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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: a bit of history, Stealing a Great Idea from the 6600
Date: Tue, 30 Apr 2024 19:42:29 -0000 (UTC)
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According to Scott Lurndal <slp53@pacbell.net>:
>mitchalsup@aol.com (MitchAlsup1) writes:
>>Maybe now with 4-or-5-bit condition codes yes,
>>But the early machines (360) with 2-bit codes were already constricted.
>
>The B3500 (contemporaneous with 360) had COMS toggles (2 bits) and OVERFLOW toggle (1 bit).

As far as I can tell, the 360 was the only machine with short encoded
condition codes. 

In their archtecture book Brooks and Blaauw say it was a mistake, but
it also would have been a problem to fit conditional branches into the
instruction set if they needed more instruction bits to say which
codes to test. As it was, the branch instructions had 4 condition bits
which let them check for any combination of two-bit conditions along
with 1111 for unconditional branch and 0000 for no-op.

-- 
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly