << 5) & 0x100)
                if (
                    previous_vertical_blank_start !== this.vertical_blank_start
                ) {
                    this.update_vga_size()
                }
                this.update_layers()
                break
            }
            case 0x8:
                dbg_log('3D5 / preset row scan write: ' + h(value), LOG_VGA)
                this.preset_row_scan = value
                this.update_layers()
                break
            case 0x9: {
                dbg_log('3D5 / max scan line write: ' + h(value), LOG_VGA)
                const previous_max_scan_line = this.max_scan_line
                this.max_scan_line = value
                this.line_compare =
                    (this.line_compare & 0x1ff) | ((value << 3) & 0x200)

                const previous_vertical_blank_start2 = this.vertical_blank_start
                this.vertical_blank_start =
                    (this.vertical_blank_start & 0x1ff) | ((value << 4) & 0x200)
                if (
                    (previous_max_scan_line ^ this.max_scan_line) & 0x9f ||
                    previous_vertical_blank_start2 !== this.vertical_blank_start
                ) {
                    this.update_vga_size()
                }

                this.update_cursor_scanline()
                this.update_layers()

                this.set_font_bitmap(false)
                break
            }
            case 0xa:
                dbg_log(
                    '3D5 / cursor scanline start write: ' + h(value),
                    LOG_VGA,
                )
                this.cursor_scanline_start = value
                this.update_cursor_scanline()
                break
            case 0xb:
                dbg_log('3D5 / cursor scanline end write: ' + h(value), LOG_VGA)
                this.cursor_scanline_end = value
                this.update_cursor_scanline()
                break
            case 0xc:
                if (((this.start_address >> 8) & 0xff) !== value) {
                    this.start_address =
                        (this.start_address & 0xff) | (value << 8)
                    this.update_layers()
                    if (~this.crtc_mode & 0x3) {
                        // Address substitution implementation depends on the
                        // starting row and column, so the pixel buffer is invalidated.
                        this.complete_replot()
                    }
                }
                dbg_log(
                    '3D5 / start addr hi write: ' +
                        h(value) +
                        ' -> ' +
                        h(this.start_address, 4),
                    LOG_VGA,
                )
                break
            case 0xd:
                if ((this.start_address & 0xff) !== value) {
                    this.start_address = (this.start_address & 0xff00) | value
                    this.update_layers()
                    if (~this.crtc_mode & 0x3) {
                        // Address substitution implementation depends on the
                        // starting row and column, so the pixel buffer is invalidated.
                        this.complete_replot()
                    }
                }
                dbg_log(
                    '3D5 / start addr lo write: ' +
                        h(value) +
                        ' -> ' +
                        h(this.start_address, 4),
                    LOG_VGA,
                )
                break
            case 0xe:
                dbg_log('3D5 / cursor address hi write: ' + h(value), LOG_VGA)
                this.cursor_address =
                    (this.cursor_address & 0xff) | (value << 8)
                this.update_cursor()
                break
            case 0xf:
                dbg_log('3D5 / cursor address lo write: ' + h(value), LOG_VGA)
                this.cursor_address = (this.cursor_address & 0xff00) | value
                this.update_cursor()
                break
            case 0x12:
                dbg_log('3D5 / vdisp enable end write: ' + h(value), LOG_VGA)
                if ((this.vertical_display_enable_end & 0xff) !== value) {
                    this.vertical_display_enable_end =
                        (this.vertical_display_enable_end & 0x300) | value
                    this.update_vga_size()
                }
                break
            case 0x13:
                dbg_log('3D5 / offset register write: ' + h(value), LOG_VGA)
                if (this.offset_register !== value) {
                    this.offset_register = value
                    this.update_vga_size()

                    if (~this.crtc_mode & 0x3) {
                        // Address substitution implementation depends on the
                        // virtual width, so the pixel buffer is invalidated.
                        this.complete_replot()
                    }
                }
                break
            case 0x14:
                dbg_log('3D5 / underline location write: ' + h(value), LOG_VGA)
                if (this.underline_location_register !== value) {
                    const previous_underline = this.underline_location_register

                    this.underline_location_register = value
                    this.update_vga_size()

