Path: utzoo!attcan!uunet!husc6!think!ames!amdahl!apple!baum From: baum@Apple.COM (Allen J. Baum) Newsgroups: comp.arch Subject: Re: More On Write-Only Control Registers Message-ID: <12656@apple.Apple.COM> Date: 22 Jun 88 19:58:45 GMT References: <439@cf-cm.UUCP> <22150@amdcad.AMD.COM> <2007@alliant.Alliant.COM> <12587@apple.Apple.COM> <5184@nsc.nsc.com> Reply-To: baum@apple.UUCP (Allen Baum) Organization: Apple Computer, Inc. Lines: 17 [] >In article <5184@nsc.nsc.com> stevew@nsc.UUCP (Steve Wilson) writes: > >Sometimes side effects are desirable, i.e. do you want to eliminate use >of input/output fifos? >As an example, all USARTS that I'm aware of clear the data read port >after they are read. Steve also listed potential problems of such a scheme, like what happens when you RETRY, or what happens on out-of-order execution. A simple fix is to allow the FIFO, but don't advance it on a simple read; advance it explicitly, with a write operation. This fixes most of the problems, which are caused by read operations with side effects. Writes are allowed to have side effects (naturally), so there are no problems there. -- {decwrl,hplabs,ihnp4}!nsc!apple!baum (408)973-3385