• # - Graphics in pmode
• # - Video timing
• # - VGA
• # - Super VGA (SVGA)
• # - Code snippets
• # - Links
• REPORT BUGS OR ERRORS IN THIS DOCUMENT

Graphics in pmode

1. You don't. Graphics programming is fun, but graphics are hardly essential for an OS. Don't get side-tracked.
2. Call the BIOS mode-set interrupt in the 16-bit boot code, before the pmode kernel starts.
3. Use protected-mode code to program the VGA directly, without using the BIOS. This works only with VGA-compatible video boards and VGA-compatible video modes.
4. Add a virtual 8086 mode monitor (VMM) to your OS. Call the BIOS mode-set interrupt in virtual 8086 mode.
5. Switch from pmode to real mode, call the BIOS mode-set interrupt in real mode, then return to pmode.
6. Write a protected-mode driver specifically for the SVGA chip used in your video board. Someone else who wants to use your OS must have the same video board (or they must write a new driver for their own video board).
7. Call VBE 3.x BIOS functions in 16-bit protected mode. VBE 3.x is supported only by very new video cards.

Video timing

• Dot clock (pixel clock)
• Character clock = dot clock divided by 8 or 9
• Horizontal sync (retrace) frequency = character clock / horizontal total
• Horizontal total / horizontal displayed ~ 1.2 (20% overscan)
• Vertical sync (retrace) frequency = horizontal sync frequency / vertical total
• Vertical total / vertical displayed ~ 1.1 (10% overscan)

VGA

• INT 10h VGA BIOS interrupts which work in real or virtual-8086 mode only, to set mode, change font, etc.
• Dot clock is one of: 28.35, 25.2, 14.175, or 12.6 MHz
• Character clock is dot clock divided by 8 or 9
• Horizontal sync frequency for very old VGA monitors is always 31.5 kHz. Other frequencies can be produced by the VGA board and are supported by newer SVGA monitors.
• Vertical sync frequency for very old VGA monitors is always 60 or 70 Hz. Other frequencies can be produced by the VGA board and are supported by newer SVGA monitors.
• No interlace. No video memory 'banks'. No color depth > 8. Only 256K usable video memory max.
• Four memory planes, 64K each: P1, P2, P4, and P8
• Text: characters occupy even bytes in plane P1, attributes occupy odd bytes in plane P2, fonts are in plane P4. Text mode uses Even/Odd addressing: bit b0 of address selects between planes P1 and P2, so you need not change planes to set the attributes.
• 4-color (CGA) graphics: 4 pixels per byte; uses planes P1 and P2. As with text, Even/Odd addressing eliminates the need to switch planes.
• 16-color graphics: four monochrome planes: blue, green, red, and intense (the 16- and 256-color palettes can be re-defined to give you any 16 colors you want). Highest graphics resolution available with plain VGA, but very difficult to write software for this mode.
• 256-color graphics: one byte per pixel. Simple software, but low resolution with plain VGA. All four planes are used with Chain-4 addressing: bits b0 and b1 of the address select the plane so you need not worry about it. Mode-X is unchained 256-color mode: difficult to program, but it lets you use up to 256K video memory for 256-color modes up to 360x480 resolution.

Super VGA (SVGA)

• INT 10h AH=4Fh BIOS interrupts (VESA BIOS extensions; VBE). VBE 1.x runs in real or virtual-8086 mode only. VBE 2.x has a few not-too-useful pmode functions. For VBE 3.x, most functions (including mode-set) are carefully written to work in real mode or 16-bit protected mode.
• More than 256K video memory
• SVGA boards support many dot clock frequencies. SVGA monitors support many sync frequencies.
• 16-bit SVGA, VBE 1.x BIOS: video memory accessible one 64K 'bank' at a time. Paging can be used to simulate a linear framebuffer, but there are some 'gotchas' with this method (see links below)
• 32-bit SVGA, VBE 2.x or 3.x BIOS: Linear framebuffer for video memory -- no banks or planes, but video memory is no longer at A000:0000

Code snippets

Set text or graphics video modes (including text font) without using the BIOS

Simple V86 mode monitor; calls 16-bit video BIOS from 32-bit pmode.

Links

Finn Thøgersen's VGADOC: http://home.worldonline.dk/finth/

Simulated linear framebuffer:

• Alexei Frounze's LFBemu: http://alexfru.narod.ru/miscdocs/pmode/lfbemu22.zip
• Chris Giese's simulated linear framebuffer: http://my.execpc.com/~geezer/os/slfb.asm
• Potential problems with simulated linear framebuffers: http://marc.theaimsgroup.com/?m=88102879813311&w=2

TO DO

(S)VGA functional units:
- Miscellaneous register
- Sequencer (SEQ)
        - In unchained modes, SEQ register 2 selects one
          (or more!) planes to write
- CRT Controller (CRTC)
- Graphics Controller (GC)
        - In unchained modes, GC register 4 selects a plane to read
- Attribute Controller (AC)
        - 16-color palette in first 16 registers of AC
- 256-color palette (DAC)

Gotchas:
- 'Lock' bits in CRTC registers 3 and 17
- Screwy CGA addressing (disabled by bit b0 in CRTC register 23)
- Palette is not compact, i.e. for the 16-color palette
        entry[i] != i

REPORT BUGS OR ERRORS IN THIS DOCUMENT