> On Apr 10, 2024, at 8:18 AM, CAREY SCHUG via cctalk <cctalk@classiccmp.org>
> wrote:
>
> Nearly all the 360s were microcoded, so adding a bit more microcode let them
> emulate 1400/7000 series computers as a standard optional feature. (well the
> model 44 emulated the 1620, ...
Um, what?
In college I used a 360/44, which ran OS/360 (PCP 19.6, all that could fit in
128 kB of memory), which was made possible by the fact that it had the
"emulator" option. But that wasn't a 1620 emulation; instead, it added the SS
instructions of the standard 360 instruction set back in, those were omitted
from the base model 44. Without SS instructions, OS/360 could not run, which
is why the model 44 had an operating system specifically for that machine
(PS/44 ? I'm not sure, I never used it).
The emulator had a separate chunk of memory and a separate IPL button;
unimplemented instructions would trap to that memory for the emulator to handle
-- very much like how subset VAX systems like MicroVAX would emulate the
missing instructions.
The emulator binary came in a card deck, a standard BPS binary deck preceded by
a single-card loader that was an amazingly clever self-modifying channel
program. The entire logic to interpret the fields of the binary cards and load
the entire deck to the right places was implemented in that one-card channel
program.
I read the relevant documentation back then and decoded the loader, but I have
never seen any of it since; even just a bare mention of the emulator feature is
nearly non-existent.
paul
