> Then how is the PROM exec able to talk with a.b. devices?

Byte read/write on a certain register.  Packet queueing, assembling
and interpretation has to be done entirely by software.

> Are the same
> instructions that Ultrix uses embedded in the EEPROM?

I guess so.

> I find it hard to
> believe that DEC would have gone through all the trouble of redesigning
> devices, hardware, and OS all for i2c/a.b. if a.b. were so difficult and
> troublesome to use. (Even they couldn't get it to work right all the time
> under Ultrix 4.3)

They seem realized the same conclusion, abandoned ACCESS.bus, and used
standard serial chip (a pair of Zilog) for DEC3000 just as VAXstations
and older DECstations.  They wanted ACCESS.bus competitive to
Macintosh ADB, HP HIL or other marketting pressures at that time.

Tohru Nishimura