From: Brian L. Stuart: Sunday, July 26, 2015 8:51 PM
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.
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.
Running the GTF instruction on my 8/M
when the inhibit FF is set results in a 0 in AC3.
Curiouser.
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 don't believe that the pre-Omnibus memory extensions
implement GTF.
The IM6100 datasheet says bits 3 and 5-11 are under control of
the memory extension. Reading the description for the IM6102,
it says GTF will place INT INH in AC3.
The HD6210 datasheet says bit 3 is the PWRON flag for GTF, and
bit 3 is unchanged by RIB.
The design of the KT8A is sufficiently opaque that I haven't figured
out yet how GTF works there.
Vince
