Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/5/84; site olivee.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!allegra!oliveb!olivea!olivee!spud
From: spud@olivee.UUCP (John Purser)
Newsgroups: net.micro.pc
Subject: Re: Trying to access vertical retrace.
Message-ID: <448@olivee.UUCP>
Date: Sat, 23-Nov-85 20:42:39 EST
Article-I.D.: olivee.448
Posted: Sat Nov 23 20:42:39 1985
Date-Received: Sun, 24-Nov-85 11:16:04 EST
References: <273@meccts.UUCP>
Organization: Olivetti ATC; Cupertino, Ca
Lines: 31


Here's what I use to find vertical retrace. The proc exits on vertical
retrace.

STATUS_PORT EQU     03DAh
V_RTRC	    EQU     8

vrt_rtrc proc near
    push    ax
    push    dx
    mov     dx,STATUS_PORT
l01:
    in	    al,dx 		; stays in this loop if you enter during
    test    al,V_RTRC		; vertical retrace
    jnz     l01
l02:
    in	    al,dx		; stays in this loop until you hit
    test    al,V_RTRC		; vertical retrace then exits
    jz	    l02
    pop     dx
    pop     ax
    ret
vrt_rtrc endp

    This works on an ATT6300 and should work on an IBM.
    The top loop is there so that I won't exit until I have a full 
    vertical retrace to use.


    John Purser
    Olivetti ATC