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[Johnny Billquist]

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