[TUHS] Memory on Lion's v6

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue Mar 1 10:44:22 AEST 2022

    > From: Will Senn

    > Does anybody know how much memory was configured on the PDP-11 that
    > Lion's used for the commentary system. Here's what the book says about
    > the system:
    > ..
    > ; PDP11/40 processor;
    > ...
    > It seems like he must have had an MMU

V6 absolutely requires an MMU; the need for it is all throughout basic
attributes of the system - e.g. user processes start their address space at 0.

(BTW, there are V6 descendants, MINI-UNIX:


and LSX, which don't use/need an MMU, and run on -11 models without memory
managament, such as -11/05's, but I don't think they were in wide use outside

    > and 128k memory

The -11/40, as originally released, only supported the MM11-L, which came in
multiples of 16KB (for a 3-board set). Use of the later MM11-U (32KB units)
required a new main power harness, which only came in on
higher-serial-numbered -11/40's.

The -11/40 (as it was at first) that I had at LCS had, to start with, I'm
pretty sure, 3 MM11-L units (i.e. one MM11-L backplane full) - i.e. 48KB. I
know this sounds incredible, and I'm having a hard time believing it myself,
wondering if my memory is failing with age; but it definitely had
extraordinarily little.

I just looked on my V6 (running in a simulator), and it appears that by
trimming all parameters (e.g. number of disk buffers) to the absolute bone,
the kernel could be trimmed to about 36KB. (I haven't actually tried it,
since I don't feel like recompiling all the kernel modules, but one can
estimate it - take the current system size [44KB], delete 10 buffers @ .5KB
gives 39KB, etc, etc.)

That would allow a maximum user process of 12KB on a 48KB machine - and
MINI-UNIX, which runs basically stock V6 user code, can manage with user
processes that small.

I see Andrew's email which reports that the Lions machine had more main
memory, 128KB (maybe 4 MM11-U's - two MM11-U backplanes full); that 
woould have made their life a lot easier.


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