Information Request: unidentified HP 9825T instructions

Tony Duell ard.p850ug1 at
Mon Feb 13 13:35:03 CST 2017

On Mon, Feb 13, 2017 at 7:30 PM, Craig Ruff <cruff at> wrote:
> Tony Duell wrote:
>> My first thought, and it's probably wrong, is that these instrucitons
>> (which differ by one bit, so might be
>> setting/reseting something) are NOPs to the CPU, but are interpretted
>> by the memory mapping hardware in those
>> 9825s that have more than 64K or RAM and ROM total.
> This seems likely.  According to your schematics, gate U47 detects the pattern 0701xx.
>  This signal feeds into the U43c flipflop, which appears to latch the state of the low 4 bits
> of the MAD bus into register U42, which sets the state of the /ForceRAM (bit 3),
> /ForceROM (bit 2), /DiagRd (bit 1) and ALLROM (bit 0) signals.  Thus, these two
> instructions appear to toggle the state of the /ForceROM signal.
> If I’ve wrapped my brain around the details it appears that 070113 deasserts the
> /ForceROM signal, and 070117 asserts it?

That seems possible, yes.

Having other hardware decode instructions that appear as NOPs to the main
processor is not uncommon in HP machines.



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