[Info-vax] RB730 Integrated Disk Controller (R80/RL02) usable with VAX-11/750?

[Johnny Billquist]

On 2011-05-03 10:34, Bob Koehler wrote:
> In article<slrnis00n9.m6.rivie at stench.no.domain>, Roger Ivie<rivie at ridgenet.net>  writes:
>> On 2011-05-03, Bob Koehler<koehler at eisner.nospam.encompasserve.org>  wrote:
>>>
>>>     IIRC, the 730 and 725 were true UNIBUS systems, using the UNIBUS
>>>     as the system bus, with all its limitations, just like my PDP-11/34
>>>     and 11/44.
>>
>> It's been a *long* time since I've used a /730, but I'm certain it had a
>> UNIBUS map. I think the one I used to use had a couple megs of RAM.
>
>     Yes, you can find a discussion of LARGO on the 'net, an 11/730 with
>     3 1MB memory modules.  Just because it's an 18 bit system bus doesn't
>     mean the system is limited to 18 bits, you just have to get
>     inventive.

Except then of course it isn't the system bus, but just an I/O bus... :-)

>> AFAIK, the only VAX without a UNIBUS map (or its equivalent) was
>> MicroVAX I, which used QBUS memory and was therefore limited to 4MB.
>
>     The 11/725 wasn't just a stripped down 730.  It's map would not
>     automatically update at page boundaries, so drivers had to break
>     down large transfers and update the map between pages.  (There's
>     no way to be sure that successive pages in virtual memory are
>     in the same part of the map in physical space.)

I'm not even sure I understand that comment.
The Unibus map is a way to map the Unibus 18-bit address into a 22-bit 
address (for large PDP-11s) or 32-bit address (for VAXen). You normally 
never updated in during DMA transfers. You set up the whole mapping for 
your DMA before starting a transfer, and the scatter/gather behavior of 
the Unibus map was passively there all the time.
Exactly how you split up the 18-bit Unibus address decides how large 
your Unibus map is. I don't remember for the on the VAX, but I suspect 
each "page" is 512 bytes (since the VAX loves that size in all weather), 
which means 512 registers in the map. A DMA runs through a range of 
addresses, and might touch one or several map registers.

The map itself is static, and is not updated at page boundaries, or on 
any other conditions either, except for programmatical writes to the 
map, which are done prior to starting a transfer. So, what do the quoted 
paragraph above actually mean?

	Johnny

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