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