[Simh] Multiple Processors

[Holger Veit]

Am 04.04.2011 23:53, schrieb Rob Jarratt:
> Threads in SIMH would be great for emulating many devices too. I suspect the
> reason they are not there and that it is polled is portability. I have never
> looked into seeing if there is a way to get threads on all the supported
> platforms, but if anyone else has then it would be interesting to hear about
> it.
>
> Regards
>
> Rob
>> On 04/04/11 5:10 PM, Bill Beech (NJ7P) wrote:
>>> All,
>>>
>>> I am in the process of writing an emulator for the Intel 310 system.
>>> I would like to know if anyone has figured a way to emulate multiple
>>> processors in one emulation?  The Intel 310 used the following
>>> processors simultaneously:
>>>
>>> Master CPU 80286
>>> Serial Board Slave 80188
>>> Ethernet Slave 80188
>>> Tape Drive Slave 8089
>>> Hard Disk Slave 8089
>>>
>>> Any thoughts?

You don't need threading in simh, because it already implements sort of 
threads by
implementing an event queue to process I/O events. Threads are just a 
light-weight
word for "scheduling in a single executable". Simh is thus very similar 
to an operating system
(as any simulator is, in principle).

What you need for multiprocessors is splitting the sim_instr() function 
into independent
handlers for each CPU. When done correctly, it will also allow multiple 
instances of
the same CPU in a single simulator.

Admittedly, doing this is much simpler with real OO concepts, where you 
just instantiate
one or more CPU classes and then, in a loop call their "sim_instr" 
methods sucessively
for a single CPU instruction and increment simulator time accordingly.

Besides, many simulators have "intelligent" I/O gear; you should 
carefully analyze whether
the tape and disk slave subsystems actually need to be emulated 
precisely, or just at their
I/O border to the main CPU. See the Altair simulator in simh, for 
instance: the built-in floppy disk
controller, including a DMA controller, might have been simulated 
precisely as if it were an independent
machine, but actually, it is sufficient to pass it certain 
read/write/format/seek commands in
special registers, and then, after some time passed, it will magically 
transfer data from an emulated
disk file directly to the emulated main memory of the Altair machine, 
and throws a "command done"
interrupt.

-- 
Holger