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VGA

Register Groups

VGA hardware is controlled through five groups of I/O-mapped registers. All access is through the kernel's outb/inb wrappers.

Group Index Port Data Port Purpose
Sequencer (SEQ) 0x3C4 0x3C5 Memory sequencing, clock, plane write masks
CRTC 0x3D4 0x3D5 Display timing, scan line counts, cursor position
Graphics Controller (GC) 0x3CE 0x3CF Read/write modes, color compare, bit masks
Attribute Controller (AC) 0x3C0 0x3C0 Palette, display enable, pixel clock
Miscellaneous Output 0x3C2 — Clock select, sync polarity, I/O address select

The Attribute Controller uses a single port for both index and data, toggled by an internal flip-flop. It must be reset by reading 0x3DA (Input Status 1) before writing.

Supported Hardware Modes

set_video_mode(mode: u8) -> bool in video/vga_hw.rs programs all five register groups for the requested mode. Returns true on success, false for an unknown mode number.

Mode Number Resolution Colors Use
Text 80×25 0x03 720×400 (character) 16 Kernel console, default
16-color 320×200 0x0D 320×200 16 Legacy planar graphics
16-color 640×480 0x12 640×480 16 Higher-res planar graphics
256-color 320×200 0x13 320×200 256 Mode 13h, linear byte-per-pixel

Mode-Switch Sequence

For all modes:

  1. Write Miscellaneous Output register (0x3C2).
  2. Write Sequencer registers (0x3C4/0x3C5), index 0–4.
  3. Unlock CRTC (clear protect bit on index 0x11), then write CRTC registers.
  4. Write Graphics Controller registers.
  5. Reset AC flip-flop (read 0x3DA), then write all 21 Attribute Controller registers (indices 0–20), finishing with a write of 0x20 to 0x3C0 to re-enable display output.
  6. For mode 0x03 only: call clear_text_buf(), restore_font(), restore_text_dac().

Mode 0x03 — Text Mode Restoration

Switching back to text mode from a graphics mode requires three additional steps that reprogram state the BIOS would normally set up.

clear_text_buf()

Fills the VGA text buffer at 0xB8000 with space characters (byte 0x20) and attribute byte 0x07 (light gray on black). Clears all 80×25 = 2000 character cells.

restore_font()

Reloads the embedded 8×16 font (VGA_FONT_8X16, 4096 bytes) into VGA plane 2 (the font plane). The sequence:

SEQ[0x00] ← 0x01   (synchronous reset)
SEQ[0x02] ← 0x04   (write to plane 2 only)
SEQ[0x04] ← 0x07   (sequential addressing, odd/even disabled)
GC[0x04]  ← 0x02   (read plane 2)
GC[0x05]  ← 0x00   (write mode 0, read mode 0)
GC[0x06]  ← 0x00   (A000-BFFF range, graph mode off)

for each glyph g (0..256):
    write 16 bytes of VGA_FONT_8X16[g*16..] to 0xA0000 + g*32

SEQ[0x02] ← 0x03   (restore write to planes 0+1)
SEQ[0x04] ← 0x03   (odd/even addressing)
GC[0x04]  ← 0x00   (read plane 0)
GC[0x05]  ← 0x10   (odd/even read)
GC[0x06]  ← 0x0E   (B800-BFFF range, text mode)
SEQ[0x00] ← 0x03   (release reset)

Each glyph occupies 32 bytes in plane 2 (only the first 16 are significant; the upper 16 are zero-padding required by VGA).

restore_text_dac()

Programs the VGA DAC (Digital-to-Analog Converter) with the standard 16-color EGA-compatible palette. The DAC is accessed via:

  • Write palette index to 0x3C8
  • Write R, G, B (6-bit, 0–63 each) sequentially to 0x3C9

The standard palette maps the 16 EGA attribute colors (0–15) to their canonical RGB values (e.g., index 0 = black 0,0,0; index 7 = light gray 42,42,42; index 15 = white 63,63,63).

Text Mode Writer (video/vga.rs)

The Writer struct manages the kernel's VGA text console.

For an EFI text session (console=fb), the same writer uses a RAM character buffer and video/console.rs draws 8x16 glyphs, foreground/background colors, scrolling, and an underline cursor on the boot framebuffer. Glyphs scale by an integer factor up to 4 to fit the display. The grid stays 80x25; stride and RGB field positions come from Multiboot2. Supported formats are 24-bit and 32-bit RGB below 4 GiB, at least 640x400. VGA cursor ports are not accessed by this output path. The kernel shell runs, Memento does not autostart, and the clock that writes directly into VGA memory is omitted.

pub struct Writer {
    col_pos:    usize,
    row_pos:    usize,
    color_code: ColorCode,
    buffer:     *mut Buffer,   // 0xB8000
}

Constants

Constant Value
BUFFER_WIDTH 80
BUFFER_HEIGHT 25
BUFFER_ADDRESS 0xB8000

Colors

Color is a 4-bit VGA attribute value (0–15). ColorCode packs foreground and background:

ColorCode = (background << 4) | foreground

Standard color indices: Black=0, Blue=1, Green=2, Cyan=3, Red=4, Magenta=5, Brown=6, LightGray=7, DarkGray=8, LightBlue=9, LightGreen=10, LightCyan=11, LightRed=12, Pink=13, Yellow=14, White=15.

write_byte(byte: u8)

Byte Action
\n (0x0A) Advance to next row (new_line())
\r / 0x08 Backspace: decrement col_pos, write space, do not advance
Other Write character cell at (col_pos, row_pos), advance col_pos; call new_line() on column overflow

new_line()

When row_pos reaches BUFFER_HEIGHT - 1, the entire buffer is scrolled up one row: rows 1–24 are copied to rows 0–23 using direct memory writes to 0xB8000. Row 24 is cleared. When row_pos < BUFFER_HEIGHT - 1, row_pos is incremented and col_pos is reset to 0.

Hardware Cursor (move_cursor())

The hardware text cursor is positioned by writing the linear cell index (row * 80 + col) as two bytes to CRTC:

CRTC[0x0E] ← (position >> 8) & 0xFF   (high byte)
CRTC[0x0F] ← position & 0xFF           (low byte)

Called after every character write that changes the cursor position.

VGA VRAM Window

The VGA VRAM window (0xA00_000 virtual, 0xA0000 physical, 64 KiB) is mapped on demand by syscall 0x14 (map_vram). The mapping uses fine-grained 4 KiB pages (a P1 table allocated from PAGE_TABLE_POOL) rather than the 2 MiB huge pages used elsewhere.

The P1 table is allocated once per process on the first map_vram call. Repeated calls are idempotent — the same P1 is reused. After mapping, userland can write directly to 0xA00_000 as Mode 13h VRAM (320×200, one byte per pixel, 64000 bytes).

The kernel's TLB is flushed by reloading CR3 after mapping.

Legacy VGA Module (src/vga/)

An older VGA module lives under src/vga/ (distinct from src/video/). It contains:

  • src/vga/buffer.rs — character/color cell types
  • src/vga/write.rs — writer implementation
  • src/vga/screen.rs — screen helper

This module predates the current video/ subsystem. The canonical implementation is src/video/vga.rs.

Related documentation: Video Overview, Kernel Internals, Video and Audio ABI.