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Assembly Source File | 1988-12-17 | 32.4 KB | 782 lines

; LVIDEO.ASM - after Microsoft Systems Journal, Nov. 1988, p. 6 ; ; *** Caution: Large Model stack bias! *** ; ; This program handles direct writes to the IBM PC video screen buffer ; located in RAM memory. Works with all 100% compatible clones, and ; correctly eliminates snow when using CGA. See the article in MSJ: ; "Building a device-independent video display I/O library in Microsoft C", ; by Jeff Prosise. Requires MASM v5.0 or MASM v5.1, per documentation. ; ; Memory model support by David C. Oshel, Ames, Iowa, Dec. 9, 1988. ; This file is essentially ASMVIDEO.ASM, modified for appropriate function ; calls and returns, and DS register management for various data objects. ; ; I've added four additional functions, all of which take a len argument, ; to Prosise's original set of three: ; ; MSJ_SetFldAttr, MSJ_DispMsgLen 12/10/88 ; MSJ_MovScrBuf, MSJ_MovBufScr 12/11/88 ; ; Added Wozniak's pattern fill to MSJ_SetFldAttr on 12/12/88, and figured ; out why I still had snow on a CGA (NCR PC 6300) on 12/13/88. See next. ; ; *** C A U T I O N *** ; ; It turns out that Prosise's XCHG AX,BX and STOSW pattern in the snow ; elimination code is a VERY time critical optimization. I actually had ; to read the article before I could expand on the original routines. Ouch. ; --- 12/13/88, d.c.oshel ; .MODEL LARGE .CODE PUBLIC _MSJ_MovBufScr, _MSJ_MovScrBuf PUBLIC _MSJ_SetFldAttr, _MSJ_DispMsgLen PUBLIC _MSJ_DispString, _MSJ_DispCharAttr, _MSJ_GetCharAttr ; ; The C interface for these functions and structures looks like this: ; ;struct MSJ_VideoInfo { ; unsigned char mode; ; unsigned char rows; ; unsigned char columns; ; unsigned char ColorFlag; ; unsigned char SnowFlag; ; unsigned int BufferStart; ; unsigned int SegAddr; ;}; ; ;extern struct MSJ_VideoInfo video; /* defined in CVIDEO.C */ ; ; ;/* MSJ, Nov. 1988 ; */ ;extern int cdecl MSJ_GetCharAttr( char row, char col, ; struct MSJ_VideoInfo * sptr ); ; ;/* MSJ, Nov. 1988 ; */ ;extern void cdecl MSJ_DispCharAttr( char ch, ; char row, char col, ; char VideoAttr, ; struct MSJ_VideoInfo * sptr ); ; ; ;/* MSJ, Nov. 1988 ; */ ;extern void cdecl MSJ_DispString( char * msg, ; char row, char col, ; char VideoAttr, ; struct MSJ_VideoInfo * sptr ); ; ; ;/* MSJ_DispMsgLen is like MSJ_DispString, but takes a length argument, ; and does not disturb screen attributes in the receiving field ; 12/10/88, d.c.oshel ; */ ;extern void cdecl MSJ_DispMsgLen( char * msg, ; char row, char col, ; int len, ; struct MSJ_VideoInfo * sptr ); ; ; ;/* MSJ_SetFldAttr takes a length argument and clears a field of the ; screen to the given char having the given attribute ; 12/10/88, d.c.oshel ; */ ;extern void cdecl MSJ_SetFldAttr( char ch, ; char row, ; char col, ; char VideoAttr, ; int len, ; struct MSJ_VideoInfo * sptr ); ; ; ;/* move count WORDS from screen to far buffer, 12/10/88, d.c.oshel ; */ ;extern void cdecl MSJ_MovScrBuf( int far * buffer, ; char row, ; char col, ; int count, ; struct MSJ_VideoInfo * sptr ); ; ; ;/* move count WORDS from far buffer to screen, 12/10/88, d.c.oshel ; */ ;extern void cdecl MSJ_MovBufScr( int far * buffer, ; char row, ; char col, ; int count, ; struct MSJ_VideoInfo * sptr ); ; ;----------------------------------------------------------------------- ; _MSJ_MovBufScr copies count words from the user's save buffer to ; the active video buffer ; 12/10/88, d.c.oshel ; ; _MSJ_MovScrBuf copies count words from