29 Sep
2021
29 Sep
'21
5:06 p.m.
On Sep 29, 2021, at 7:31 PM, Matt Burke via cctalk
<cctalk at classiccmp.org> wrote:
I've been restoring a PDP-11/05 recently and after replacing several
faulty ICs I have it mostly working. I've run into a bit of a problem
whilst running MAINDEC-11-D0NB (T14 TRAP TEST) though.
The failing instruction sequence is:
7200: MOV #6340,R0
7204: MOV R0,(R0)+
7206: CMP 6340,#6342
7214: BEQ 7220
7216: HALT
This halts at 7216 with:
R0 = 6342
6340 = 6340
I tried this same set of instructions on a PDP-11/84 and also on Simh
and the result is:
R0 = 6342
6340 = 6342
which is what the diagnostic seems to expect.
Either the diagnostic has a major bug, or you're running it on a machine it
doesn't support, or there is a setup to tell it the difference which you missed.
What you're seeing is the very first entry in the table "PDP-11 family
differences", appendix B of the PDP-11 architecture handbook. It is the famous
pattern that the assembler rejects with a Z (machine dependency alert) error code.
This particular pattern produces 6342 only on 11/23, 11/20, 11/35, 11/70, J-11, and T-11.
On all others, including the 11/05 and the PDP-11 emulation in the VAX, it produces 6340
as you observed.
paul