Front panel switches - what did they do?

Paul Koning paulkoning at
Wed May 25 08:50:36 CDT 2016

> On May 24, 2016, at 10:05 PM, Jon Elson <elson at> wrote:
> On 05/24/2016 02:13 PM, Chuck Guzis wrote:
>> I seem to recall that reworking the 360/30 microprogramming was preferred by tinkerers over the 360/40 was primarily that CROS was easier to work with than TROS. I don't recall what the RCA Spectrolas used.
> And the 360/25 had all writeable control store.  The control store was just the top 16 KB of main core memory!  To change emulators, restore from a microprogram crash, etc. you loaded the emulator from a card deck!

The 360 model 44, with the emulation option, is somewhat similar.  The base machine had a trimmed down instruction set, shades of microVAX: no decimal or string operations.  So you could not run OS/360.  Instead, you had to run PS/44 (or some such name).

Alternatively, you could get the emulation option.  That added some extra memory and an emulation mode, where reserved instructions would trap to the emulator and get emulated there.  So now you could run OS/360, and PL/I or COBOL applications (albeit quite slowly).

The emulator was loaded, on those rare occasions where the memory got wiped, using the "Emulator IPL" button, from a binary card deck.  That deck was pretty slick: it was a channel program loop.  No CPU code involved at all; the first card was a 4 (?) entry channel program that would read the remaining cards, which were a standard assembler output (object deck).  Self modifying channel code: since each object card contained the address and length for its data, the channel program would pick up those two fields and drop them into the third channel command, which would transfer that number of bytes to that address.


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