[Simh] RA70/73 simh support

[Bob Armstrong]

>Tim Shoppa wrote
>>  I'm afraid I don't actually know what "RCTS" stands for ....
>
>Replacement Caching TableS I think.

  I realized as soon as I posted that it was a foolish question; we were
just talking about the RCTs after all.  For simh, using the RA70 data I just
posted, I assume that

	#define RA70_RCTS		198	/* [RLA] 198 sectors per RCT
*/
	#define RA70_RCTC		7	/* [RLA] and 7 copies of the
RCT per drive */

  It's funny, though - the other RA series drives all seem to have RCTC=1 in
simh; surely this can't be right?

  I still don't know what the XBNs and DBNs are, nor do I know the model
type byte for a RA70, but even with these all set to zero it appears to be
enough to fool 2.11bsd and RSTS.  Interestingly, RSTS is even able to
identify the drive as a RA70 even without the model type byte; I guess it
must be using the media ID to identify it instead.

  With those limitations and a few other minor changes the RA70 emulation
seems to work - how does one apply to the simh maintainers to incorporate
this addition back into the master source ?

Bob