Anders Magnusson <ragge@ludd.luth.se> wrote:
> PROM control store? Isn't all microcode loaded from the RX01 floppy?

According to the VAX Hardware Handbook (not sure which year; I don't have
it handy), the 11/780 has 4K*99 of non-writable control store and 2K*99
of WDCS.  The KU780 option addes 2K*99 of UWCS (user-writable control
store).  I imagine the reasons for using PROM were that it was cheaper at
the time than fast static RAM, and that it required less support circuitry
since there didn't need to be a write data path (or, for that matter, a
readback data path).

I don't know how the 11/785's control store is structured, which is
why I asked.

A friend claims that he was told that the 11/785 was a good machine for
user-written microcode; if true that implies that UWCS was available, at
least as an option.

> Well, that should work in just the same way, but I think it will be hard
> for you to find any OS that supports it :-)

If I can get it, presumably the modified BSD 4.1 that ran on the dual 780
should run on a dual 785 as well.  AFAIK, there are no software-visible
differences between the 780 and 785; if I'm wrong, they are probably only
small differences that are easily accounted for.

Cheers,
Eric