Path: utzoo!attcan!uunet!husc6!mailrus!ames!oliveb!amdahl!nsc!stevew From: stevew@nsc.nsc.com (Steve Wilson) Newsgroups: comp.arch Subject: Re: More On Write-Only Control Registers Message-ID: <5178@nsc.nsc.com> Date: 20 Jun 88 16:56:47 GMT References: <439@cf-cm.UUCP> Reply-To: stevew@nsc.UUCP (Steve Wilson) Organization: National Semiconductor, Sunnyvale Lines: 51 In article <439@cf-cm.UUCP> mch@computing-maths.cardiff.ac.uk (Major Kano) writes: >In my earlier posting, I cannot have explained clearly what I meant. The few >replies that I saw/recieved suggested that there was no point in putting >write-only or read-only registers at different addresses. >Now that's all very well, but what I was trying to get at was why the **** have >write-only control registers or read-only status registers AT ALL ? > >I do not like the idea of not being able to read from a register that I've >written to. Maybe this is just paranoid and OS's should keep a copy ANYWAY >of whatever they load into a chip, but you can bet your next three pay checks >that SOMEDAY there will be a situation where a register must be read from; >eg., a faulty privileged program has used a chip before it was killed, and you >(the OS or super-user) have to know EXACTLY what it did. > >Any comments ? > >Regards, >-- >Martin C. Howe, University College Cardiff | "Can YOU do the Milano Mosh ?" >mch@vax1.computing-maths.cardiff.ac.uk. | As has been explained by others on the net, giving read-back capability is expensive in real-estate. If the decision comes down to providing read-back capability or making the design producable, guess which wins. This may sound extreme, but EVERY design I've done in the last 9 years has had this issue raised. Normally, the first place that the issue occurs isn't when considering production code, but in examining the design for testability via software. More often than I care to admit, I've had to design out testing features (read this as read-back capability) because there just wasn't enough pcb space to fit it in. In my last design, it would have cost an additional 20% in logic to provide read-back for all of the registers utilizing hugh muxes. I did come up with a solution for the testing side of the problem, though the applications using the board couldn't make use of the feature. It may be hard to believe, but hardware guys don't normally go out of their way to make life difficult for programmers. Consider that hardware guys usually would prefer having the read-back feature because it makes our life easier during initial design verification. This is an area where design trade-offs are made by hardware engineers. When the cost is not having the product versus not having read-back of ALL of the registers, the designer is forced into making decisions. It is not a trade-off that is made lightly, but it is one that occurs quite often. Steve Wilson National Semiconductor [The opinions stated previously are mine, not those of my employer.]