[HECnet] VMS/RSX Guest accounts --> 2020 Unibus(ses)

[R. Voorhorst]

L.S.

Well, look in Tops10 monitor sources and in the engineering stuff: multiple Unibus adapters (up to 4, standard 2 provided) each which its own mapping hardware and registers.
It can easily expanded to even handle 4 MW memory space (I run here some 2 MW variants), but the current Tops10 single section monitor cannot handle the needed page mapping in monitor mapping space very well.
1 MW works fine though. 1 MW variants surfaced somewhat later than the 512k ones.


R.V.

-----Original Message-----
From: owner-hecnet at Update.UU.SE [mailto:owner-hecnet at Update.UU.SE] On Behalf Of Johnny Billquist
Sent: Wednesday, 29 April, 2020 08:14
To: hecnet at Update.UU.SE
Subject: Re: [HECnet] VMS/RSX Guest accounts

I should point out that I actually suspect the 2020 have a Unibus map, just as the big PDP-11s or any VAXen have.
Which then remaps the Unibus address space to the larger address space of the machine, so that DMA can go anywhere.

   Johnny

On 2020-04-29 08:10, Johnny Billquist wrote:
> Nope. Unibus can only address 128Kword, and in this context that is 
> 128K of 18-bit words. So you cannot even DMA into a full 256K of 36 bit words.
> 
>    Johnny
> 
> On 2020-04-29 05:51, Thomas DeBellis wrote:
>> You know, I have been racking my brains from time to time and I can't 
>> remember a thing about the ADP modifications to the 2020.
>>
>> So, the 2020 came with a maximum of 512K words, 2**9.  An addition 
>> bit would have brought it up to a full megaword, 2**10, which is 
>> quite reasonable for the target audience (some kind of installation 
>> that didn't hold stock in the local power utility).
>>
>> I guess there may have been modifications to the 2020 build of the 
>> monitor to allow for the extra bit.  I don't know if the Unibus 
>> devices could do DMA into the full address space.
>>
>> Did you pick up any software with these little jewels?  The monitor 
>> changes might be interesting.
>>
>>> --------------------------------------------------------------------
>>> ---- On 4/24/20 12:12 AM, Dave McGuire wrote:
>>>    Nice!
>>>
>>>    One of our 2020s is in the brown color scheme.  You know what 
>>> that
>>> means: ADP, and one more address bit.
>>>> -------------------------------------------------------------------
>>>> -----
>>>>
>>>> On 4/23/20 6:15 PM, Thomas DeBellis wrote:
>>>>
>>>> The solution for 4.1 was one of the finest hacks I have ever heard 
>>>> of; while the 2020 doesn't support extended addressing, it does 
>>>> support multiple address spaces, so what they did was move all the 
>>>> symbols into a separate address space.  This was called 'hiding' 
>>>> symbols and I thought it was great because it made them harder to 
>>>> smash.  However, all of that went out the window with 5.0, which 
>>>> fully supported extended addressing.
> 

-- 
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