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

[Johnny Billquist]

On 2011-05-04 10:22, Bob Koehler wrote:
> In article<ips1gq$7i2$1 at Iltempo.Update.UU.SE>, Johnny Billquist<bqt at softjar.se>  writes:
>> On 2011-05-04 06:32, Bob Koehler wrote:
>>> In article<ipqpfi$qvl$1 at Iltempo.Update.UU.SE>, Johnny Billquist<bqt at softjar.se>   writes:
>>>>
>>>> Except then of course it isn't the system bus, but just an I/O bus... :-)
>>>
>>>      If everything except the CPU-memory connection is on the same
>>>      bus, and CPU and memory are also on that bus, then which is the
>>>      system bus?
>>
>> Uh? The memory is not on that bus, or even near it. I thought we had
>> just established that.
>
>     If the memory was not on the UNIBUS, then I/O peripherals could not
>     do DMA.  There might have been some PDP somewhere that I haven't
>     looked at that couldn't do DMA, but all VAXen certainly can.

That's what the Unibus Map is there for. The Unibus map sits between the 
Unibus and the memory, and remaps DMA from the Unibus into the larger 
address space of the memory bus. Same both on the 22-bit capable PDP-11s 
and all VAXen. The exact details of the Unibus map differs between a 
PDP-11 and a VAX, though, but the principles are exactly the same.

	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