PDP-8 GTF question

Brian L. Stuart blstuart at bellsouth.net
Mon Jul 27 09:48:35 CDT 2015

On Mon, 7/27/15, Vincent Slyngstad <v.slyngstad at frontier.com> wrote:
> > I need to pick the brains of some PDP-8 experts.  According to
> > the references I can find, especially the Small Computer Handbook,
> > the GTF instruction should include the M837 interrupt inhibit bit
> > in AC3.
> I can see where this happens in the M837 schematic (E50), 
> whenever DF is gated to AC9-11.  That, in turn, seems to 
> be for GTF or RIB.

Interesting.  By the time I got this far last night, I decided to put
off looking at the schematics to today.  Thanks for the pointer.
> > However, maindec 8E-D1HA test 05 seems to depend
> > on this not being true. 
> Can you tell us where you are seeing this?  A pointer to the 
> copy of the diagnostic you're using, and perhaps an address?
> There seem to be several GTF tests, if 
> http://svn.so-much-stuff.com/svn/trunk/pdp8/src/maindec/8e/d1ha-d.pdf
> matches what you are running.

It does indeed.  In particular, in Test 05 on Page 35 of that PDF,
at address 2342, we load the AC with 5200, do an RTF, set the
AC to 7777, and then do a GTF and compare the result to 5200.
Although AC3 isn't set when executing the RTF instruction, one
of the effects of the RTF is to always set the interrupt inhibit.  So
in my original code, the GTF resulted in a 5600, rather than a 5200.
> > Running the GTF instruction on my 8/M
> > when the inhibit FF is set results in a 0 in AC3. 
> Curiouser.

Indeed.  In trying to sort this out, I single stepped through this
part of the the maindec on my my actual machine making particular
note of the inhibit shown in the "no int" indicator in the status
switch setting.  It does get set by the RTF, but after the GTF,
the accumulator has 5200 in it.
> I don't believe that the pre-Omnibus memory extensions 
> implement GTF.

Good point.  As I was about to head to bed last night, I took a
look in the '67 small computer handbook and noticed that the
same instruction (6004) was used for a A/D converter in the
straight 8.


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