PDP-8 GTF question
bqt at update.uu.se
Mon Jul 27 04:39:07 CDT 2015
On 2015-07-27 08:16, Vincent Slyngstad wrote:
> From: Brian L. Stuart: Sunday, July 26, 2015 8:51 PM
>> Running the GTF instruction on my 8/M
>> when the inhibit FF is set results in a 0 in AC3.
To Stuart - are you sure your code does it right. Could you post it?
>> 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.
Correct. Pre-Omnibus machines had a rather different set of instructions
in in those opcodes.
> 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.
he KT8A is very different. If someone is curious, the manuals do explain
it, or ask and I can try and explain it as well.
But essentially, AC3 in GTF in the KT8A is targeted for use to detect if
an IOT was executed while a delayed CIF was in effect in user mode.
Essentially saying to the OS that the user mode program is barfed and
cannot be restarted.
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