Um.. in the case of the 750, at least, this isn't strictly speaking
true. 
The 750 will run quite happily from just the rom microcode. The 750
started
out as a rom-only machine, with the L0005 CCS control store module. 
Subsequently, all 750s should have been field-upgraded to use the L0008 
Patchable Control Store, which provides 1K * 80 bits of ram, along with 
a one-bit wide 'map', which enables particular locations to be fetched
from ram instead of rom, so that bugs in the rom can be fixed by loading 
patch microcode into the PCS. 

For the L0005, there was an add-on daughter board which contained a 1K
WCS. For the L0008, there were three variants of the board: -YA was the
standard PCS, with just the 1K of patch ram. -YB had an additional 1K
of writable control store, and was equivalent to the L0005 with the 
daughter-board. -YC, which as far as I know was never manufactured,
had another 1K of WCS. If you're handy with a soldering iron, and
you have a supply of the necessary fast static rams, you can upgrade
a standard YA to a YB or YC. (The main problem is the fact that the 
board is flow soldered, so all the unoccupied chip positions are filled
with solder.)

All the ram banks on the PCS/WCS are accessible in the top 2megs of the
physical address space, and it's relatively easy (on 4.3BSD, at least
:-))
to write a driver to access it. On the other hand, there isn't much
point,
as there doesn't seem to be a microcoder's manual for the 750 (if
anybody's
got one, I would just love to buy a photocopy). 

James

allisonp@world.std.com wrote:
> 
> > > The 700 series in general use writeable ucode stores (780, 782, 785,
> > > 750 and 730 to be sure I may have missed a few).
> >
> > The question wasn't whether the 780 had WCS, it was whether the 785 had
> > it.
> 
> Your being oblique.  All of them have it.  The ucode in prom was only the
> core instructions and hardly complete.
> 
> > The 725/730 and 750 use writable control store but do not support
> > user-written microcode.
> 
> Define support.  If you can get it on the tape from the system (easy)
> you can at BOOT time input that to the WCS that is there.  Its not BUS
> accessable as a realtime option however.
> 
> it wasn't widely done as the ucode was not trivial and the data paths were
> anything but simple.  But Guys like UCSD, LLL, fermi and friends did that
> as it bought them enough for theirt fancy work.
> 
> Allison