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Graphics Programming Black Book (Special Edition)
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chapter52
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MODE.ASM
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Assembly Source File
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1994-12-17
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4KB
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89 lines
; Mode X (320x240, 256 colors) mode set routine. Works on all VGAs.
; ****************************************************************
; * Revised 6/19/91 to select correct clock; fixes vertical roll *
; * problems on fixed-frequency (IBM 851X-type) monitors. *
; ****************************************************************
; C near-callable as:
; void Set320x240Mode(void);
; Tested with TASM 4.0.
; Modified from public-domain mode set code by John Bridges.
SC_INDEX equ 03c4h ;Sequence Controller Index
CRTC_INDEX equ 03d4h ;CRT Controller Index
MISC_OUTPUT equ 03c2h ;Miscellaneous Output register
SCREEN_SEG equ 0a000h ;segment of display memory in mode X
.model small
.data
; Index/data pairs for CRT Controller registers that differ between
; mode 13h and mode X.
CRTParms label word
dw 00d06h ;vertical total
dw 03e07h ;overflow (bit 8 of vertical counts)
dw 04109h ;cell height (2 to double-scan)
dw 0ea10h ;v sync start
dw 0ac11h ;v sync end and protect cr0-cr7
dw 0df12h ;vertical displayed
dw 00014h ;turn off dword mode
dw 0e715h ;v blank start
dw 00616h ;v blank end
dw 0e317h ;turn on byte mode
CRT_PARM_LENGTH equ (($-CRTParms)/2)
.code
public _Set320x240Mode
_Set320x240Mode proc near
push bp ;preserve caller's stack frame
push si ;preserve C register vars
push di ; (don't count on BIOS preserving anything)
mov ax,13h ;let the BIOS set standard 256-color
int 10h ; mode (320x200 linear)
mov dx,SC_INDEX
mov ax,0604h
out dx,ax ;disable chain4 mode
mov ax,0100h
out dx,ax ;synchronous reset while setting Misc Output
; for safety, even though clock unchanged
mov dx,MISC_OUTPUT
mov al,0e3h
out dx,al ;select 25 MHz dot clock & 60 Hz scanning rate
mov dx,SC_INDEX
mov ax,0300h
out dx,ax ;undo reset (restart sequencer)
mov dx,CRTC_INDEX ;reprogram the CRT Controller
mov al,11h ;VSync End reg contains register write
out dx,al ; protect bit
inc dx ;CRT Controller Data register
in al,dx ;get current VSync End register setting
and al,7fh ;remove write protect on various
out dx,al ; CRTC registers
dec dx ;CRT Controller Index
cld
mov si,offset CRTParms ;point to CRT parameter table
mov cx,CRT_PARM_LENGTH ;# of table entries
SetCRTParmsLoop:
lodsw ;get the next CRT Index/Data pair
out dx,ax ;set the next CRT Index/Data pair
loop SetCRTParmsLoop
mov dx,SC_INDEX
mov ax,0f02h
out dx,ax ;enable writes to all four planes
mov ax,SCREEN_SEG ;now clear all display memory, 8 pixels
mov es,ax ; at a time
sub di,di ;point ES:DI to display memory
sub ax,ax ;clear to zero-value pixels
mov cx,8000h ;# of words in display memory
rep stosw ;clear all of display memory
pop di ;restore C register vars
pop si
pop bp ;restore caller's stack frame
ret
_Set320x240Mode endp
end