PDP-8 GTF question

Johnny Billquist 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.
> Curiouser.

To Stuart - are you sure your code does it right. Could you post it?

>> 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.

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.


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

More information about the cctalk mailing list