Re: what's a register, The Design of Design

According to Scott Lurndal <slp53@pacbell.net>:

It seems that the /360 was the first machine which put many
registers into a (conceptually) separate space, leaving them open
to implementing them either in memory or as faster logic.

>
PDP-8 had both (auto-increment index registers in memory) and
the separate accumulator and link registers.

The auto-increment "registers" in the PDP-8 and its 18-bit cousins
were just regular memory locations with a hack that incremented them
when you used them as an indirect address. They weren't anything like
what we call index registers, since you couldn't combine them with
anything else to get an address.  (I speak from direct experience.)

Much later the 18-bit PDP-15 added a real index register, but there is
no way you could have done that on a PDP-8 since there was no spare
place in the instruction word to put an index bit. And by that time it
was clear that the PDP-8 was headed for niche process control
applications and oblivion while the PDP-11 was the future.

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