the active video buffer to ; the user's save buffer ; 12/10/88, d.c.oshel ;----------------------------------------------------------------------- ; ; Arguments on the stack (LARGE MODEL): ; ; [bp+18] <--- seg\ struct MSJ_VideoInfo far * sptr ; [bp+16] <--- ofs/ ; [bp+14] <--- int count ; [bp+12] <--- char col ; [bp+10] <--- char row ; [bp+8] <--- seg\ int far * buffer ; [bp+6] <--- ofs/ ; _MSJ_MovBufScr PROC push bp ;save regs & establish stack frame mov bp,sp push si push di push es push ds ;large model uses data segments anywhere ; COUNT: load CX with the number of bytes to move ; mov cx,[bp+14] ;CX <-- count cmp cx,1 ;we're already done if CX <= 0 jl exitmbs ; DESTINATION: set ES:DI to point to screen buffer absolute address ; note - this changes DS to video structure's segment ; lds bx,[bp+16] ;DS:BX <-- ptr to video structure test byte ptr [bx+4], 1 ;is video.SnowFlag clear? pushf ;save answer for later mov dh,[bp+10] ;DH <-- row mov dl,[bp+12] ;DL <-- col call AddressOf ;DI <-- screen offset mov es,[bx+8] ;ES <-- video.SegAddr (screen segment) ; SOURCE: set DS:SI to point to user's save buffer ; lds si,[bp+6] ;DS:SI <-- ptr to source cld popf jnz hardway ; BEST CASE, move the string from DS:SI to ES:DI, with immediate writes ; rep movsw ;move all immediately jmp exitmbs hardway: lodsw ;get char from buffer xchg ax,bx cli ;disable interrupts mov dx,3DAh ;DX <- ptr to Status Register st1: in al,dx ;if we've arrived during a test al,1 ;horizontal blanking interval, jnz st1 ;must wait for the start of next! st2: in al,dx ;bit one is horizontal retrace test al,1 ;bit three is vertical retrace jz st2 ;if nz, horizontal blanking is NOW. ; Move up to 16 bits ONLY to the screen, taking advantage ; of the 8088's 4-byte prefetch buffer to save time (!), ; during the 7 microseconds of horizontal blanking; ; Prosise says timing is so tight we MUST use the XCHG ; instruction to get AX back, and nothing else will do. ; ; There are 200 horizontal retrace intervals per frame, ; only one vertical retrace interval per frame; i.e., 200 ; lines on the screen, horizontal retrace is the zag ; following raster scan's zig, vertical retrace is the ; corner-to-corner diagonal path back to top of screen. ; ; For reference, 65 clock cycles are about 14 µsec. on ; a 4.77 mHz 8088. See the article for details. ; ; In general, "snow" is caused by the CGA trying to access ; video RAM at the same time as the CPU (us!) is trying ; to change the screen. CGA is ALWAYS accessing video ; RAM, except during horizontal or vertical retrace, or ; unless the port is turned off. Prosise's discussion, ; en precis. NOTE: Not all hardware that uses CGA modes ; is actually a CGA! The snow elimination algorithm is ; so slow (relatively speaking), it should be turned off ; unless actually needed to eliminate snow. For example, ; my ATI Graphics Solution card emulates CGA but does not ; require snow elimination. ; ; XCHG AX,BX, STOSW is 4+11 clocks (ca. 3.23 microseconds), ; while MOVSW is 18 clocks (ca. 3.88 microseconds), according ; to the Intel Programmer's Pocket Reference Guide. ; xchg ax,bx stosw ;from buffer to screen sti ;enable interrupts dec cx ;decrement byte counter jcxz exitmbs ;all done, CX has gone to 0 jmp hardway ;loop back for more exitmbs: pop ds ;restore original DS, etc. pop es pop di pop si pop bp ret _MSJ_MovBufScr ENDP _MSJ_MovScrBuf PROC push bp ;save regs & establish stack frame mov bp,sp push si push di push es push ds ;large model uses data segments anywhere ; COUNT: load