[TUHS] who invented the link register

[Robert Clausecker]

Am Tue, Oct 25, 2022 at 03:24:01PM -0700 schrieb Bakul Shah:
> On Oct 12, 2022, at 12:01 PM, ron minnich <rminnich at gmail.com> wrote:
> > 
> > I know branch and link was in the 360; was it earlier? And ... anybody
> > know who invented it?
> > 
> > This came up in a risc-v meeting just now :-) My claim is that if anybody
> > knows, they will  be in this group.
> 
> Zuse Z4 had instructions to jump to a subprogram and back. Unclear if
> they were in the original Z4 (1945) or were added later. Or how it was done.

The Z4's programs were stored on a number of tapes.  Initially, there
was only one tape reader and no branch instructions.  Later, the design
was modified by adding additional tape readers and instructions to
switch to a different tape reader.  Thus, sub programs can be realised
by storing the subprogram on a different, possibly looped, tape and then
switching to the other tape reader when the sub program is called.  The
subprogram's final instruction would switch back to the main tape
reader.  So not quite a link register.

An instruction set reference for a later (1950) model can be found in the
ETH Zurich library:

https://www.e-manuscripta.ch/zut/doi/10.7891/e-manuscripta-98601

Yours,
Robert Clausecker

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