4 Mar
2023
4 Mar
'23
7:57 a.m.
From: Douglas McIlroy
Typo, in v3 through v6 ...
26^3 16-bit trigram counts didn't fit in the PDP-11 memory
Being mildly curious, I fed '26 3 ^p' into 'dc' to see just how big it
was -
and got "17576", a 16-bit word array of which would fit into a PDP-11 64KB
address space.
I think the answer is in the first line - V3 didn't use the PDP-11 memory
management, so the kernel _and_ the application had to fit into 56KB. So
there may well have not been 36KB available to hold a 26^3 array of 16-bit
words.
The other possible explanation is that it was perfectly possible to run UNIXes
of that era (V4 on) on machines without enough main memory to hold the kernel
and a 'full-sized' process simultaneously. (Our original machine, an -11/40,
started out without a lot of memory; I don't recall exactly how much, though.
It had, I'm pretty sure, 3 banks of core; I was thinking it was 3 MM11-L core
units, which would be 3x16KB, or only 48KB, but my memory must be wrong;
that's not really enough.)
Noel