Re: NetBSD on SIMH, but not microvax3900

On 07/03/2024 21:14, Paul Koning wrote:
> There's also the question of what the memory bus on each VAX implementation can handle.  I forgot the 780 answer but chances are its address bus width is not nearly enough to support 512 MB.
>
> 	paul

Apologies if this is a repeat but I got a bounce from the list. I had 
written:
[

I have 64MB as the maximum memory for the VAX-11/780, but I didn't 
record where I found that, so I have no idea how reliable it is. 
Wikipedia has 128MB and they've referenced it, but it's basically the 
SIMH documentation so it certainly doesn't mean that you could ever 
configure that much memory on a system. The MS780-E/F memory subsystem 
brought the VAX-11/780 up to 16MB (or maybe 32MB if you could fit two of 
them in there). There was a later MS780-H and the FMPS suggests that it 
could control 40MB, so you'd think that you could get to 80MB that way. 
But the 1986 VAX Hardware Handbook says that with the MS780-H you could 
only get to 64MB. It explicitly says that the MS780-E controlled up to 
16MB of memory and that you could fit a second MS780-E to get up to 
32MB. There's no mention of a second MS780-H.

To answer the "what could I have physically configured using DEC parts 
in 1983/4" I think you'll need a  Systems & Options Catalogue from that era.

]

But after sending that I read a bit further in the Hardware Handbook and 
found:

"The 28-bit SBI longword address field allows access to up to 512 Mbytes of
main memory. The hardware supports a maximum of 64 Mbytes of main
memory. Physical memory operations are performed when bit 27 of the SBI
addresses is zero; 1/0 operations occur when bit 27 is one.

So SBI could do 256MB of which half was for I/O."

So there seems to be something that caps physical memory at 64MB (plus 
perhaps a bit for multiport memory, but that was always just a handful 
of MB).

Antonio



-- 
Antonio Carlini
[email protected]