[pups] Interesting PDP/Xenix History

Frank Wortner frank at wortner.com
Tue Mar 26 01:18:28 AEST 2002

> I don't understand how any paging system could simulate split I/D space?
> Do you remember any details?

Sure.  (Perhaps I used an incorrect term,  but "paging" seemed reasonable.)
The executing image consisted of 3 parts:  2 8K segments of instructions,
and 1 48K segment of data/stack.  One of the 8K instruction segments was
always resident in core and contained some program code and sort of jump
table.  As the program executed,  it used the jump table to map appropriate
8K sections of code into the other 8K instruction segment.  The 48K data
segment was always resident in memory.  The net effect was a slow pseudo
split I&D area with 56K instruction space and 48K data space.  For the most
part,  this was adequate to run system utilities.

The link edit scheme was fairly complicated.  You first had to build two
executables:  a standard shared text executable,  and a standard split I&D
executable.  Then you had to run a special program which took those two
binaries as input and constructed the final image.  If any of your
subroutines couldn't fit into 8K,  you were out of luck.  Fortunately,  the
C compiler driver had a option to do this silliness automatically.

I remember being frustrated by the fact that Microsoft had,  for reasons
unknown,  forgotten to build /usr/bin/sort as one of these special binaries.
Instead,  it was a conventional split I&D executable.  I had to convince a
friend of mine whose institution had a source license to let me "borrow" the
source to sort so I could build an executable that ran on a PDP 11/23.

Robert Tillyard asked "Would SCO->Caldera have copies of this [PDP/11
XENIX]?"  I remember asking that same question on this very list several
years ago.  Warren's answer was essentially "No."  If I remember correctly,
he said that they really had no archival material at all.

I don't remember what happened to the copy I worked with.  It probably
vanished into a dumpster when the company I worked for went "belly up."

If anyone *does* still have a copy,  it would be great to see it in the PUPS
Archives,  since it is probably the "rarest" PDP/11 Unix variant.

"They haven't got Brains,  any of them,  only grey fluff that's blown into
their heads by mistake,  and they don't Think."
* Eeyore,  "The House at Pooh Corner"

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