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LISTING 31.2 L31-2.ASM

; Program to demonstrate the two pages available in 320x400
; 256-color modes on a VGA.  Draws diagonal color bars in all
; 256 colors in page 0, then does the same in page 1 (but with
; the bars tilted the other way), and finally draws vertical
; color bars in page 0.
;
VGA_SEGMENT        equ    0a000h
SC_INDEX           equ    3c4h        ;Sequence Controller Index register
GC_INDEX           equ    3ceh        ;Graphics Controller Index register
CRTC_INDEX         equ    3d4h        ;CRT Controller Index register
MAP_MASK           equ    2           ;Map Mask register index in SC
MEMORY_MODE        equ    4           ;Memory Mode register index in SC
MAX_SCAN_LINE      equ    9           ;Maximum Scan Line reg index in CRTC
START_ADDRESS_HIGH equ    0ch         ;Start Address High reg index in CRTC
UNDERLINE          equ    14h         ;Underline Location reg index in CRTC
MODE_CONTROL       equ    17h         ;Mode Control register index in CRTC
GRAPHICS_MODE      equ    5           ;Graphics Mode register index in GC
MISCELLANEOUS      equ    6           ;Miscellaneous register index in GC
SCREEN_WIDTH       equ    320         ;# of pixels across screen
SCREEN_HEIGHT      equ    400         ;# of scan lines on screen
WORD_OUTS_OK       equ    1           ;set to 0 to assemble for
                                      ; computers that can't handle
                                      ; word outs to indexed VGA registers
;
stack        segment        para stack 'STACK'
             db             512 dup (?)
stack        ends
;
; Macro to output a word value to a port.
;
OUT_WORDmacro
if WORD_OUTS_OK
      outdx,ax
else
        out  dx,al
        inc  dx
        xch  gah,al
        out  dx,al
        dec  dx
        xch  gah,al
endif
        endm
;
; Macro to output a constant value to an indexed VGA register.
;
CONSTANT_TO_INDEXED_REGISTERmacroADDRESS, INDEX, VALUE
        mov  dx,ADDRESS
        mov  ax,(VALUE shl 8) + INDEX
        OUT_WORD
        endm
;
Code    segment
        assume        cs:Code
Start   proc   near
;
; Set 320x400 256-color mode.
;
callSet320By400Mode
;
; We're in 320x400 256-color mode, with page 0 displayed.
; Let's fill page 0 with color bars slanting down and to the right.
;
        sub   di,di        ;page 0 starts at address 0
        mov   bl,1         ;make color bars slant down and
                           ; to the right
        call   ColorBarsUp ;draw the color bars
;
; Now do the same for page 1, but with the color bars
; tilting the other way.
;
        mov   di,8000h        ;page 1 starts at address 8000h
        mov   bl,-1           ;make color bars slant down and
                              ; to the left
        call  ColorBarsUp     ;draw the color bars
;
; Wait for a key and flip to page 1 when one is pressed.
;
      callGetNextKey
      CONSTANT_TO_INDEXED_REGISTER CRTC_INDEX,START_ADDRESS_HIGH,80h
                               ;set the Start Address High register
                               ; to 80h, for a start address of 8000h
;
; Draw vertical bars in page 0 while page 1 is displayed.
;
        sub  di,di              ;page 0 starts at address 0
        sub  bl,bl              ;make color bars vertical
        call ColorBarsUp        ;draw the color bars
;
; Wait for another key and flip back to page 0 when one is pressed.
;
        callGetNextKey
        CONSTANT_TO_INDEXED_REGISTER CRTC_INDEX,START_ADDRESS_HIGH,00h
                                ;set the Start Address High register
                                ; to 00h, for a start address of 0000h
;
; Wait for yet another key and return to text mode and end when
; one is pressed.
;
        call   GetNextKey
        mov    ax,0003h
        int    10h                ;text mode
        mov    ah,4ch
        int    21h                ;done
;
Start endp
;
; Sets up 320x400 256-color modes.
;
; Input: none
;
; Output: none
;
Set320By400Modeprocnear
;
; First, go to normal 320x200 256-color mode, which is really a
; 320x400 256-color mode with each line scanned twice.
;
        mov   ax,0013h           ;AH = 0 means mode set, AL = 13h selects
                                 ; 256-color graphics mode
        int   10h                ;BIOS video interrupt
;
; Change CPU addressing of video memory to linear (not odd/even,
; chain, or chain 4), to allow us to access all 256K of display
; memory. When this is done, VGA memory will look just like memory
; in modes 10h and 12h, except that each byte of display memory will
; control one 256-color pixel, with 4 adjacent pixels at any given
; address, one pixel per plane.
;
      mov     dx,SC_INDEX
      mov     al,MEMORY_MODE
      out     dx,al
      inc     dx
      ina     l,dx
      and     al,not 08h                   ;turn off chain 4
      ora     l,04h                        ;turn off odd/even
      out     dx,al
      mov     dx,GC_INDEX
      mov     al,GRAPHICS_MODE
      out     dx,al
      inc     dx
      ina     l,dx
      and     al,not 10h                   ;turn off odd/even
      out     dx,al
      dec     dx
      mov     al,MISCELLANEOUS
      out     dx,al
      inc     dx
      ina     l,dx
