Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!husc6!think!ames!amdahl!nsc!voder!apple!baum
From: baum@Apple.COM (Allen J. Baum)
Newsgroups: comp.arch
Subject: Re: why people create write-only registers
Message-ID: <12539@apple.Apple.COM>
Date: 20 Jun 88 21:28:08 GMT
References: <12441@apple.Apple.COM> <12135@ut-sally.UUCP>
Reply-To: baum@apple.UUCP (Allen Baum)
Organization: Apple Computer, Inc.
Lines: 33

[]
>In article <12441@apple.Apple.COM>, aek@Apple.COM (Al Kossow) writes:
> 
> Sometimes when you're running tight on pcb (or chip) real-estate
> you have to weigh how important it is to have lots of area tied up in
> space-hogging buses just for reading a register back.
> 

I've designed hardware both ways. In my "youth" I designed systems that had
write only registers. It saved chips. We didn't build very many of the beasts,
but they were moderately complex. I had to build them, debug them, and write
diagnostics for them. I immediately started believing that the registers should
be read/write. Debugging and diagnostics were hell to write. It wouldn't have
made that big a difference to our budget; we didn't build very many, after all.
But it would have made the project go a much faster, and thats worth a LOT of
chips.

On the other hand, I've built cards that I just couldn't afford the extra chips
to do the readback. It was a fairly simple board (20 chips+RAM), and lots were
built. I've never felt bad about the tradeoff- the board was simple enough
that there wasn't that much to test anyway, and simple enough that the write
only register could be worked around and tested.

I've also built a more complex board, with its own microprocessor and self-test
diagnostics, and made sure that the microprocessor could read every bit of
state and every register in the machine. It was a godsend, and I haven't 
regretted that decision either.

So, sometimes you can get away with it, and sometimes you can't;
you just have to know when the right time is.


--
{decwrl,hplabs,ihnp4}!nsc!apple!baum		(408)973-3385