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From: ldo@waikato.ac.nz (Lawrence D'Oliveiro, Waikato University)
Newsgroups: comp.arch
Subject: Re: Bitfield instructions--a good idea?
Message-ID: <1991Apr17.174918.3458@waikato.ac.nz>
Date: 17 Apr 91 05:49:18 GMT
References: <1991Apr15.193425.3436@waikato.ac.nz> <712@seqp4.UUCP>
Organization: University of Waikato, Hamilton, New Zealand
Lines: 24

In article <712@seqp4.UUCP>, jdarcy@seqp4.ORG (Jeff d'Arcy) mentions
the bitfield instructions on the Motorola 88K processor family.

Someone else has kindly e-mailed me a description of the 88K bitfield
instructions. They are truly remarkable animals, except for some reason
the variable versions take *both* the offset and field width (as bitfields)
from the same register.

In other words, if you were dealing with a variable bitfield, you'd need
another sequence of bitfield instructions (using fixed, immediate values for
the width and offset) to set up the operands for the first one! I'd venture
to suggest this would nullify any performance gain from trying to use bitfield
instructions in the first place, except that Jeff D'Arcy also mentions that
the 88K doesn't have any shift or mask operators, so you don't have the
choice.

Why did they design the 88K instructions to pack two operands into
one register?

Lawrence D'Oliveiro                       fone: +64-71-562-889
Computer Services Dept                     fax: +64-71-384-066
University of Waikato            electric mail: ldo@waikato.ac.nz
Hamilton, New Zealand    37^ 47' 26" S, 175^ 19' 7" E, GMT+12:00
Be kind to plants--eat more herbivores.