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Re: QBUS Prototyping

Cross-posted to 3 groups.

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On 2011-04-18 06:00, Bob Koehler wrote:
> In article<FgC+HafqKJY6@eisner.encompasserve.org>, koehler@eisner.nospam.encompasserve.org (Bob Koehler) writes:
>>
>>     I never had a PDP-11 with less than 18 bit address space.
>>     Instructions could only access 16 bits of it at a time, but the
>>     hardware was there to make use of the rest of it.
>
>     Maybe you didn't, but many of us did.  And on PDP-11 with 18
>     bit UNIBUS, we had some systems which were limitted to 16 bit RAM.
>     (By design, not by purchase).
>
>     No APRs, no need for them.

The Unibus is always 18 address bits. A PDP-11 without MMU can only use 
16 bits of addressing.

All PDP-11s start running with MMU turned off, which means that you only 
have 16-bit address space. When you turn on the MMU you get 18 bit, or 
22 bit, depending on model, and how you set up the MMU.

The same is true for both Unibus and Q-bus. Although at 22-bit mode, the 
Unibus and Q-bus machines (obviously) differ, since Q-bus have 22 
address bits available, while the Unibus don't.

That's why Unibus machines that support 22-bit physical addressing have 
a Unibus map. (The CPU can access all 22 bits of memory, but Unibus DMA 
devices can not.)

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt@softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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Re: QBUS Prototyping koehler@eisner.nospam.encompasserve.org (Bob Koehler) - 2011-04-18 08:00 -0500
  Re: QBUS Prototyping Johnny Billquist <bqt@softjar.se> - 2011-04-18 08:06 -0700

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