Re: NetBSD/vax vm for github testing

On 2025-01-11 16:00, Mouse wrote:
>> Also, emulating multiprocessor support in general using a
>> multiprocessor machine is extremely difficult, since you need to
>> synchronise the CPUs so that they emulate at exactly the same speed.
> 
> Why?
> 
> I'm reasonably sure this isn't true for values of "in general"
> including non-VAX machines, as (for exmaple) SPARC-20s can have
> processors with wildly different clock speeds.
> 
> As for VAXen, what is there that requires the two processors to be
> running at exactly the same speed?  In the one case I know enough about
> to be confident of - the MicroVAX-II - I'm fairly sure it's not true of
> real iron; different processor boards have independent clock
> oscillators, which can never be _exactly_ the same speed, even if they
> are running at the same nominal frequency.

I would not expect that they need to run at exactly the same speed. But 
I think simulations might generally have behavior that is bad enough to 
cause problems. But it does also depend on how OSes behave.

MicroVAX-II was never used as Multiprocessor systems. In general, the 
Qbus wasn't designed for multiprocessor systems, and while it is 
possible, it do require some things to be done differently on the other 
boards than the main CPU, which is the bus master. Also, the MicroVAX-II 
have the memory on a private bus, so other CPUs would not be able to 
access it, except via DMA and mappings managed by the master, so it 
would be really hard to to MP uVAX-II.

   Johnny

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