[TUHS] PDP-11/70 SPL

Johnny Billquist bqt at update.uu.se
Tue Mar 29 02:37:01 AEST 2016

Thanks for some additional information.

On 2016-03-28 18:16, Milo Velimirović wrote:
>> On Mar 28, 2016, at 9:44 AM, Johnny Billquist <bqt at update.uu.se> wrote:
>> On 2016-03-28 16:18, Noel Chiappa wrote:
>>>      > From: Dave Horsfall <dave at horsfall.org>
> [ Wait & RK discussion snipped.]
>>>      > I know that Kevin Dawson (I think) tried it on my /40 as well
>>> The 11/40 does not have the SPL instruction; see the '75-'76 PDP-11 Processor
>>> Handbook, pg. 4-5. (Again, sorry, just want to be accurate.)
>> This is also a pretty important point. But one which also begs the question how the splxxx() functions in Unix worked back then. Or did Unix not use this pattern and these functions back when the 11/40 was relevant?
> These functions existed in V6 and can be found in the file, m40.s, that was assembled with the rest of the kernel to generate a unix that would run on a /40 class machine.

Aha. Great. Thanks. Yes, BIS and BIC on the PSW obviously works, but 
this would definitely not block interrupts for the next instruction. So 
at least in that case, a WAIT could result in the kernel sitting around 
waiting for the next interrupt. I don't really think DEC intend WAIT to 
be used in the way Unix uses it, and it don't really have the properties 
that would be ideas for Unix. Also somewhat indicated by the fact that 
DEC did not use WAIT this way themselves.


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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