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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: branch address architecture, not interactive use, The Design of Design
Date: Sat, 11 May 2024 20:54:53 -0000 (UTC)
Organization: Taughannock Networks
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According to Thomas Koenig  <tkoenig@netcologne.de>:
>John Levine <johnl@taugh.com> schrieb:
>
>> Brooks said it was ugly that some instructions (RX) had both base and
>> index registers while others (SS) only had base registers, which I
>> expect made it even harder to do what you suggested.
>
>Depending on base registers for both data and branches was one
>of the ideas that did not age well, I think. 

Yup. S/390 added relative versions of all the branches with a 16 bit
signed offset. Since instructions are aligned on two byte boundaries,
the offset is shifted bit left to allow 64K in either direction.
zSeries added long versions of most branches with a 32 bit offset.

Do we know who invented relative branches? The PDP-11 had them in 1969
but I don't think they were new then. They feel like one of those
things that are obvious in retrospect, but not at the time. (Why do
you want to make branch addressing different? And run them all through
an adder? Do you think gates grow on trees?)

-- 
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