Re: branch address architecture, not interactive use, The Design of Design

John Levine <johnl@taugh.com> schrieb:

Do we know who invented relative branches? The PDP-11 had them in 1969
but I don't think they were new then.

Very good question.

The Nova offered this possibility.  They had three addressing modes:
zero page with an 8-bit unsigned, PC-realtive with an 8-bit signed
offset, and one of the two index registers with a signed 8-bit offset.
This worked for load/store and for jumping.  They implemented
conditional via skipping over jump instructions.

So, PC-relative jumps already existed before the PDP-11 at least,
although not as branches as such.

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?)

Obviously the Nova designers thought they could do so (but they
only had a single 4-bit adder originally.  This is why the load
and store instructions were so slow, apparently, see appendix F of
http://bitsavers.org/pdf/dg/015-000023-03_NOVA_PgmrRefMan_Jan76.pdf

Hmm... browsing the appendix E, loading and storing bytes really
was a hack on that machine, the PDP-11 (and the S/360) were much
faster there.  I wonder why deCastro didn't just put his byte
pointer bit in the lowest position, it would have saved programmers
a lot of grief (I assume...)