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

[Johnny Billquist]

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.
Also, the CPU is not exactly just sitting on the Unibus either. The CPU 
generates a 28-bit physical address through the MMU (as do all VAXen, 
except some NVAX CPUs which can generate a 34-bit physical address). The 
18-bit Unibus address space is then mapped into some part of this 28-bit 
physical address space. And the actual memory is mapped into some other 
part of this physical address space.

It's really simple. The Unibus is just an I/O bus for the CPU. The CPU 
don't sit straight on the Unibus.

>> 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).
>
>     It's a matter of how big the "map" is.  Drivers had to break up
>     large transfers, so I assume the "map" was only one entry (more
>     like an APR than a map).  Or maybe it was just a few, I forget
>     the upper limit, but I recall it was only for the 11/725.

Unfortunately the 11/725 documents don't seem to be on bitsavers, so I 
can't read up on this. But it do sound weird. The 11/730 atleast have a 
full Unibus map, which covers the full 256K.

	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