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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

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