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From: HBO043%DJUKFA11.BITNET@cunyvm.cuny.edu (Christoph van Wuellen)
Newsgroups: comp.os.minix
Subject: 68000 clr instruction -- useless?
Message-ID: <47175@nigel.ee.udel.edu>
Date: 11 Mar 91 11:10:38 GMT
Sender: mmdf@ee.udel.edu
Lines: 16

I learned that the 68000 reads the operand which it overwrites with
zeroes later upon a clr instruction.

This may have unwanted effects when programming interface registers --
so I will withdraw the clr instruction from the peephole optimizer of
c68.

There is only a single case where clr is faster than move #0.
this is clr.b or clr.w Dn. There are cases where clr is slower:
clr.w -(An) and clr.l -(An)

The advantage of the clr instruction is that it is shorter.

The next c68 patch will generate only clr.b and clr.w to data registers.

C.v.W.