Path: utzoo!attcan!uunet!xavax!alvitar
From: alvitar@xavax.com (Phillip Harbison)
Newsgroups: comp.arch
Subject: Re: unexpected CPU behavior [was 486 bugs -- it's in there!]
Message-ID: <1990May27.110726.17007@xavax.com>
Date: 27 May 90 11:07:26 GMT
Sender: alvitar@xavax.com
Organization: Xavax
Lines: 19

In article <2813@medusa.informatik.uni-erlangen.de>, csbrod@medusa (Claus
Brod) writes:
> Motorola's 68000 had a kind of quirk that caused a clr command to
> first read the addressed location and then clear it. This led
> to numerous difficulties when clearing I/O registers. This one,
> however, has always been documented by Motorola, so it's more
> a feature than a bug. Subsequent 680x0s perform a clr operation
> as God intended.

I never considered this to be a big problem.  As you noted, Motorola
documented this behavior, and anyone who really cared would use the
"move quick" alternative (MOVEQ #0,destination).  Since the constant
value (8 bits signed) was included in the instruction word, I don't
believe there was any advantage to CLR versus MOVEQ.

-- 
Live: Phil Harbison, Xavax, P.O. Box 7413, Huntsville, AL 35807
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