The Strange Birth and Long Life of Unix - IEEE Spectrum

[Scott Allen]

On 6 December 2011 16:23, James Knott <james.knott-bJEeYj9oJeDQT0dZR+AlfA at public.gmane.org> wrote:
> Other computers had separate I/O instructions and signals on the bus.  Since
> the PDP-11 didn't have those, it had to use memory mapped I/O.  However, you
> could generally use memory mapped I/O on any CPU.

True; unless there are timing or caching problems, memory mapped I/O
just appears as regular memory to a CPU.

When I designed an I/O board for my 8080 based system (back around
1978) I made all the I/O registers both I/O and memory mapped at the
same time. In some cases, you could write more efficient assembler if
you could choose between using a memory or I/O instruction to access a
register.

-- 
Scott
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