Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!batcomputer!eacj
From: eacj@batcomputer.tn.cornell.edu (Julian Vrieslander)
Newsgroups: comp.sys.mac.programmer
Subject: If Apple implements user mode, how will we do uninterrupted I/O?
Keywords: user mode interrupt
Message-ID: <8538@batcomputer.tn.cornell.edu>
Date: 2 Aug 89 02:11:22 GMT
Reply-To: eacj@tcgould.tn.cornell.edu (Julian Vrieslander)
Organization: Cornell Theory Center, Cornell University, Ithaca NY
Lines: 45

Tech Note #85 says that we should not disable interrupts from within an
application.  There is an ominous admonition that "there will come a time
... when you won't be able to change the interrupt mask because the
processor is running in user mode."  This may be putting me in a real
quandary, and it may be bad news for a lot of other people who are writing
hardware interfacing code.

I have been writing software to drive a data acquisition card sitting on
the Nubus in a Mac II.  But my application requires the CPU to do high
speed uninterrupted polling of the card, sometimes for several seconds at
a time.  It may be impossible to implement these routines with
interrupt-driven code, since the interval between service requests could
be as little as 5 microseconds.  And an interrupt by someone else's code
(like a VBL task) could cause me to miss some data.

Note that the applications that I am working on (waveform synthesis and
precision event timing) are quite common jobs for laboratory
microcomputers.  And remember that general purpose data acquistion boards
do not have much buffering or on-card intelligence.  So there are lots of
other folks out there who are writing code that temporarily disables
interrupts.  The following questions should be of interest to all of them:

What does Apple really mean when they say that the interrupt mask may
become inaccessible in some future System version, or on some future Mac?
My reading of the Motorola manuals indicates that applications running in
user mode will not be able to directly modify the interrupt mask in the
status register of a 680xx CPU.  If applications must run in user mode,
will Apple provide a trap (or another mechanism) that will allow an
application to disable interrupts?

If not, what is the recommended way of doing uninterrupted I/O?  I believe
that it is possible to turn off individual interrupt sources by flipping
bits in the Interrupt Enable registers of the VIA chip(s). But this
hardware is quite machine dependent - different Mac models have either 1
or 2 VIA chips, in different memory locations, and with different register
bit mappings.  So the code would have to special case each computer that
it runs on, and might break on a future model.

Perhaps I am overlooking something obvious (wouldn't be the first time),
but I don't see a clean solution to this problem.
-- 
Julian Vrieslander 
Neurobiology & Behavior, W250 Mudd Hall, Cornell University, Ithaca NY 14853    
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