Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!usc!apple!baum
From: baum@Apple.COM (Allen J. Baum)
Newsgroups: comp.arch
Subject: Instruction Restart and I/O
Message-ID: <34591@apple.Apple.COM>
Date: 8 Sep 89 18:20:24 GMT
References: <1790@cs-spool.calgary.UUCP>
Reply-To: baum@apple.UUCP (Allen Baum)
Organization: Apple Computer, Inc.
Lines: 18

[]
There seems to be a problem with instruction restart and memory mapped I/O.,
for example, in article <blahblahblah> deraadt@enme3.UUCP(Theo Deraadt) writes:
>Gad. How about a hardware FIFO? How about a serial receive buffer on
>your generic serial chip?

If you re-execute, then you might dump something into the FIFO twice.
Or, (more likely), you'll take something out twice.

This is only because of a design practice that should be abolished, which is
side-effects upon reading. Without this mis-feature, extraneous reads do not
change state, and the problems don't occur.

Writes generally will not get re-executed anyway, since if you execute
the write, then you've sucessfully completed the instruction and you
don't have to do it again.  --
		  baum@apple.com		(408)974-3385
{decwrl,hplabs}!amdahl!apple!baum