                    if ((previous_underline ^ value) & 0x40) {
                        // Doubleword addressing changed. Pixel buffer invalidated.
                        this.complete_replot()
                    }
                }
                break
            case 0x15:
                dbg_log(
                    '3D5 / vertical blank start write: ' + h(value),
                    LOG_VGA,
                )
                if ((this.vertical_blank_start & 0xff) !== value) {
                    this.vertical_blank_start =
                        (this.vertical_blank_start & 0x300) | value
                    this.update_vga_size()
                }
                break
            case 0x17:
                dbg_log('3D5 / crtc mode write: ' + h(value), LOG_VGA)
                if (this.crtc_mode !== value) {
                    const previous_mode = this.crtc_mode

                    this.crtc_mode = value
                    this.update_vga_size()

                    if ((previous_mode ^ value) & 0x43) {
                        // Word/byte addressing changed or address substitution changed.
                        // Pixel buffer invalidated.
                        this.complete_replot()
                    }
                }
                break
            case 0x18:
                dbg_log('3D5 / line compare write: ' + h(value), LOG_VGA)
                this.line_compare = (this.line_compare & 0x300) | value
                this.update_layers()
                break
            default:
                if (this.index_crtc < this.crtc.length) {
                    this.crtc[this.index_crtc] = value
                }
                dbg_log(
                    '3D5 / CRTC write ' + h(this.index_crtc) + ': ' + h(value),
                    LOG_VGA,
                )
        }
    }

    port3D5_write16(register: number): void {
        dbg_log('16-bit write to 3D5: ' + h(register, 4), LOG_VGA)
        this.port3D5_write(register & 0xff)
    }

    port3D5_read(): number {
        dbg_log('3D5 read ' + h(this.index_crtc), LOG_VGA)

        switch (this.index_crtc) {
            case 0x1:
                return this.horizontal_display_enable_end
            case 0x2:
                return this.horizontal_blank_start
            case 0x7:
                return (
                    ((this.vertical_display_enable_end >> 7) & 0x2) |
                    ((this.vertical_blank_start >> 5) & 0x8) |
                    ((this.line_compare >> 4) & 0x10) |
                    ((this.vertical_display_enable_end >> 3) & 0x40)
                )
            case 0x8:
                return this.preset_row_scan
            case 0x9:
                return this.max_scan_line
            case 0xa:
                return this.cursor_scanline_start
            case 0xb:
                return this.cursor_scanline_end
            case 0xc:
                return this.start_address & 0xff
            case 0xd:
                return this.start_address >> 8
            case 0xe:
                return this.cursor_address >> 8
            case 0xf:
                return this.cursor_address & 0xff
            case 0x12:
                return this.vertical_display_enable_end & 0xff
            case 0x13:
                return this.offset_register
            case 0x14:
                return this.underline_location_register
            case 0x15:
                return this.vertical_blank_start & 0xff
            case 0x17:
                return this.crtc_mode
            case 0x18:
                return this.line_compare & 0xff
        }

        if (this.index_crtc < this.crtc.length) {
            return this.crtc[this.index_crtc]
        } else {
            return 0
        }
    }

    port3D5_read16(): number {
        dbg_log('Warning: 16-bit read from 3D5', LOG_VGA)
        return this.port3D5_read()
    }

    port3DA_read(): number {
        dbg_log('3DA read - status 1 and clear attr index', LOG_VGA)

        const value = this.port_3DA_value

        // Status register, bit 3 set by update_vertical_retrace
        // during screen-fill-buffer
        if (!this.graphical_mode) {
            // But screen-fill-buffer may not get triggered in text mode
            // so toggle it manually here
            if (this.port_3DA_value & 1) {
                this.port_3DA_value ^= 8
            }
            this.port_3DA_value ^= 1
        } else {
            this.port_3DA_value ^= 1
            this.port_3DA_value &= 1
        }
        this.attribute_controller_index = -1
        return value
    }

    port1CE_write(value: number): void {
        this.dispi_index = value
    }

    port1CF_write(value: number): void {
        dbg_log(
            '1CF / dispi write ' + h(this.dispi_index) + ': ' + h(value),
            LOG_VGA,
        )