CX with the number of bytes to move ; mov cx,[bp+14] ;CX <-- count cmp cx,1 ;we're already done if CX <= 0 jl exitmsb ; SOURCE: set DS:SI to point to screen buffer absolute address ; note - this changes DS to video structure's segment ; lds bx,[bp+16] ;DS:BX <-- ptr to video structure test byte ptr [bx+4], 1 ;is video.SnowFlag clear? pushf ;save answer for later mov dh,[bp+10] ;DH <-- row mov dl,[bp+12] ;DL <-- col call AddressOf ;DI <-- screen offset mov es,[bx+8] ;ES <-- video.SegAddr (screen segment) push es ;save ES:DI, screen address push di ;will pop as DS:SI just below! ; DESTINATION: set ES:DI to point to user's save buffer ; les di,[bp+6] ;ES:DI <-- ptr to destination pop si pop ds ;DS:SI <-- ptr to source cld ;clear direction flag popf ;was video.SnowFlag clear? jnz hw ;no, do it the hard way ; BEST CASE, move the string from DS:SI to ES:DI, with immediate writes ; rep movsw ;all at once! jmp exitmsb ; move the string from DS:SI to ES:DI, with retrace delay ; ; we're going from SCREEN to BUFFER, so must wait for blanking interval ; before READING the char/attribute hw: cli ;disable interrupts mov dx,3DAh ;DX <- ptr to Status Register st3: in al,dx ;if we've arrived during a test al,1 ;horizontal blanking interval, jnz st3 ;must wait for the start of next! st4: in al,dx ;bit one is horizontal retrace test al,1 ;bit three is vertical retrace jz st4 ;if nz, horizontal blanking is NOW. ; TIME CRITICAL, only have 7 µsec to read 16 bits into AX ; lodsw ; But going from AX to buffer can take as long as it takes ; stosw sti ;enable interrupts dec cx ;decrement byte counter jcxz exitmsb ;all done, CX has gone to 0 jmp hw ;loop back for more exitmsb: pop ds ;restore original DS, etc. pop es pop di pop si pop bp ret _MSJ_MovScrBuf ENDP ;------------------------------------------------------------------------ ; _MSJ_SetFldAttr writes count copies of char/attribute to video buffer ; 12/12/88, d.c.oshel ;------------------------------------------------------------------------ ; ; Arguments on the stack (LARGE MODEL): ; ; [bp+18] <--- seg\ struct MSJ_VideoInfo far * sptr ; [bp+16] <--- ofs/ ; [bp+14] <--- int count ; [bp+12] <--- char VideoAttr ; [bp+10] <--- char col ; [bp+8] <--- char row ; [bp+6] <--- char ch ; _MSJ_SetFldAttr PROC push bp ;save regs & establish stack frame mov bp,sp push si push di push es push ds ;large model uses data segments anywhere mov cx,[bp+14] ;CX <-- count cmp cx,1 ;we're already done if CX <= 0 jl exitsfa ; DESTINATION: set ES:DI to point to screen buffer absolute address ; note - this changes DS to video structure's segment ; lds bx,[bp+16] ;DS:BX <-- ptr to video structure mov dh,[bp+8] ;DH <-- row mov dl,[bp+10] ;DL <-- col call AddressOf ;DI <-- screen offset mov es,[bx+8] ;ES <-- video.SegAddr (screen segment) ; SOURCE: load attribute and character ; mov ah,[bp+12] ;AH <-- attr mov al,[bp+6] ;AL <-- ch ; move the FIRST word from AX to DESTINATION, with possible retrace delay ; cld ;clear direction flag test byte ptr [bx+4], 1 ;is video.SnowFlag clear? jz quick ;yes ;Rats. Got CGA, so do it slow. Just repeats. ; ;Although slow, this is still faster than the ;hardway pattern fill, which would have to wait for a ;horizontal blanking interval for BOTH reads and writes, ;since both source and destination are up in video RAM. ; slow: xchg ax,bx ;save char/attr mov dx,3DAh ;DX <- ptr to Status Register cli ;disable interrupts at1: in al,dx ;wait for non-retrace period test al,1 jnz at1 at2: in al,dx ;wait for next horz retrace