PDP-8 diagnostic tests

Doug Ingraham dpi at dustyoldcomputers.com
Wed Aug 24 12:26:37 CDT 2016

Kudos for working on a VHDL model.  Which 8 variant are you trying to model?

As others have stated, the IAC does affect the link.  Of the other
instructions that one might think could change the Link, the ISZ
instruction does not affect the link and indirection that uses the
autoincrement memory locations 0010 through 0017 do not affect the Link.
This is because in those cases the increment is done in the MB register and
not the Link, AC.  Some of the EAE instructions also change the Link.  I
have paper listings of many of the diagnostics and one of my projects is to
get these re-keyed if I can't find a copy and scanned as PDF's so they can
be uploaded to bitsavers and on my own website when I get around to
bringing it up.  I have done Maindec-801-1 and both of the EAE Maindec.
 (801-3a and 801-3b I think they are called.)

The PDP-8 is an extremely simple machine.  If you are confused by some
aspect of implementation you are probably not doing it "the 8 way" because
they way they did it was almost always the simplest way.

Best wishes on your project!

On Tue, Aug 23, 2016 at 1:31 PM, Scott Baker <scott.l.baker at gmail.com>

> Hi,
> I have written a PDP-8 VHDL model and I have it running in an FPGA
> https://github.com/scottlbaker/PDP8-SOC
> At this time it has passed a basic DEC diagnostic instruction test but
> I found some interesting things when getting that instruction test to pass.
> For example:
> The following segment of code implies that IAC instruction affects the Link
> bit
>  1797             /GROUP 1 OPERATE TEST 33
>  1798 02626 7300          CLA CLL         /AC=0000 LINK=0
>  1799 02627 1053          TAD K2525               /AC=2525
>  1800 02630 7261          CLA CMA CML IAC /TEST COMBINATION
>  1801 02631 7420          SNL
>  1802 02632 7430          SZL
>  1803 02633 7402          HLT             /CLA CMA CML IAC FAILED, AC
>  1804                                     /BE 0000, LINK SHOULD BE ZERO
> but the PDP-8 Handbook  ; DEC copyright 1966; page 14
> says nothing about the Link bit being affected by the IAC instruction.
> The simh PDP-8 simulator also shows that L is affected by IAC.
> If I change this test line from
>  1800 02630 7261          CLA CMA CML IAC /TEST COMBINATION  << link=0
> to:
>  1800 02630 7261          CLA CMA CML  /TEST COMBINATION  << link=1
> Can anyone point me to:
> 1) DEC documentation which more fully describes all the instruction set (in
> more detail  than the PDP-8 handbook)
> 2) Some more instruction tests in assembler source code format. I have
> found lots of binary files but I would prefer assembler source code format.
> I am using a pal compatible cross assembler.
> Thanks and Regards,
> Scott

Doug Ingraham
PDP-8 SN 1175

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