[Simh] PDP11 reset

[Bob Supnik]

With help from Tim Litt, I think we've arrived at the right answer:

1. cpu_reset should CLEAR PS for all processors EXCEPT T11.

2. boot routines should jam PS = 340, which is what is expected today.

#1 is based on an examination of specs, manuals, and schematics for all 
the PDP11 CPUs. In particular, all TTL systems wire the flops/registers 
that implement PS<7:5> to clear on processor init, which is triggered by 
the START switch. The LSI11, F11, J11 clear PS on microODT GO 
(equivalent of hitting the front panel START switch). The T11 sets PS = 
340 on init.

#2 is based on an examination of the official boot ROM listings. While 
there may be paths through the boot ROMs that don't set PS = 340, the 
primary path does that, and it is known to work with all the existing 
boot ROMs in the simulator.

The PDP11 cpu will implement a new routine:

cpu_set_boot_pc_pw (int32 pc)

It will set saved_PC to the argument and set PS = 340. 16 boot routines 
will need to be modified (gulp).

/Bob