On Mon, Feb 13, 2017 at 7:30 PM, Craig Ruff <[email protected]> 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. -tony > > >
