PDP 11 RL11/01 Address confusion.

[Johnny Billquist]

"Tim  Shoppa" <tshoppa at wmata.com> skrev:

> Johnny writes:
> 
>> The PDP-11 CPU is a 16-bit machine, so all code will always just deal 
>> with 16-bit values.
>>
>> The PDP-11 can have different buses, with different behaviours, but 
>> basically we can divide them into three groups.
>> 16-bit bus
>> 18-bit bus
>> 22-bit bus
>>
>> Machines with a simple 16-bit bus is almost nonexistant, but there was 
>> some early Q-bus CPUs with a pure 16-bit address range, I seem to remember.
>>
>> 18-bit means all Unibus machines, as well as some Q-bus machines.
>>
>> 22-bit means most Q-bus machines.
> 
> Obviously written from a 11M+ user's perspective. Ignore all those puny Unibus and Q-bus machines without MMU, will you? But they still run RT-11 just fine!

Guilty as charged on the 11M+ part. :-)
However, I think I covered machines without MMUs as well, I just didn't spell it 
out.
Without MMUs are the same as machines with the MMU disabled.
The one thing that differs is who puts out the extra bits on the address bus if 
there are more than 16 address bits, when you don't have an MMU.
The CPU do, in those machines.

Oh well. And I thought I'd done a real proper job. :-)

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
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