[Simh] UC15 development is done (for now)

[Bob Supnik]

At long last, I've finished testing the PDP15/UC15 combination, and it 
works well enough to run a full XVM/DOS-15 sysgen. I've sent an "RC1" 
package to Mark fP. or trial integration.

The configuration consists of two separate simulators - the PDP15 and a 
stripped down PDP11 called the UC15. It uses the "shared memory" 
facility that Mark P. created for both the shared memory between the 
PDP15 and the PDP11 and the control link state. Getting decent 
performance requires multiple cores and tight polling, so this initial 
implementation has a number of restrictions:

- Windows and Linux only.
- The host system must have at least two cores or processors.
- The host system needs wall power, because the two simulated processes 
run flat out, without idling.

There are a couple of clear areas for future development:

- Implementation of the 'shmem' simulator library on other host 
platforms (Mac, VMS).
- Augmentation of the shared memory capability with some sort of 
directed interrupt, so that the two simulator processes can sleep/idle 
when nothing is happening. (Specifically, the simulator needs a 'sleep 
until <a> or <b>' capability, where the conditions are expiration of the 
sleep timer or receipt of a signal from another process.)
- Identification of the idle loop on the PDP15 side (the PDP11 sits at a 
WAIT instruction).

Solving the idling problem will help make this configuration 'mobile 
friendly,' which its certainly is not at the moment.

When 3.10 is ready to go, I'll post it on the SimH web site, along with 
a design paper about the UC15 configuration, and additional instructions 
for running XVM/DOS-15 in a UC15 configuration.

/Bob Supnik