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From: Lawson.English@p88.f15.n300.z1.fidonet.org (Lawson English)
Newsgroups: comp.sys.mac.programmer
Subject: VIA woes!
Message-ID: <4301.27D4F060@stjhmc.fidonet.org>
Date: 5 Mar 91 16:57:18 GMT
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todd matthew morin writes in a message to All

TMM> I've inherited an assembly language routine that was written 
TMM> a few years ago and writes directly to the mac VIA to enable 
TMM> millisecond accurate timing. Unfortunately, this code predates 
TMM> all but the MacPlus and SE (and the 512K, etc). The problem is 
TMM> that on newer machines, most notably the SE/30, unexpected crashes 
TMM> occur as a result of this code. Using MacsBug I've been able 
TMM> to figure that the program is crashing on an RTS instruction. 
TMM>  The program itself (in which this timer is included), displays 
TMM> files on the screen synchronously with the vertical interrupt, 
TMM> then starts the timer. The SE/30 does not consistently crash 
TMM> at the same file being displayed, but only consistently on the 
TMM> RTS instruction. This leads me to believe that the SE/30 is doing 
TMM> something with VIA1 during the millitimer routine


I too have some code that breaks with the newer systems. It doesn't seem to
break due to hardware problems, but because we were using a low-memory global
as a scratch address which is now overwritten. As far as I can tell, VIA1 should
work as it always does. It does for my code, I simply have to rework the scratch
address to work with post System 4.1 systems. 

It is an interactive profiler that examines all address space and graphs memory
usage "on the fly." It uses the VIA to set an interupt every millisecond. When
it was working, we could zoom into any heavily used area of memory and map the
most used sections. It had accuracy from 1/256 of the total address space down
to 2 bytes, so we could see single-machine-instruction bottlenecks.


I suspect that your problem isn't with the VIA, it's System-dependent like mine
was(is). The other possibility is a stack problem. There may be differences
in the way the stack RTS is handled returning from an interupt. 



Lawson
 

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