        const was_enabled = this.svga_enabled

        switch (this.dispi_index) {
            case 0:
                if (value >= 0xb0c0 && value <= 0xb0c5) {
                    this.svga_version = value
                } else {
                    dbg_log('Invalid version value: ' + h(value), LOG_VGA)
                }
                break
            case 1:
                this.svga_width = value
                if (this.svga_width > MAX_XRES) {
                    dbg_log(
                        'svga_width reduced from ' +
                            this.svga_width +
                            ' to ' +
                            MAX_XRES,
                        LOG_VGA,
                    )
                    this.svga_width = MAX_XRES
                }
                break
            case 2:
                this.svga_height = value
                if (this.svga_height > MAX_YRES) {
                    dbg_log(
                        'svga_height reduced from ' +
                            this.svga_height +
                            ' to ' +
                            MAX_YRES,
                        LOG_VGA,
                    )
                    this.svga_height = MAX_YRES
                }
                break
            case 3:
                this.svga_bpp = value
                break
            case 4:
                // enable, options
                this.svga_enabled = (value & 1) === 1
                if (this.svga_enabled && (value & 0x80) === 0) {
                    this.svga_memory.fill(0)
                }
                this.dispi_enable_value = value
                break
            case 5:
                dbg_log('SVGA bank offset: ' + h(value << 16), LOG_VGA)
                this.svga_bank_offset = value << 16
                break
            case 8:
                // x offset
                dbg_log('SVGA X offset: ' + h(value), LOG_VGA)
                if (this.svga_offset_x !== value) {
                    this.svga_offset_x = value
                    this.svga_offset =
                        this.svga_offset_y * this.svga_width +
                        this.svga_offset_x
                    this.complete_redraw()
                }
                break
            case 9:
                // y offset
                dbg_log(
                    'SVGA Y offset: ' +
                        h(value * this.svga_width) +
                        ' y=' +
                        h(value),
                    LOG_VGA,
                )
                if (this.svga_offset_y !== value) {
                    this.svga_offset_y = value
                    this.svga_offset =
                        this.svga_offset_y * this.svga_width +
                        this.svga_offset_x
                    this.complete_redraw()
                }
                break
            default:
                dbg_log(
                    'Unimplemented dispi write index: ' + h(this.dispi_index),
                    LOG_VGA,
                )
        }

        if (this.svga_enabled && (!this.svga_width || !this.svga_height)) {
            dbg_log(
                'SVGA: disabled because of invalid width/height: ' +
                    this.svga_width +
                    'x' +
                    this.svga_height,
                LOG_VGA,
            )
            this.svga_enabled = false
        }

        dbg_assert(this.svga_bpp !== 4, 'unimplemented svga bpp: 4')
        dbg_assert(
            this.svga_bpp === 4 ||
                this.svga_bpp === 8 ||
                this.svga_bpp === 15 ||
                this.svga_bpp === 16 ||
                this.svga_bpp === 24 ||
                this.svga_bpp === 32,
            'unexpected svga bpp: ' + this.svga_bpp,
        )

        if (this.svga_enabled) {
            dbg_log(
                'SVGA: enabled, ' +
                    this.svga_width +
                    'x' +
                    this.svga_height +
                    'x' +
                    this.svga_bpp,
                LOG_VGA,
            )
        } else {
            dbg_log('SVGA: disabled', LOG_VGA)
        }

        if (this.svga_enabled && !was_enabled) {
            this.svga_offset = 0
            this.svga_offset_x = 0
            this.svga_offset_y = 0

            this.graphical_mode = true
            this.screen.set_mode(this.graphical_mode)
            this.set_size_graphical(
                this.svga_width,
                this.svga_height,
                this.svga_width,
                this.svga_height,
                this.svga_bpp,
            )
        }

        if (was_enabled && !this.svga_enabled) {
            const is_graphical = (this.attribute_mode & 0x1) !== 0
            this.graphical_mode = is_graphical
            this.screen.set_mode(is_graphical)
            this.update_vga_size()
            this.set_font_bitmap(false)
            this.complete_redraw()
        }

