PDP-8 GTF question

Brian L. Stuart blstuart at bellsouth.net
Sun Jul 26 22:51:00 CDT 2015

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.  However, maindec 8E-D1HA test 05 seems to depend
on this not being true.  Running the GTF instruction on my 8/M
when the inhibit FF is set results in a 0 in AC3.  Furthermore,
both simh and Doug Jones' emulator omit the interrupt inhibit
bit in the GTF instruction.  In fact Dr Jones' emulator has a /*?*/
comment in the code.  Does anyone know the origin of this
discrepancy?  Was the behavior the same on pre-Omnibus
versions of the 8, or did earlier address extensions include
that bit in the GTF result?

I came across this oddity in the course of writing an emulator
of my own, and I'd like understand what's going on here a little
deeper than "ignore the documentation, and make it work like
the real hardware."

Thanks in advance,

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