Early Programming Books

[Van Snyder]

On Mon, 2021-06-21 at 16:13 -0400, Paul Koning via cctalk wrote:
> The registers may actually be implemented as memory (PDP-6 and PDP-10 
> without the "fast registers" feature), and perhaps the Philips PR8000
> which had 8 sets of 8 registers, one per interrupt level. 
> Independently, registers may appear in the memory address space, as
> in the PDP-10 or the Electrologica X8.

Univac 1108 had two sets of registers, selected by a bit in the
Processor Status Word. The "interrupt" set was automatically selected
when an interrupt occurred, so interrupt-service code didn't need to
save registers. They were made from thin-film magnetic memory, not
semiconductors. They occupied "core" locations starting at zero. There
was "core" behind them that could be accessed by setting the indirect-
address bit in the instruction word -- which actually did indirect
addressing if the address wasn't in the register file.

In each set, there were 16 index (X) registers, although X0 always
contained zero, 16 accumulator (A) registers, and 16 general-purpose
(R) registers. R0 was the real-time clock. R1 was for block
instructions. R2 was for mask instructions. X12-X15 and A0-A3 were
actually the same overlapping registers so you could do "accumulator
stuff" to calculate index registers, and then not need to move the
result.