        if (!this.svga_enabled) {
            this.svga_bank_offset = 0
        }

        this.update_layers()
    }

    port1CF_read(): number {
        dbg_log('1CF / dispi read ' + h(this.dispi_index), LOG_VGA)
        return this.svga_register_read(this.dispi_index)
    }

    svga_register_read(n: number): number {
        switch (n) {
            case 0:
                return this.svga_version
            case 1:
                return this.dispi_enable_value & 2 ? MAX_XRES : this.svga_width
            case 2:
                return this.dispi_enable_value & 2 ? MAX_YRES : this.svga_height
            case 3:
                return this.dispi_enable_value & 2 ? MAX_BPP : this.svga_bpp
            case 4:
                return this.dispi_enable_value
            case 5:
                return this.svga_bank_offset >>> 16
            case 6:
                // virtual width
                if (this.screen_width) {
                    return this.screen_width
                } else {
                    return 1 // seabios/windows98 divide exception
                }

            case 8:
                // x offset
                return this.svga_offset_x
            case 9:
                return this.svga_offset_y
            case 0x0a:
                // memory size in 64 kilobyte banks
                return (this.vga_memory_size / VGA_BANK_SIZE) | 0
            default:
                dbg_log(
                    'Unimplemented dispi read index: ' + h(this.dispi_index),
                    LOG_VGA,
                )
        }

        return 0xff
    }

    // Transfers graphics from VGA Planes to the Pixel Buffer
    // VGA Planes represent data stored on actual hardware.
    // Pixel Buffer caches the 4-bit or 8-bit color indices for each pixel.
    vga_replot(): void {
        // Round to multiple of 8 towards extreme
        const start = this.diff_plot_min & ~0xf
        const end = Math.min(
            this.diff_plot_max | 0xf,
            VGA_PIXEL_BUFFER_SIZE - 1,
        )

        const addr_shift = this.vga_addr_shift_count()
        const addr_substitution = ~this.crtc_mode & 0x3

        const shift_mode = this.planar_mode & 0x60
        const pel_width = this.attribute_mode & 0x40

        for (let pixel_addr = start; pixel_addr <= end; ) {
            let addr = pixel_addr >>> addr_shift
            if (addr_substitution) {
                let row = (pixel_addr / this.virtual_width) | 0
                const col = pixel_addr - this.virtual_width * row

                switch (addr_substitution) {
                    case 0x1:
                        // Alternating rows using bit 13
                        // Assumes max scan line = 1
                        addr = (row & 0x1) << 13
                        row >>>= 1
                        break
                    case 0x2:
                        // Alternating rows using bit 14
                        // Assumes max scan line = 3
                        addr = (row & 0x1) << 14
                        row >>>= 1
                        break
                    case 0x3:
                        // Cycling through rows using bit 13 and 14
                        // Assumes max scan line = 3
                        addr = (row & 0x3) << 13
                        row >>>= 2
                        break
                }

                addr |=
                    ((row * this.virtual_width + col) >>> addr_shift) +
                    this.start_address
            }

            let byte0 = this.plane0[addr]
            let byte1 = this.plane1[addr]
            let byte2 = this.plane2[addr]
            let byte3 = this.plane3[addr]

            const shift_loads = new Uint8Array(8)
            switch (shift_mode) {
                // Planar Shift Mode
                // See http://www.osdever.net/FreeVGA/vga/vgaseq.htm
                case 0x00:
                    // Shift these, so that the bits for the color are in
                    // the correct position in the for loop
                    byte0 <<= 0
                    byte1 <<= 1
                    byte2 <<= 2
                    byte3 <<= 3

                    for (let i = 7; i >= 0; i--) {
                        shift_loads[7 - i] =
                            ((byte0 >> i) & 1) |
                            ((byte1 >> i) & 2) |
                            ((byte2 >> i) & 4) |
                            ((byte3 >> i) & 8)
                    }
                    break

