PDP11: 11/70 read-only registers must not return NXM on write #306
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@rms47 What are you thoughts on this? |
17 777 740 - 17 777 742, read-only error address registers, and 17 777 764, a read-only System ID register, and are not handled in the CPU70_wr() routine, which means for these addresses the routine returns NXM, which then translates to "bus timeout" (no response to address), and then, as a result, trap to vector 4. That is incorrect, IMO. These locations are read-only yet the address gets decoded, and even though writing does not have any effect, the write routine for these addresses should return SCPE_OK.
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17 777 740-17 777 742, read-only error address registers, and17 777 764, a read-only System ID register,are not handled in the
CPU70_wr()routine, which means for these addresses the routine returnsNXM, which then translates to "bus timeout" (no response to address), and then, as a result, trap to vector 4.That is incorrect, IMO.
These locations are read-only yet the address gets decoded, and even though writing does not have any effect, the write routine for these addresses should return
SCPE_OK.This patch fixes the issue.
Background:
I was trying to run DEC/X11 diagnostic and, instead of getting the "
CMD>" prompt, when started it was throwing a lot of VECTOR 4 errors for the following instruction, endlessly (only the simulator stop and reboot would help):177740-42is an 11/70's error address register. Obviously, DEC/X11 was checking that the register exists (because 11/70 was a declared configuration), but the address appeared "unresponsive".When I added a NO-OP handling for both
177740and177742in simh, the DEC/X11 diagnostics showed me the prompt and I was able to (successfully) test the simulated hardware. I suppose that returningNXMfor an address that actually exists but read-only, is not the correct behavior, and the fix just proves it. Going a bit further, and using the same logic, I figured out that yet another 11/70's register, the System ID register, was not handled in the write routine (same, returningNXM) so I added the NO-OP handling for that, too. With this change, it looks like all the special 11/70 registers (per the Handbook that I have) are covered in the simh code now, as "recognized addresses".