Mike,
You should get yourself a good intro to the PDP-11, for example one of the
PDP-11 Processor Handbook copies you can find on Bitsavers. It answers that
question and many others.
The key point is that you have to distinguish virtual addresses from physical
ones. The I/O device addresses are physical addresses. The program uses
virtual addresses. Physical addresses can be 16, 18, or 22 bits depending on
the model; virtual addresses are always 16 bits.
The power on default mapping is that physical addresses 0-157777 map to
physical addresses with leading zeroes; virtual addresses 160000 and up map to
physical adddresses with leading ones. So, for example,
mov #101, @#177566
moves the value 0101 (decimal 65, the ASCII code for "A") to physical address
17 777 566 which should result in A appearing on the console output.
On machines with an MMU (which is most of them) the mapping from virtual to
physical can be changed, and for example the "I/O page" (top 4kW of physical
memory) might not be accessible. But by universal software convention, in
kernel mode 160000 and up (MMU page 7) are always mapped to the physical I/O
page address range.
paul
> On Feb 16, 2018, at 10:05 AM, Mike Stramba <[email protected]> wrote:
>
> Greetings,
>
> I'm trying to figure out how console I/O works with the PD11.
>
> Or more generally I/O addressing
>
> E.g.
> A "sh TTO" gives :
>
> TTO, address=17777564-17777567, vector=64, 7p
>
> And 17777564 (octal) is 4,194,164 dec, which is obviously out of range
> of the 64K addressable by any MOV instruction.
>
> But various diagnostic routines, e.g. below, are using
> movb rX,@#177566 ; output to console
>
> https://www.ak6dn.com/PDP-11/M9312/23-248F1/23-248F1.mac
>
> Are I/O devices always at the "top 64k" of memory ?
>
> If so, why doesn't SH DEV display :
>
> xxx xxx-177 777 ?
>
> Mike
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