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

[Johnny Billquist]

On 2011-05-05 07:02, Bob Koehler wrote:
> In article<ipt5am$d6v$3 at Iltempo.Update.UU.SE>, Johnny Billquist<bqt at softjar.se>  writes:
>>
>> 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.
>
>     Too me, that map is part of the memory subsystem.

The Unibus map is the very important piece that maps Unibus addresses to 
memory bus addresses for DMA. As such, is it a part of the memory 
system, or the Unibus? It sits on both...

Either way, it is absolutely necessary in order to have DMA from the 
Unibus work on a machine where the memory does *not* sit on the Unibus.

On lowly PDP-11 models, where the CPU only address 18 bits (or less), 
the memory really sits on the Unibus, and DMA can be performed directly 
there, and memory is responding to Unibus signals. This is (obviously) 
very different from when you have machines with a Unibus map.

Thus, on any VAX, the memory does not sit on the Unibus, and the Unibus 
is only used as an I/O bus. As someone else mentioned, the MicroVAX I is 
the only design where you really only have one bus, the Q-bus in that 
case. The uVAX I therefor is limited to the physical address space of 
the Q-bus (which is 22 bits), and DMA can be performed directly between 
peripherials and memory, which both sits on the same bus.

A short additional note. On PDP-11s the Unibus map is located in 
different places depending on model. For the 11/44 it's in the MMU. The 
11/70 have it along the cache. The 11/84 and 11/94 have the Unibus map 
on the Unibus adapter which is a separate card on those machines.
For all of them, DMA from the Unibus "speaks" (that is, do all the 
Unibus handshaking and signalling) with whatever have the Unibus map. 
And the Unibus map in turn then "speaks" with the actual memory.
When the CPU speaks to memory, the Unibus is totally unaware, and 
remains quiescent.
(For a machine like the 11/70, and VAXen, when devices not on that 
Unibus speaks to memory, the Unibus is also quiescent.)

So no, the memory is *not* on the Unibus. Memory can exist on the 
Unibus, and do so for 18-bit PDP-11 models, and that is obviously very 
different.

	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