PDP 11 RL11/01 Address confusion. - test-drb@ccmp.vtda.org

2 Apr 2008

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.