                // Packed Shift Mode, aka Interleaved Shift Mode
                // Video Modes 4h and 5h
                case 0x20:
                    shift_loads[0] = ((byte0 >> 6) & 0x3) | ((byte2 >> 4) & 0xc)
                    shift_loads[1] = ((byte0 >> 4) & 0x3) | ((byte2 >> 2) & 0xc)
                    shift_loads[2] = ((byte0 >> 2) & 0x3) | ((byte2 >> 0) & 0xc)
                    shift_loads[3] = ((byte0 >> 0) & 0x3) | ((byte2 << 2) & 0xc)

                    shift_loads[4] = ((byte1 >> 6) & 0x3) | ((byte3 >> 4) & 0xc)
                    shift_loads[5] = ((byte1 >> 4) & 0x3) | ((byte3 >> 2) & 0xc)
                    shift_loads[6] = ((byte1 >> 2) & 0x3) | ((byte3 >> 0) & 0xc)
                    shift_loads[7] = ((byte1 >> 0) & 0x3) | ((byte3 << 2) & 0xc)
                    break

                // 256-Color Shift Mode
                // Video Modes 13h and unchained 256 color
                case 0x40:
                case 0x60:
                    shift_loads[0] = (byte0 >> 4) & 0xf
                    shift_loads[1] = (byte0 >> 0) & 0xf
                    shift_loads[2] = (byte1 >> 4) & 0xf
                    shift_loads[3] = (byte1 >> 0) & 0xf
                    shift_loads[4] = (byte2 >> 4) & 0xf
                    shift_loads[5] = (byte2 >> 0) & 0xf
                    shift_loads[6] = (byte3 >> 4) & 0xf
                    shift_loads[7] = (byte3 >> 0) & 0xf
                    break
            }

            if (pel_width) {
                // Assemble from two sets of 4 bits.
                for (let i = 0, j = 0; i < 4; i++, pixel_addr++, j += 2) {
                    this.pixel_buffer[pixel_addr] =
                        (shift_loads[j] << 4) | shift_loads[j + 1]
                }
            } else {
                for (let i = 0; i < 8; i++, pixel_addr++) {
                    this.pixel_buffer[pixel_addr] = shift_loads[i]
                }
            }
        }
    }

    // Transfers graphics from Pixel Buffer to Destination Image Buffer.
    // The 4-bit/8-bit color indices in the Pixel Buffer are passed through
    // the internal palette (dac_map) and the DAC palette (vga256_palette) to
    // obtain the final 32 bit color that the Canvas API uses.
    vga_redraw(): void {
        const start = this.diff_addr_min
        const end = Math.min(this.diff_addr_max, VGA_PIXEL_BUFFER_SIZE - 1)
        const buffer = new Int32Array(
            this.cpu.wasm_memory.buffer,
            this.dest_buffet_offset,
            this.virtual_width * this.virtual_height,
        )

        let mask = 0xff
        let colorset = 0x00
        if (this.attribute_mode & 0x80) {
            // Palette bits 5/4 select
            mask &= 0xcf
            colorset |= (this.color_select << 4) & 0x30
        }

        if (this.attribute_mode & 0x40) {
            // 8 bit mode

            for (let pixel_addr = start; pixel_addr <= end; pixel_addr++) {
                const color256 =
                    (this.pixel_buffer[pixel_addr] & mask) | colorset
                const color = this.vga256_palette[color256]

                buffer[pixel_addr] =
                    (color & 0xff00) |
                    (color << 16) |
                    (color >> 16) |
                    0xff000000
            }
        } else {
            // 4 bit mode

            // Palette bits 7/6 select
            mask &= 0x3f
            colorset |= (this.color_select << 4) & 0xc0

            for (let pixel_addr = start; pixel_addr <= end; pixel_addr++) {
                const color16 =
                    this.pixel_buffer[pixel_addr] & this.color_plane_enable
                const color256 = (this.dac_map[color16] & mask) | colorset
                const color = this.vga256_palette[color256]

                buffer[pixel_addr] =
                    (color & 0xff00) |
                    (color << 16) |
                    (color >> 16) |
                    0xff000000
            }
        }
    }

    screen_fill_buffer(): void {
        if (!this.graphical_mode) {
            // text mode
            // Update retrace behaviour anyway - programs waiting for signal before
            // changing to graphical mode
            this.update_vertical_retrace()
            return
        }

