Path: utzoo!attcan!utgpu!watmath!clyde!att!chinet!ward
From: ward@chinet.chi.il.us (Ward Christensen)
Newsgroups: comp.sys.ibm.pc
Subject: Re: assorted questions
Message-ID: <7351@chinet.chi.il.us>
Date: 2 Jan 89 16:11:57 GMT
References: <5082@phoenix.Princeton.EDU>
Reply-To: ward@chinet.chi.il.us (Ward Christensen)
Organization: Chinet - Public Access Unix
Lines: 55

In article <5082@phoenix.Princeton.EDU> jwbirdsa@phoenix.Princeton.EDU 
	(James Webster Birdsall) writes:
>   1) I need a sophisticated way to determine where the screen begins in
>RAM. I have looked in the interrupt list recently posted and didn't find
>anything that looked useful. For example, how does Turbo C (which makes
>a point of doing direct screen writes for speed) do it?
  The folowing asm routine will find the screen base address by
using an int 10 function to determine the screen mode.  It also
sets a SNOW flag if it finds a color board.  NOTE that an EGA or VGA
is not differentiated from a CGA.  In the routine that uses this
routine, I generally have a run-time flag for setting snow false.
  Note that mono "hangs" if you force snow to true, since the bit
  doesn't toggle:
GetVideoMode proc near  ;Function GetVideoMode:boolean;
 push bp
 mov bp,sp
 push es
 mov ah,0fh  ;get video mode
 int 10h  ; al = 7 => mono
 mov ah,1  ;snow := true;
 mov screen_base,0b800h ;Screen base addr if not mono
 cmp al,7  ;is it mono?
 jnz GetvRet
 mov ah,0  ;snow := false
 mov screen_base,0b000h  ;set it to mono
GetVRet:
 mov al,ah
 mov ah,0  ;set ax := snow
 pop es
 pop bp
 ret
GetVideoMode endp

>
>   2) Saw this posted a while ago but can't find it again... what port
>should I watch to avoid snow on the CGA? And am I right in believing
>that this problem is limited to the CGA?
  here is the snow-check routine:
 MOV BX,AX  ;Store video word in BX
 MOV AH,9   ;Move horizontal & vertical retrace mask into AH
 CLI        ;Nointerrupts now
WaitH: IN AL,DX  ;Get 6845 status
 RCR AL,1   ;Wait for horizontal
 JC WaitH   ; retrace
WaitV: IN AL,DX  ;Get 6845 status again
 AND AL,AH  ;Wait for vertical
 JZ WaitV   ; retrace
 MOV AX,BX  ;Move word back to AX...
 STOSW      ; and then to screen
 STI        ;Allow interrupts!
  ----------------
  This came from a FASTWRITE routine; can't seem to find original
author's name (sorry!)
  I use this in a Turbo Pascal routine used by thousands of people across
the country and have had no reports of problems or incompatabilities.