PDP-8 GTF question
v.slyngstad at frontier.com
Mon Jul 27 01:16:23 CDT 2015
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
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.
> 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
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.
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