Newsgroups: comp.sys.mac.programmer
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!snorkelwacker.mit.edu!bloom-picayune.mit.edu!athena.mit.edu!captkidd
From: captkidd@athena.mit.edu (Ivan Cavero Belaunde)
Subject: Re: TicksCount
Message-ID: <1991Jun13.163254.27912@athena.mit.edu>
Sender: news@athena.mit.edu (News system)
Organization: Massachusetts Institute of Technology
References: <1991Jun11.092513.14329@cs.uri.edu> <1991Jun11.185747.9300@athena.mit.edu> <1991Jun13.111156.25658@cs.uri.edu>
Date: Thu, 13 Jun 91 16:32:54 GMT
Lines: 51

In article <1991Jun13.111156.25658@cs.uri.edu> reynhout@cs.uri.edu (Andrew Reynhout) writes:
>In article <1991Jun11.185747.9300@athena.mit.edu> captkidd@athena.mit.edu (Ivan Cavero Belaunde) writes:
>>In article <1991Jun11.092513.14329@cs.uri.edu> reynhout@cs.uri.edu (Andrew Reynhout) writes:
>>>   Unfortunately, you'd need discrete hardware to do this.  The Mac gets its
>>>clock signals from the 60Hz AC input.  Of course, it would be very *simple*
>>>hardware...perhaps there is something out there.
>>
>>Actually, the Mac gets its clock signals from crystals (I shudder to think
>>of a Mac getting its working frequencies from Boston Edison ;-), which
>>generate interrupts at specified intervals via counters in the VIAs.
>
>   Yes, several people have taken the time to correct me here.  To clarify, I
>was under the impression that the Mac got its source for TicksCount (not the
>"clock" as I said previously) from the vertical retrace of the monitor, which
>I, for some reason, believed to be synchronized with the 60Hz of the AC 
>power line.

Actually, I think you're right in that TickCount comes from the vertical
retrace signal (at least on non-Nubus macs - in Nubus Macs I think [I might
be wrong here] that each slot can have a different VBL frequency for
interrupts). It's not that harebrained a scheme either; I seem to remember
that the Apple IIc's serial port's timing signals were generated from the
AC 60Hz signal and Apple had to release a fix/motherboard upgrade/somesuch,
since it drifted so much.

>   That's not how it's done, and I apologize for opening my mouth so soon.
>Apparently, the Time Manager is documented in IM VI.  I'm still wading through
>the first four volumes and coding on a Plus.  I guess it's time for me to 
>update myself, eh?  :-)

I'm pretty sure the Time Manager is documented pre-IM V/VI, it's just buried
since it's not a commonly used part of the OS (unlike, say, the Resource Mgr).

I'd wait on updating, though, until the 2nd Edition comes out (BTW, anyone
know when that will be?).

>   (So I was still wrong, but at least I have a reasonable explanation for 
>my error.  :-)  Thanks for not flaming too hotly.]

Ack. Flaming people over factual inaccuracies is IMHO a *bad* idea (flamewar
fuel).

Now religious flames (my Mac is better than your Amiga which is better than
your NeXT which is worse than my IBM ad nauseaum) are *so* much fun... ;-)

Later,

-Ivan Cavero Belaunde
Digital Video Applications (DiVA) Corp.
Internet: captkidd@ATHENA.MIT.EDU