M9301-YB Bootstrap/Terminator ROM dumps / listings?

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue Apr 28 10:12:53 CDT 2015

    > From: Charles Dickman

    > You can find a dump and partial disassembly

So I decided, just for grins (this may be duplicative of what's on a fiche
somewhere) to complete the disassembly, and there was something that was
puzzling me.

There a bunch of entry points (for auto-booting from different devices) which
all are of the form:

BR	foo

where foo is the same location. The code doesn't save anything before the
branch, it just does it right off. So how does it know, when it gets there,
which entry point was used?

Well, it turns out that the answer, I am pretty sure, is that with the M9312,
location 773024 is even more magic than it first appears.

(Slight digression, for those who didn't get the reference: the way the M9312
forces the 11/04 or /34 CPU into the ROM, on power-fail or manual restart, is
that when the CPU tries to fetch the PC/PS pair from location 024 - the
power-fail/restart vector - the card jams the high UNIBUS address lines to
773000 for two bus cycles, directing the fetch of the new PC/PS to locations
773024-6. Next, when the CPU fetches the contents of location 773024, the
basic contents of that location, in the ROM - 173000 - are or'd with switch
S1-3 (bit 8, 4xx) through S1-10 (bit 1, 2). So that's how the code jumps to
different entry points, based on the contents of S1.)

However, from reading the code (yet to verify this on prints, but it _has_ to
work this way, the code doesn't make any sense otherwise), it appears that
_any_ reference to location 773024, at _any time_, produces the contents of
ROM location 773024 or'd with the contents of switch S1!!

In other words, location 773024 is effectively a hacky register that allows
S1 to be read. (And the M9312 Maint Manual does _not_ make this clear. It
talks about how the contents are modified during the power-on PC/PS fetch
cycle, but _not_ about the later references part.)

Very clever, though!


PS: Josh, from what I've seen so far, the code tends to loop on error, not
halt. So if your machine's halting...

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