AWCTTYPA@UIAMVS.BITNET ("David A. Lyons") (03/25/89)
>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