24 Aug
2016
24 Aug
'16
10:26 a.m.
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>
wrote:
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
SHOULD
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