      and     al,not 02h                   ;turn off chain
      out     dx,al
;
; Now clear the whole screen, since the mode 13h mode set only
; cleared 64K out of the 256K of display memory. Do this before
; we switch the CRTC out of mode 13h, so we don't see garbage
; on the screen when we make the switch.
;
CONSTANT_TO_INDEXED_REGISTER SC_INDEX,MAP_MASK,0fh
                                        ;  enable writes to all planes, so
                                        ; we can clear 4 pixels at a time
        mov     ax,VGA_SEGMENT
        mov     es,ax
        sub     di,di
        mov     ax,di
        mov     cx,8000h                ;# of words in 64K
        cld
        rep     stosw                   ;clear all of display memory
;
; Tweak the mode to 320x400 256-color mode by not scanning each
; line twice.
;
        mov     dx,CRTC_INDEX
        mov     al,MAX_SCAN_LINE
        out     dx,al
        inc     dx
        in      al,dx
        and     al,not 1fh                ;set maximum scan line = 0
        out     dx,al
        dec     dx
;
; Change CRTC scanning from doubleword mode to byte mode, allowing
; the CRTC to scan more than 64K of video data.
;
        mov     al,UNDERLINE
        out     dx,al
        inc     dx
        ina     l,dx
        and     al,not40h                ;turn off doubleword
        out     dx,al
        dec     dx
        mov     al,MODE_CONTROL
        out     dx,al
        inc     dx
        in      al,dx
        or      al,40h                   ;turn on the byte mode bit, so memory is
                                         ; scanned for video data in a purely
                                         ; linear way, just as in modes 10h and 12h
        out     dx,al
        ret
Set320By400Mode  endp
;
; Draws a full screen of slanting color bars in the specified page.
;
; Input:
;        DI = page start address
;        BL = 1 to make the bars slant down and to the right, -1 to
;        make them slant down and to the left, 0 to make
;        them vertical.
;
ColorBarsUpprocnear
        mov     ax,VGA_SEGMENT
        mov     es,ax                ;point to display memory
        sub     bh,bh                ;start with color 0
        mov     si,SCREEN_HEIGHT     ;# of rows to do
        mov     dx,SC_INDEX
        mov     al,MAP_MASK
        out     dx,al                ;point the SC Index reg to the Map Mask reg
        inc     dx                   ;point DX to the SC Data register
RowLoop:
        mov     cx,SCREEN_WIDTH/4
                                     ;4 pixels at each address, so
                                     ; each 320-pixel row is 80 bytes wide
                                     ; in each plane
        pus h   bx                   ;save the row-start color
ColumnLoop:
MAP_SELECT = 1
        rept  4                      ;do all 4 pixels at this address with
                                     ; in-line code
        mov     al,MAP_SELECT
        out     dx,al                ;select planes 0, 1, 2, and 3 in turn
        mov     es:[di],bh           ;write this plane's pixel
        inc     bh                   ;set the color for the next pixel
MAP_SELECT = MAP_SELECT shl 1
        endm
        inc     di                    ;point to the address containing the next
                                      ; 4 pixels
        loop    ColumnLoop            ;do any remaining pixels on this line
        pop     bx                    ;get back the row-start color
        add     bh,bl                 ;select next row-start color (controls
                                       ; slanting of color bars)
        dec     si                     ;count down lines on the screen
        jnz     RowLoop
        ret
ColorBarsUpendp
;
; Waits for the next key and returns it in AX.
;
GetNextKeyprocnear
WaitKey:
        mov     ah,1
        int     16h
        jz      WaitKey                ;wait for a key to become available
        sub     ah,ah
        int     16h                    ;read the key
        ret
GetNextKey      endp
;
Codeends
;
endStart

When you run Listing 31.2, note the extremely smooth edges and fine gradations of color, especially in the screens with slanting color bars. The displays produced by Listing 31.2 make it clear that 320×400 256-color mode can produce effects that are simply not possible in any 16-color mode.

Something to Think About

You can, if you wish, use the display memory organization of 320×400 mode in 320×200 mode by modifying Set320×400Mode to leave the maximum scan line setting at 1 in the mode set. (The version of Set320×400Mode in Listings 31.1 and 31.2 forces the maximum scan line to 0, doubling the effective resolution of the screen.) Why would you want to do that? For one thing, you could then choose from not two but four 320×200 256-color display pages, starting at offsets 0, 4000H, 8000H, and 0C000H in display memory. For another, having only half as many pixels per screen can as much as double drawing speeds; that’s one reason that many games run at 320×200, and even then often limit the active display drawing area to only a portion of the screen.


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Graphics Programming Black Book © 2001 Michael Abrash