PDP-8 GTF question

Vincent Slyngstad 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. 


> 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.


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