[TUHS] PDP-11/70 SPL (was: Early non-Unix filesystems?)

Johnny Billquist bqt at update.uu.se
Sun Mar 27 20:05:35 AEST 2016

On 2016-03-27 03:50, Dave Horsfall<dave at horsfall.org> wrote:
> On Fri, 25 Mar 2016, Johnny Billquist wrote:
>>> > >Some instructions inhibit the "check for interrupts at the end of this
>>> > >instruction" check.  I'm most familiar with the 8080 EI instruction,
>>> > >which enabled interrupts after the following instruction (so things
>>> > >like EI;HLT didn't have a window).  It seems the PDP-11 SPL behaves
>>> > >the same.
>> >
>> >I don't think it should on the PDP-11, and the documentation do not
>> >mention any such thing.
> It most certainly did, at least on the 11/70 that I used...  Do you have
> experience otherwise?

I do not have any experience either way. I have never checked this. I'm 
just saying that it don't make sense in my head, and the processor 
handbook do not describe such a property of SPL. But now that I know, 
I'm going to try and find out.

It might be correct. I'm just surprised if so, since there is no 
technical need for SPL to act that way. And having SPL behave 
differently than all other instructions means extra work for the people 
who wrote the microcode.

It would also be interesting if anyone can come up with a good reason 
why SPL should work that way.


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

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