12.1  INTRODUCTION AND PREPARATION

Mode 13h is so widely used for graphics applications in DOS because it
is very easy to use.  The screen is constantly being redrawn by the
video card.  To affect what the card draws, it is necessary to write to
the screen buffer.  The screen buffer in mode 13h is always at
segment:offset = A000:0000.  Thus, to set up drawing directly to the
video buffer, this is what you'd most often first do:

  ;Change the video mode to 13h
    xor  ah, ah         ;VIDEO Function 00h: Change screen
    mov  al, 13h        ;Put the desired graphics mode into AL
    int  10h            ;Call VIDEO

  ;Prepare for writing to the video buffer
    mov  di, 0a000h     ;Put the video segment into DI
    mov  es, di         ; so it can easily be put into ES
    xor  di, di         ;Start writing at coordinates (0,0)

12.2  WRITING PIXELS TO THE SCREEN

Why is Mode 13h so popular?  To understand, you must know a few basic
facts.  In Mode 13h, the screen is 320 by 200, or 320 pixels across and
200 pixels down.  In each pixel, there's a possibility of 256 colors,
which can be fit into one byte.  Thus, 320*200*1 = 64000 bytes, about
the size of one segment.  Think of the screen as an array of colors.
The first row takes up addresses A000:0000 to A000:013F (decimal 319),
the second row takes up addresses A000:0140 to A000:027F (decimal 639),
and so on.  To plot a pixel, assuming ES=A000:

  ;Plot a pixel in video mode 13h, where
  ;PixelAddress = (320 * Y) + X
    mov  ax, 320        ; Prepare for the multiplication
    mul  [Y]            ; Assuming that Y is defined in the data segment
                        ;   earlier in the program
    mov  di, ax         ; Put in into the pointer to the offset of ES
    add  di, [X]        ; Assuming that X is defined in the data segment
                        ;   earlier in the program
    mov  al, [Color]    ; Assuming that Color is defined in the data
                        ;   segment earlier in the program
    stosb               ; Write it to the screen!

See how easy that was?  Something to remember is that it is zero-based.
The upper-left corner is (0,0), and the lower-right is (319,199).  A
complete TASM Ideal mode procedure might look something like this (it
assumes that the video card is already set to mode 13h):

PROC WritePixel BASIC   ; Or whatever language you might want to link
                        ;  it to
    USES es, di         ; It's always a good idea to preserve ES and DI
    ARG  X:word, Y:word, Color:BYTE
    mov  di, 0a000h     ; Put the video segment into DI
    mov  es, di         ;   so it can easily be put into ES
    mov  ax, 320        ; Prepare for the multiplication
    mul  [Y]            ; Offset pointer by the Y value passed in
    mov  di, ax         ; Put in into pointer to the offset of ES
    add  di, [X]        ; Offset the pointer by the X value passed in
    mov  al, [Color]    ; Put color to be written to the screen in AL
    stosb               ; Write it to the screen!
    ret
ENDP WritePixel

To write a horizontal line, just put the length in CX, and replace the
STOSB with a REP STOSB.  Writing a vertical line is only a little
trickier.  Observe the following TASM Ideal mode procedure:

PROC VerticalLine BASIC ; Or whatever language you might want to link
                        ;  it to
    USES es, di         ; It's always a good idea to preserve ES and
                        ;  DI
    ARG  X:word, Y:word, Color:BYTE, Length:word
    mov  di, 0a000h     ; Put the video segment into DI
    mov  es, di         ; so it can easily be put into ES
    mov  ax, 320        ; Prepare for the multiplication
    mul  [Y]            ; Offset the pointer by the Y value passed in
    mov  di, ax         ; Put in into the pointer to the offset of ES
    add  di, [X]        ; Offset the pointer by the X value passed in
    mov  al, [Color]    ; Put the color to be written to the screen
                        ; in AL
    mov  cx, [Length]   ; Prepare for the loop
YLoop:
    stosb               ; Write it to the screen!
    add  di, 319        ; Move down one row (DI has already advanced
                        ;  once because of the STOSB, thus the 319)
    loop YLoop
    ret
ENDP VerticleLine

Observe how there is a tight loop that moves DI down one row each
iteration.

In short, the easiest way to write directly to the Mode 13h video buffer
is to think of the screen as just a 320 by 200 array of bytes, starting
at A000:0000.

Author: Michael Averbuch (mikeaver@prairienet.org)
Last Change: 29 Dec 94

Some parts © 2009 Advameg, Inc.