On 2019-04-07 01:14, Matt Burke wrote:
On 06/04/2019 23:50, Timothy Stark wrote:
Does simh use threads to handle MP processors? I have other SIMH versions
(vaxmp and turbovax) that uses threads to take advantage of CPU cores.
I looked through your 782 code and did not see any threads handle that vaxmp
and turbovax uses.
For the VAX 8300/8350 I just run both processors in the same thread and
alternate between them on each instruction cycle. Things get a lot more
complicated when threads are involved because you have to think about
locking and synchronisation.
The VAX-11/782 is quite different as you actually run two separate
copies of the VAX-11/780 simulator and they communicate with each other
using a shared memory interconnect (simulated by shared memory on the
host operating system). One VAX-11/780 runs as the master and this is
the one that you interact with. It has access to all the I/O devices
such as disks/tapes. The other VAX-11/780 is non-interactive and can
only process workloads which are handed over from the master. This is
asymmetric multiprocessing.
Unless you also have some synchronization mechanisms in there, this
might not actually work right, but it can take time for issues to show
up. Some VAX instructions can be rather complex, and are guaranteed to
be atomic to memory. I believe that holds true also on the 11/782, which
means that instructions like the interlocked queue instructions will not
naturally be interlocked if you just have two threads with shared memory
for a simulation. And so, you run the risk that if both CPUs would want
to modify the same data interlocked, the simulation will actually end up
corrupting memory. But this is probably a rare occurrence, so chances
are that you have gotten away with it, even if you don't do proper
interlocking.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: b...@softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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