> On Nov 19, 2016, at 3:38 PM, Jon Elson <[email protected]> wrote:
>
>> ...
> The /20 was very limited, and not a general purpose 360. There was also the
> 360/22 and 360/25 that were variants of the /30 model. One was cheaper, one
> was a little faster. But, if they were doing mostly RPG work, then a /20
> could do that.
Another limited machine was the 360 model 44, which omitted the string and
decimal instructions. So it couldn't run OS/360; it had its own operating
system (PS/44?). But there was a fix for that: an optional "emulation" feature
would add a separate chunk of memory in which an instruction emulation library
would run that would simulate the missing instructions. Think of it like the
full VAX instruction emulation done in some microVAX models. It's a very
obscure feature; I've looked for descriptions but not found much at all. Our
360/44 in college had this, and as a result, it did run OS/360 (19.6 PCP). And
it ran PL/I applications -- VERY slowly.
The emulator was loaded from a binary card deck, only if needed (core memory --
it wasn't normally done). It was an interesting bit of magic; the actual load
was done by a complex channel program, which would actually pick apart the
fields of the object deck to extract the load address and length fields of each
binary card, modify the channel program from those fields, then read the data
portion of the card into the correct location. A pretty impressive combination
of command and data chaining.
paul