        if (this.image_data!.data.byteLength === 0) {
            // wasm memory resized
            const buffer = new Uint8ClampedArray(
                this.cpu.wasm_memory.buffer,
                this.dest_buffet_offset,
                4 * this.virtual_width * this.virtual_height,
            )
            this.image_data = new ImageData(
                buffer,
                this.virtual_width,
                this.virtual_height,
            )
            this.update_layers()
        }

        if (this.svga_enabled) {
            let min_y = 0
            let max_y = this.svga_height

            if (this.svga_bpp === 8) {
                // XXX: Slow, should be ported to rust, but it doesn't have access to vga256_palette
                // XXX: Doesn't take svga_offset into account
                const buffer = new Int32Array(
                    this.cpu.wasm_memory.buffer,
                    this.dest_buffet_offset,
                    this.screen_width * this.screen_height,
                )
                const svga_memory = new Uint8Array(
                    this.cpu.wasm_memory.buffer,
                    this.svga_memory.byteOffset,
                    this.vga_memory_size,
                )

                for (let i = 0; i < buffer.length; i++) {
                    const color = this.vga256_palette[svga_memory[i]]
                    buffer[i] =
                        (color & 0xff00) |
                        (color << 16) |
                        (color >> 16) |
                        0xff000000
                }
            } else {
                this.cpu.svga_fill_pixel_buffer(this.svga_bpp, this.svga_offset)

                const bytes_per_pixel =
                    this.svga_bpp === 15 ? 2 : this.svga_bpp / 8
                min_y =
                    ((((this.cpu.svga_dirty_bitmap_min_offset[0] /
                        bytes_per_pixel) |
                        0) -
                        this.svga_offset) /
                        this.svga_width) |
                    0
                max_y =
                    (((((this.cpu.svga_dirty_bitmap_max_offset[0] /
                        bytes_per_pixel) |
                        0) -
                        this.svga_offset) /
                        this.svga_width) |
                        0) +
                    1
            }

            if (min_y < max_y) {
                min_y = Math.max(min_y, 0)
                max_y = Math.min(max_y, this.svga_height)

                this.screen.update_buffer([
                    {
                        image_data: this.image_data,
                        screen_x: 0,
                        screen_y: min_y,
                        buffer_x: 0,
                        buffer_y: min_y,
                        buffer_width: this.svga_width,
                        buffer_height: max_y - min_y,
                    },
                ])
            }
        } else {
            this.vga_replot()
            this.vga_redraw()
            this.screen.update_buffer(this.layers)
        }

        this.reset_diffs()
        this.update_vertical_retrace()
    }

    set_font_bitmap(font_plane_dirty: boolean): void {
        const height = this.max_scan_line & 0x1f
        if (height && !this.graphical_mode) {
            const width_dbl = !!(this.clocking_mode & 0x08)
            const width_9px = !width_dbl && !(this.clocking_mode & 0x01)
            const copy_8th_col = !!(this.attribute_mode & 0x04)
            this.screen.set_font_bitmap(
                height + 1, // int height, font height 1..32px
                width_9px, // bool width_9px, True: font width 9px, else 8px
                width_dbl, // bool width_dbl, True: font width 16px (overrides width_9px)
                copy_8th_col, // bool copy_8th_col, True: duplicate 8th into 9th column in ASCII chars 0xC0-0xDF
                this.plane2, // Uint8Array font_bitmap[64k], static
                font_plane_dirty, // bool bitmap_changed, True: content of this.plane2 has changed
            )
        }
    }

    set_font_page(): void {
        // bits 2, 3 and 5 (LSB to MSB): VGA font page index of font A
        // bits 0, 1 and 4: VGA font page index of font B
        // linear_index_map[] maps VGA's non-liner font page index to linear index
        const linear_index_map = [0, 2, 4, 6, 1, 3, 5, 7]
        const vga_index_A =
            ((this.character_map_select & 0b1100) >> 2) |
            ((this.character_map_select & 0b100000) >> 3)
        const vga_index_B =
            (this.character_map_select & 0b11) |
            ((this.character_map_select & 0b10000) >> 2)
        this.font_page_ab_enabled = vga_index_A !== vga_index_B
        this.screen.set_font_page(
            linear_index_map[vga_index_A],
            linear_index_map[vga_index_B],
        )
        this.complete_redraw()
    }
}