[Simh] Older Model VAX simulators, what can you do with them, and how

[Bob Supnik]

Another issue with the 750, 730, and 8600 is that they are not as exact 
at the 780 and CVAX simulators, because certain critical documents 
(microcode, some subsystem specifications, some schematics) are not 
available. For example, the 750's simulated Massbus and Unibus adapters 
started out as clones of the 780s and have received only minimal 
modifications. Both simulated versions contain <too much> logic and 
error checking; the 750 was done with 400 gate TTL gate arrays, and 
gates were precious. VMS is not sensitive to the remaining differences; 
other operating environments might be. (For example, the 750 didn't 
check that register accesses were longword and aligned, and this was 
used in both the 750 boot ROMs and the diagnostic supervisor.)

For the 750, the UBA750 spec is online (although the register 
descriptions in it are obscure). The RH750 register secs can be inferred 
from the Emulex workalike, the SC750.

/Bob