test al,1 jz at2 ;if ready, fall into... ; TIME CRITICAL, must accomplish move in 7 µsec or less ; xchg ax,bx stosw ;di advances to next word sti ;enable interrupts dec cx ;decrement byte counter jcxz exitsfa ;all done, if CX went to 0 jmp slow ;loop back until done ; Pattern Fill: ES:DI is destination of next place to be filled ; DS:SI is original ES:DI, one word back from current ES:DI ; DS:[SI] now contains the pattern! ; CX contains the count of words remaining in the pattern ; ; Example with count == 5; this is one of Steve Wozniak's old tricks: ; s d ; \ | ; ..... <- fill range of 5 places with Z's ; Z... <- source, after loading Z (move 4 one place right) ; .... <- destination at start of move ; Z... <- destination after one move is obvious ; ZZ.. <- source after one move is subtle! ; ZZZZZ <- final result ; ; This works because the source modifies its own next byte by moving its ; first byte to an overlapping destination. It's efficient because in the ; best case, we can move the entire range-1 using REP MOVSW. ; quick: push es ;save starting destination, ES:DI push di stosw ;load first word into pattern, DI advances pop si ;pop starting destination as source, DS:SI! pop ds dec cx ;decrement byte counter jcxz exitsfa ;all done, if CX went to 0 rep movsw ;magic pattern fill ... ! exitsfa: pop ds ;restore original DS, etc. pop es pop di pop si pop bp ret _MSJ_SetFldAttr ENDP ;----------------------------------------------------------------------- ; _MSJ_DispMsgLen writes len bytes from msg to the active video buffer ; without disturbing the receiving attribute field ; 12/9/88, d.c.oshel ;----------------------------------------------------------------------- ; ; Arguments on the stack (LARGE MODEL): ; ; [bp+18] <--- seg\ struct MSJ_VideoInfo far * sptr ; [bp+16] <--- ofs/ ; [bp+14] <--- int len ; [bp+12] <--- char col ; [bp+10] <--- char row ; [bp+8] <--- seg\ char far * msg ; [bp+6] <--- ofs/ ; _MSJ_DispMsgLen PROC push bp ;save regs & establish stack frame mov bp,sp push si push di push es push ds ;large model uses data segments anywhere mov cx,[bp+14] ;CX <-- length of string cmp cx,1 ;we're already done if CX <= 0 jl dexit ; DESTINATION: set ES:DI to point to screen buffer absolute address ; note - this changes DS to video structure's segment ; lds bx,[bp+16] ;DS:BX <-- ptr to video structure test byte ptr [bx+4], 1 ;is video.SnowFlag clear? pushf ;save result, will jz later mov dh,[bp+10] ;DH <-- row mov dl,[bp+12] ;DL <-- col call AddressOf ;DI <-- screen offset mov es,[bx+8] ;ES <-- video.SegAddr (screen segment) ; SOURCE: set DS:SI to point to null-terminated ASCII string, load attribute ; note - this changes DS to msg's segment, old value not needed ; lds si,[bp+6] ;DS:SI <-- ptr to msg cld ;clear direction flag popf ;was video.SnowFlag set? jnz dmsg1 ;yes, do it the hard way ; move the string from SOURCE to DESTINATION, with immediate writes ; dmsg2: movsb ;move character from string to screen inc di ; and skip over attribute on screen dec cx ;decrement byte counter jcxz dexit ;all done, CX has gone to 0 jmp dmsg2 ;loop back for more ; move the string from SOURCE to DESTINATION, with retrace delay ; dmsg1: lodsb ;move character from string to AL xchg ax,bx cli mov dx,3DAh ;DX <- ptr to Status Register wt1: in al,dx ;wait for non-retrace period test al,1 jnz wt1 wt2: in al,dx ;wait for next horz retrace test al,1 jz wt2 ;if ready, fall into... ; TIME CRITICAL, must accomplish move in 7 µsec or less! ; xchg ax,bx stosb ;move character from AL