Re: Emulator idle detect [was Re: Clock Error]

On 2023-12-20 03:05, Mouse wrote:
> [bqt@]
>> Unless I misremember, it raises the IPL to 1.  A level that is
>> otherwise meaningless, so not something done by mistake, and not
>> something used by anything.
> 
> But setting IPL to 1 being "otherwise meaningless" or "not [] used by
> anything" are very OS-specific things.

Oh. It is very OS specific. That's why, in simh, you have to tell it to 
do NetBSD idle detection, which is different than VMS idle detection.

With VMS, it is, as far as I can remember, just detecting a specific 
loop pattern in the kernel.

> I suppose that's at least one reason why, as quoted below, simh has a
> BSD-specific internal flag controlling idle detection.

Yes. Not sure if Ken copied it verbatim from the later version of simh, 
or if he embellished it. But either way, that's basically it.

> [kwellsch@]
>> ... and FWIW, this it the hackish patch I made to SIMH 3.12-4 to
>> carry over the correct IDLE detection code that is part of open-simh
>> (4.1).
> 
>>       case MT_IPL:                                        /* IPL */
>>           PSL = (PSL & ~PSL_IPL) | ((val & PSL_M_IPL) << PSL_V_IPL);
>> +        if ((VAX_IDLE_BSD & cpu_idle_mask) &&           /* New NetBSD and OpenBSD */
>> +            (0 != (PC & 0x80000000)) &&                 /* System Space (Not BOOT ROM) */
>> +            (val == 1))                                 /* IPL 1 */
>> +            cpu_idle();                                 /* idle loop */
>>           break;
> 
> Ah, glad I asked!  Just from the kernel code jbglaw@ quoted, I would
> not have inferred the "PC must be in system space" condition.

Considering that a user space program usually can't even raise the IPL, 
it sortof already gives that this would be in the kernel. Of course, any 
code that can switch to running in kernel mode can play around with 
this, so it might not be strictly in the kernel itself, but unless you 
really just want to mess with the simh idle detection, why would anyone 
want to go to IPL 1? There is no hardware that does interrupts at that 
level, so it's pretty much an IPL level that is not used/useful.

   Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
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