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

[Johnny Billquist]

On 2011-05-03 06:13, Bob Koehler wrote:
> In article<ipo238$id3$1 at Iltempo.Update.UU.SE>, Johnny Billquist<bqt at softjar.se>  writes:
>>
>> What??? Are you sure? The Unibus only have 18 address lines, which means
>> 256K addressable. And some of that is needed for I/O. I would be very
>> surprised if memory really sat on the Unibus.
>
>     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.

The 11/44 were not really that "true". It had Memory on a private 
extension bus, and it did not sit on the Unibus. That's how it could 
have a max of 4 MB.
The 11/34 have a limit of 248 KB, with a 8 KB I/O page.

>     With virtual memory, who needs more than 265KB?  (These systems
>     were not meant to be fast, real word processing doesn't need
>     much.)

Not entirely true. Most the kernel needs to be in physical memory (even 
if it is in virtual address space). It's just user processes that use 
demand paging. (Virtual memory itself only means you are remapping one 
set of addresses to another set, it does not reduce the memory 
requirements at all, that's what the demand paging does for you.)

I'm pretty sure the physical memory limit was more than 248K of memory. 
I doubt many version of VMS can even run with that little physical memory.

	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