On 06/18/2015 03:44 AM, Timothy Sipples wrote:
Mike Schwab writes:
Well, if you are going to convert software, there is the RPG for MVT
that could be ported to 360/20 subset.
I thought of that, but could that compiler run usefully on a 4K Model 20?
Swapping, paging, or some other memory shuffling approach would be quite
ugly via card I/O. :-) (Glen's museum's Model 20 doesn't have disk or tape,
apparently.) The 4K (or 8K) storage was/is really the governing constraint.
<SNIPPAGE>
The RPG compiler was a single pass compiler. Even IESRPG is a
single pass (OS/360).
The initial code loads, processes the Header Spec (if there is
one), then starts processing the File Specs (which I seem to
remember is handled by the main routine). Each type after that is
handled by its own overlay (of part of the main code) until all
the input has been loaded and the array/table inputs at the end
are made part of the object deck produced.
There is no swapping, paging, etc. because these were REAL
machines, not virtual. To re-iterate, they used program overlay
technology because of limited real storage.
Come to think of it, the model 20 didn't have storage protect
keys either, because only one program could be loaded and run at
a time, since it used a "monitor", not an operating system.
If you were to try to port IESRPG to a S360/20, you will have to
change the code to recognize that certain of the registers have
pre-set values (R3-R7?). Then you will have to change to
recognize that the registers were only 16bits wide. For I/O you
will have to do EXCP and understand how the "channel control"
instructions work (man, I can't believe the things that I can
remember like this). You will also be using the "original" BAS /
BASR instructions (there was no hi-order to save bits in!!).
And you will need to fix the base program and overlays to fit
inside of 4K if they don't already.
It would/will be a lot of work.
Regards,
Steve Thompson
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