┌──────────────────────────────────────┐
                 │ Xtended Mode - Unchained 640x400x256 │
                 └──────────────────────────────────────┘

                 Written for the PC-GPE by Mark Feldman
            e-mail address : u914097@student.canberra.edu.au
                             myndale@cairo.anu.edu.au

                    Please read the file SVGINTRO.TXT
                (Graphics/SVGA/Intro PC-GPE menu option)

               ┌───────────────────────────────────────────┐
               │      THIS FILE MAY NOT BE DISTRIBUTED     │
               │ SEPARATE TO THE ENTIRE PC-GPE COLLECTION. │
               └───────────────────────────────────────────┘


┌────────────┬───────────────────────────────────────────────────────────────
│ Disclaimer │
└────────────┘

I assume no responsibility whatsoever for any effect that this file, the
information contained therein or the use thereof has on you, your sanity,
computer, spouse, children, pets or anything else related to you or your
existance. No warranty is provided nor implied with this information.


┌──────────────┬─────────────────────────────────────────────────────────────
│ Introduction │
└──────────────┘

I am calling this mode Xtended mode simply because I don't know if it 
already has a name. It is a variation of mode x and it has worked on every 
SVGA I have tried it on. It seems very very unlikely that I was the first 
person to try this, so if this mode has already been documented elsewhere I 
would very much like to hear about it.

Xtended mode is 640x400x256 and will only work on SVGA's supporting the
"regular" 640x400x256 mode. It's advantage is that it requires no
bank switching to access the entire display memory and, like mode x, polygon
fill graphics can be up to 4 times faster.

┌──────────────────────┬─────────────────────────────────────────────────────
│ Setting Xtended Mode │
└──────────────────────┘

Xtended mode is set similar to the way unchained mode 13h is set, the only
difference is that you you call BIOS to set the 640x400x256 graphics mode
instead of mode 13h. The 640x400x256 mode number varies from card to card.
The following table lists the mode number for each of the 7 "standard"
SVGAs:

           640x400x256 mode numbers for various SVGA cards
               ┌─────────────────────────────────────┐
               │      SVGA Chip          Mode Number │
               ├─────────────────────────────────────┤
               │ ATI                        61h      │
               │ Chips & Technologies       78h      │
               │ Genoa                      7Eh      │
               │ Paradise                   5Eh      │
               │ Trident                    5Ch      │
               │ Tseng                      2Fh      │
               │ Video 7                    66h      │
               └─────────────────────────────────────┘

Alternatively the mode can be set with the VESA Set Super VGA Mode BIOS
call, the VESA SVGA mode number is 100h. Refer to the file "VESASP12.TXT"
for more information on VESA BIOS calls.

The following Pascal procedure will set Xtended mode for a card with a VESA
driver:

const VIDEO     = $10;  { Video interrupt number                    }
      CRTC_ADDR	= $3d4; { Base port of the CRT Controller (color)   }
      SEQU_ADDR	= $3c4; { Base port of the Sequencer                }

procedure InitXtended;
begin

  { Set VESA 640x400x256 mode }
  asm
    mov ax, $4F02
    mov bx, $100
    int VIDEO
  end;

  { Turn the VGA screen off }
  Port[SEQU_ADDR] := 1;
  Port[SEQU_ADDR + 1] := Port[SEQU_ADDR + 1] or $20;

  { Turn off the Chain-4 bit (bit 3 at index 4, port 0x3c4) }
  PortW[SEQU_ADDR] := $0604;

  { Turn off word mode, by setting the Mode Control register
    of the CRT Controller (index 0x17, port 0x3d4) }
  PortW[CRTC_ADDR] := $E317;

  { Turn off doubleword mode, by setting the Underline Location
    register (index 0x14, port 0x3d4) }
  PortW[CRTC_ADDR] := $0014;

  { Clear entire video memory, by selecting all four planes, then writing
  color 0 to the entire segment. Stoopid FillChar fills 1 byte too short! }
  PortW[SEQU_ADDR] := $0F02;
  FillChar(Mem[$A000 : 0], $8000, 0);
  FillChar(Mem[$A000 : $8000], $8000, 0);

  { Give a small delay to let the screen sort itself out }
  Delay(100);

  { Turn the screen back on }
  Port[SEQU_ADDR] := 1;
  Port[SEQU_ADDR + 1] := Port[SEQU_ADDR + 1] and $DF;
end;

┌─────────────────┬─────────────────────────────────────────────────────────
│ Drawing a Pixel │
└─────────────────┘

Drawing a pixel in Xtended mode is similar to drawing one in unchained mode
13h or mode x, we just have to keep in mind that the display is now twice
as wide. Also keep in mind that 640x400 has 4 times as many pixels as
320x200, so there is only one page in Xtended mode.

This example Pascal routine will draw a pixel at any screen position. I'll
let you do the job of converting it to assembly:

procedure XtendedPutPixel(x, y : word; color : byte);
begin
  { Set map mask to select proper plane }
  PortW[SEQU_ADDR] := $100 shl (x and 3) + 2;

  { Calculate address (y * 160 + x div 4) and write pixel }
  Mem[$A000 : y shl 7 + y shl 5 + x shr 2] := color;
end;