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