Re: Does the KA630 have a TB?

[Paul Koning <paulkoning%comcast.net@localhost>]

> On Sep 25, 2023, at 2:06 PM, Andrew Ball <asbatwrk%gmail.com@localhost> wrote:
> 
> Hello Paul,
> 
>   M> I have not.  I grabbed a copy of simh some time ago, but I never
>> got around to convincing it to build.
> 
> ASB> It built and runs on my amd64 machine.  It's not fast and of
>> course even when the simulated VAX isn't busy the host machine
>> is kept busy emulating the idle VAX.  It's one of the few things
>> I do that make me want a faster desktop computer.
> 
>  PK> SIMH can handle idling and does so very well, provided it knows
>> the idling scheme used.  On some machines, like the PDP-11, the
>> ISA has a WAIT instruction.  On VAX, there isn't one, so there
>> the code uses various schemes to recognize the idle loop of the
>> OS.  It needs to know what OS you're running, and of course it
>> needs to have sufficiently accurate logic matching what your OS
>> actually does.
> 
>    I wonder why VAX lacked that when the PDP-11 had it.  My emulated
> VAX runs NetBSD/vax 9.3.

The VAX architects apparently didn't see the need for WAIT, or they felt it was too much trouble to implement.  In any case, it isn't part of the instruction set.  Instead, when there is nothing else to do, you tend to be in a loop in the scheduler looking for action.  What that looks like depends on the OS.

There's precedent for that; even on the PDP-11, the SJ variant of RT-11 does the same sort of thing, spinning waiting for work rather than executing a WAIT instruction.  So every other PDP-11 OS will idle well in SIMH but RT-11/SJ will not.

	paul