The Strange Birth and Long Life of Unix - IEEE Spectrum

[James Knott]

D. Hugh Redelmeier wrote:
> This is an interesting example of devices being addressed as if they
> were memory, a noteable feature of the PDP-11.

Actually, most CPUs can use memory mapped I/O.  DEC had the "feature" of 
not having any separate I/O instructions, which meant that I/O, not 
having it's own address space, occupied addresses that might otherwise 
be used for memory.  I believe the Motorola mircroprocessors also did that.

BTW, anyone else here remember the Data General I/O, where you could 
branch on busy & done flags?

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