On Sat, 30 Apr 2005, Eric Smith wrote:

> Johnny wrote:
> > Uh, the 11/780 and 11/785 is the same design.

>
> There's only slightly more truth to that claim than there would be to
> a claim that the 11/45 and 11/70 are the same design.


Well, the 11/45 and 11/70 are very similar. :-)

> > Neither loaded the microcode from disk.

>
> The 11/785 most certainly did.


You're right. I should learn to check before writing.

> The JCS (Joint Control Store) module (M7475) has 512x104 bits of PROM (13
> 512x8 PROMs) for control store, and 4Kx99 bits of RAM (99 2147 chips, 4Kx1
> each). The microword is 96 bits plus 3 bits of parity. The PROM supplies
> a "spare" bit, as well as 4 extra bits that are completely unused.
>
> The PROM control store to help the machine boot; 512 words isn't enough
> to make it a VAX. AFAIK once the real microcode is loaded, the PROM
> control store isn't used.


Pulled out my VAX Hardware Handbook (I know, should have done it from the
start), and the 11/785 actually have 0.5 Kword of ROM and 7.5 Kword of
RAM. (99 bit words). The actual hardware have some extra bits, which I'm
sure is what you're mentioning above.

> > Heck, they only have one RX01 drive.

>
> You're claiming that a 256 Kbyte RX01 floppy disk doesn't have
> sufficient capacity to store 48 Kbytes of microcode?


Well, I would have thought the microcode was bigger. After all, I checked
the size of the microcode in the 8600, and went from there. Admittedly,
it's a more complex implementation, but appearantly it's much more complex
compared to the 11/785 than I thought.
(Interestingly my book says the 8600 have 8 Kword or 86 bit words, which
is odd considering how large the microcode file is... I wonder what kind
of format that file is in...?)

> > They might have had some patch ram in the same way as the 11/750. Not sure
> > about that one...

>
> The 11/780 does. The 11/785 doesn't need it, since the entire main
> microcode store is RAM.


True.

I stand corrected.

Johnny

Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt@update.uu.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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