to screen inc di ; and skip over attribute on screen sti ;enable interrupts dec cx ;decrement byte counter jcxz dexit ;all done, CX has gone to 0 jmp dmsg1 ;loop back for more dexit: pop ds ;restore original DS, etc. pop es pop di pop si pop bp ret _MSJ_DispMsgLen ENDP ;------------------------------------------------------------ ; _MSJ_DispString writes a string to the active video buffer ;------------------------------------------------------------ ; ; Arguments on the stack (LARGE MODEL): ; ; [bp+18] <--- seg\ struct MSJ_VideoInfo far * sptr ; [bp+16] <--- ofs/ ; [bp+14] <--- char VideoAttr ; [bp+12] <--- char col ; [bp+10] <--- char row ; [bp+8] <--- seg\ char far * msg ; [bp+6] <--- ofs/ ; _MSJ_DispString PROC push bp ;save regs & establish stack frame mov bp,sp push si push di push es push ds ;large model uses data segments anywhere ; DESTINATION: set ES:DI to point to screen buffer absolute address ; note - this changes DS to video structure's segment ; lds bx,[bp+16] ;DS:BX <-- ptr to video structure mov dh,[bp+10] ;DH <-- row mov dl,[bp+12] ;DL <-- col call AddressOf ;DI <-- screen offset mov es,[bx+8] ;ES <-- video.SegAddr (screen segment) test byte ptr [bx+4], 1 ;is video.SnowFlag clear? pushf ;save result, will jz later ; SOURCE: set DS:SI to point to null-terminated ASCII string, load attribute ; note - this changes DS to msg's segment, old value not needed ; lds si,[bp+6] ;DS:SI <-- ptr to msg mov ah,[bp+14] ;AH <-- VideoAttr popf ;get flags back, with result of test jz nowait ;i.e., if video.SnowFlag is clear ; move the string from SOURCE to DESTINATION, with retrace delay ; cld ;clear DF dstr1: lodsb ;AL <-- DS:[SI] get next character or al,al ;SI is auto-incremented by 1 jz dstr3 ;found '\0', ASCIIZ string terminator xchg ax,bx ;save char/attr mov dx,3DAh ;DX <- ptr to Status Register cli ;disable interrupts wai1: in al,dx ;wait for non-retrace period test al,1 jnz wai1 wai2: in al,dx ;wait for next horz retrace test al,1 jz wai2 ; TIME CRITICAL, must accomplish move in 7 µsec or less! ; xchg ax,bx ;retrieve char/attr stosw ;write to video memory at ES:[DI] sti ;enable interrupts jmp dstr1 ;DI is auto-incremented by 2 ; move the string from SOURCE to DESTINATION, with immediate writes ; nowait: cld ;clear DF dstr2: lodsb ;AL <-- DS:[SI] get next character or al,al ;SI is auto-incremented by 1 jz dstr3 ;found '\0', ASCIIZ string terminator stosw ;ES:[DI] <-- AX, write immediately to screen jmp dstr2 ;DI is auto-incremented by 2 dstr3: pop ds ;restore original DS, etc. pop es pop di pop si pop bp ret _MSJ_DispString ENDP ;----------------------------------------------------------------------- ; _MSJ_DispCharAttr writes a character and attribute to the active ; video buffer ;----------------------------------------------------------------------- ; ; Arguments on the stack (LARGE MODEL): ; ; [bp+16] <--- seg\ struct MSJ_VideoInfo far * sptr ; [bp+14] <--- ofs/ ; [bp+12] <--- char VideoAttr ; [bp+10] <--- char col ; [bp+8] <--- char row ; [bp+6] <--- char ch ; _MSJ_DispCharAttr PROC push bp ;save regs & establish stack frame mov bp,sp push di push es push ds ; DESTINATION: set ES:DI to point to screen buffer absolute address ; note - this changes DS to video structure's segment ; lds bx,[bp+14] ;DS:BX <-- ptr to video structure mov dh,[bp+8] ;DH <-- row mov dl,[bp+10] ;DL <-- col call AddressOf ;DI <-- screen offset mov es,[bx+8] ;ES <-- video.SegAddr (screen segment) ; SOURCE: load character & attribute from stack ; mov ah,[bp+12] ;AH <-- VideoAttr mov al,[bp+6] ;AL <-- ch test byte ptr [bx+4], 1 ;is DS:video.SnowFlag clear? jz dca1 ;yes, write direct xchg ax,bx ;save char/attr mov dx,3DAh ;DX <- ptr to Status Register cli ;disable interrupts wai3: in al,dx ;wait for non-retrace period test al,1 jnz wai3 wai4: in al,dx ;wait for next horz retrace test al,1 jz wai4 ; TIME CRITICAL, must accomplish move in 7 µsec or less! ; xchg ax,bx ;retrieve char/attr stosw ;write to video memory at ES:[DI] sti ;enable interrupts jmp dca2 dca1: stosw ;ES:[DI] <-- AX a screen write dca2: pop ds pop es pop di pop bp ret _MSJ_DispCharAttr ENDP ;--------------------------------------------------------------- ; _MSJ_GetCharAttr returns the character and attribute at the ; specified row & column; i.e., ; On Exit: AH <-- attribute, AL <-- character ;--------------------------------------------------------------- ; ; Arguments on the stack (LARGE MODEL): ; ; [bp+12] <--- seg\ struct MSJ_VideoInfo far * sptr ; [bp+10] <--- ofs/ ; [bp+8] <--- char col ; [bp+6] <--- char row ; _MSJ_GetCharAttr PROC push bp ;save regs & establish stack frame mov bp,sp push di push es push ds ; SOURCE: set ES:DI to point to screen buffer absolute address ; note - this changes DS to video structure's segment ; lds bx,[bp+10] ;DS:BX <-- ptr to video structure test byte ptr [bx+4],1 ;is video.SnowFlag clear? pushf mov dh,[bp+6] ;DH <-- row mov dl,[bp+8] ;DL <-- col call AddressOf ;DI <-- screen offset mov es,[bx+8] ;ES <-- video.SegAddr (screen segment) push es pop ds push di pop si ;move DS:SI <- ES:DI cld popf ;was video.SnowFlag clear? jz gca3 ;yes, skip over snow buster mov dx,3DAh ;DX <- PORT ptr to CRTC status register cli gca1: in al,dx ;wait for non-retrace period test al,1 jnz gca1 gca2: in al,dx ;wait for next horizontal retrace test al,1 jz gca2 ;if ready, fall into... ; DESTINATION: move screen buffer contents to AX register ; gca3: lodsw ;AH <- attr, AL <- char sti pop ds pop es pop di pop bp ret _MSJ_GetCharAttr ENDP ;----------------------------------------------------------------------------- ; AddressOf returns the video buffer address that ; corresponds to the specified row & column ; ; Entry: DS:BX - address of VideoInfo structure ; DH,DL - row, column ; Exit: DI - offset address ;----------------------------------------------------------------------------- AddressOf PROC NEAR mov al,[bx+2] ;columns per row mul dh ;multiply by row number xor dh,dh ;add column number add ax,dx shl ax,1 ;mul by 2 add ax,[bx+6] ;add offset of active page mov di,ax ;DI <- result ret AddressOf ENDP ;---------------------------------------------------------------- ; WriteWithWait writes a character and attribute to a CGA during ; the horizontal blanking interval; this is the ; Microsoft-approved *** SNOWBUSTER *** routine! ; ; Entry: ES:DI - video buffer address ; AH,AL - character, attribute ; Exit: CX and DX are trashed ; ; I mainlined this routine to save time, above. 12/13/88, dco ;---------------------------------------------------------------- ;WriteWithWait PROC NEAR ; mov cx,ax ;save char/attr ; mov dx,3DAh ;DX <- ptr to Status Register ; cli ;disable interrupts ;www1: in al,dx ;wait for non-retrace period ; test al,1 ; jnz www1 ;www2: in al,dx ;wait for next horz retrace ; test al,1 ; jz www2 ; ; ; TIME CRITICAL, must accomplish move in 7 µsec or less! ; ; MOV AX,CX will *NOT* do the job! Must be XCHG. ; ; ; xchg ax,cx ;retrieve char/attr ; stosw ;write to video memory at ES:[DI] ; sti ;enable interrupts ; ret ;WriteWithWait ENDP END