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From: dmt@mtgzz.UUCP (d.m.tutelman)
Newsgroups: net.micro.pc
Subject: Re: Help with Video Retrace Code
Message-ID: <1019@mtgzz.UUCP>
Date: Sat, 3-Aug-85 22:08:42 EDT
Article-I.D.: mtgzz.1019
Posted: Sat Aug  3 22:08:42 1985
Date-Received: Mon, 5-Aug-85 07:38:38 EDT
References: <85@drux1.UUCP>
Organization: AT&T Information Systems Labs, Middletown NJ
Lines: 28
Cc: dmt

> Does anyone know the assembler code to do a check on the horizontal
> retrace flag for the mono and color display on the PC to prevent
> fuzz?
> 
Actually, you could even do it in C, though the code examples I have
are assembler.  The important thing to know is that the status
register of the 6845 display chip includes a bit (bit 1) for horizontal
retrace status. (The 6845 is used in both the mono and color boards.)

The code that follows is adapted from the Micromint MPX-16 BIOS, but is very
similar to the BIOS code published in the IBM PC Tech Ref:

	mov	dx,video_base	; 3B4H or 3D4H
	add	dx,6		; status register
waitlow:in	al,dx		; get the status reg
	test	al,1		; horizontal retrace bit on?
	jnz	waitlow
	cli			; don't want to be interrupted here
waithi:	in	al,dx		; now wait till it goes on again
	test	al,1
	jz	waithi
;NOW you can write for a period equal to the retrace interval.

			Dave Tutelman
			Physical - AT&T Information Systems
				   Holmdel, NJ 07733
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