PDP 11 RL11/01 Address confusion.

[Tim Shoppa]

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!

Lots of Unibus and Q-bus and not-anything-bus-11's all do quite fine with 16 bit address spaces.

While the Unibus electrically has 18 address bits from the get-go, the Unibus 11's without memory management (or without the memory management option) don't use the last two. Even "later" Unibus CPU's like the 11/04 still came without MMU (and it was not an option, either!)

Obviously "early" and "later" are taken to be Lasnerian.

Tim.