Article View: comp.sys.mac
Article #15181Re: Mac SE - compatibility (where's the horizontal retrace bit?)
From: jww
Date: Sat, 21 Mar 1987 03:45
Date: Sat, 21 Mar 1987 03:45
27 lines
1724 bytes
1724 bytes
In article <458@batcomputer.tn.cornell.edu>, eacj@batcomputer.tn.cornell.edu (Julian Vrieslander) writes:
> On the Plus, you can read bit 6 in VIA register B to get at the horizontal synch
> signal, which is also the clock for the sound generator DAC.
The Mac II has a vertical retrace every 1/67th of a second.
Quoting from IM Draft Volume V, ch 29:
'With the advent of slots, a variety of screens are available,
each with different vertical retrace periods.
... For compatibility with existing software, a special
system-generated interrupt handles the execution of tasks
previously performed during the vertical retrace Interrupt.'
i.e., there is no *REAL* vertical retrace interrupt, it is only emulated.
Now, from *DRAFT* Macintosh Family Hardware Reference:
VIA Register B
Bit Name Description
6 vH4 1 = SCSI IRQ Mask disabled
and there is no horizontal blanking bit anywhere.
Moral: when Apple says don't access the hardware, they mean it.
I wish I could be more help.
--
Joel West
{ucbvax,ihnp4}!sdcsvax!jww (ihnp4!gould9!joel once I fix news)
jww@sdcsvax.ucsd.edu if you must
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Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: arc1-ca.pugleaf.net!utzoo!utgpu!water!watnot!watmath!clyde!cbatt!ucbvax!sdcsvax!jww From: jww@sdcsvax.UUCP Newsgroups: comp.sys.mac Subject: Re: Mac SE - compatibility (where's the horizontal retrace bit?) Message-ID: <2888@sdcsvax.UCSD.EDU> Date: Fri, 20-Mar-87 22:45:44 EST Article-I.D.: sdcsvax.2888 Posted: Fri Mar 20 22:45:44 1987 Date-Received: Sun, 22-Mar-87 18:16:27 EST References: <458@batcomputer.tn.cornell.edu> Organization: Western Software Technology, Vista, CA Lines: 27 Keywords: Mac SE, compatibility, horizontal retrace