Path: utzoo!attcan!uunet!lll-winken!csd4.milw.wisc.edu!mailrus!ukma!tut.cis.ohio-state.edu!ucbvax!UIAMVS.BITNET!AWCTTYPA
From: AWCTTYPA@UIAMVS.BITNET ("David A. Lyons")
Newsgroups: comp.sys.apple
Subject: vertical blanking
Message-ID: <8903242100.aa28618@SMOKE.BRL.MIL>
Date: 25 Mar 89 01:59:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 28

>Date:         Fri, 24 Mar 89 01:30:00 EST
>From:         John Taylor <V131Q5CG@UBVMSC.CC.BUFFALO.EDU>
>Subject:      Two Subjects-- VBI and Constructive Criticism of Apple
>
>Topic #1: Vertical Blanking Interval
>
>Recently there was some talk of this and I kept forgetting to throw
>my two cents in, so here it is: If location -16359 is >128 then a
>screen-refresh is happening; <128 means the vertical blanking
>interval is occuring.
>
>Of course, this probably dosen't apply to the GS and it's probably
>been mentioned already (and I missed it) so my two cents is reduced
>to a peso or a ruble or something...

Take the trouble to get the Tech Notes.  "Probably"s are not
required.

Apple IIgs Tech Note #40 says that bit 7 of $C019. reads the VBL
state on the IIe, IIc (and IIc+, I assume), as well as the GS.  The
meaning of bit 7 is reversed on the GS, though.  On the GS bit 7 is
set during the vertical-blanking time, but the others are just the
opposite.

 --David A. Lyons              bitnet: awcttypa@uiamvs
   DAL Systems                 CompuServe:  72177,3233
   P.O. Box 287                GEnie mail:    D.LYONS2
   North Liberty, IA 52317     AppleLinkPE: Dave Lyons