Several comments on crashed drive in PDP-11/23

der Mouse mouse at Rodents.Montreal.QC.CA
Sun Jun 19 16:03:28 CDT 2005

>>> What is the "modern way" to get software into the PDP11?
>> Phew.  Tape normally.  Small, modern PDP11s usually have TK50, and
>> software is distributed on TK50.
> TK50...???

It's a DEC tape.  The tapes are square boxy things, about 105x105x25mm,
and the drives can be cantankerous.  (In particular, the mechanism that
picks up the tape and gets it started in the drive tends to stop
working; it's a plastic leader that hooks into a hole in the tape, and
the drive firmware is too stupid to recover from failure to hook in any
way but "try again".  If the tape hole isn't in quite the right place,
the pickup plastic flaps around again and again and wrecks itself if
you don't catch it fairly soon.)

> And a simple question: How does the addressing in pdp11 work?  R7
> seems to be only 16 bit wide.

It is.

> And when I boot up the system, the MMU should be off and the memory
> mapping should be direct.  How do I access the upper bits of my PC
> (i.e. "changing the segment")?

You don't.  There are no "upper bits" to the PC (nor any of the other
registers); the PDP-11 is not a segmented architecture in the sense the
8086 is.

With the MMU off, you can access only 64K (I think - I/D splitting
happens only with the MMU on, as I recall, even on machines that have
split I and D space).  On most machines, that's 56K of RAM and 8K of
I/O space.

With the MMU on, the 17 bits of address coming from the CPU (16 bits
that are usually thought of as address plus the I/D space bit) are
mapped to however many your hardware supports; you can access anything
you have, but you're restricted to 64K of instruction space and 64K of
data space at any given moment.  (Some implementations may not actually
have all these lines present in hardware; this is a conceptual
description.  In particular, modesl without split I and D space don't
implement the I/D space bit.  And there may be other bits I've
forgotten about.)

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