PDP-11/05 Fault?

[Paul Koning]

> 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