[TUHS] Memory on Lion's v6

Warner Losh imp at bsdimp.com
Wed Mar 2 12:15:43 AEST 2022

On Tue, Mar 1, 2022, 6:23 PM Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:

>     > From: Andrew Hume
>     > the actual configuration of Lions; PDP 11/40 was
>     >        128 Kbytes of core memory
>     > ...
>     > but note that because ... of addressing weirdness (the top 8KB were
>     > memory-mapped to I/O registers), Lions' PDP actually had 112KB of
> main
>     > memory
> I think that '112KB' must be an error; the 8KB for the 'I/O page' (as DEC
> eventually named ir, long after the rest of the world had started using the
> term :-) were deducted from the _UNIBUS_ address space, meaning a UNIBUS
> -11
> (the 'pure' UNIBUS -11's, i.e. other than the -11/70, -11/44, etc) could
> have
> a maximum of 248KB of main memory (which is on the UNIBUS).
> A pure UNIBUS -11 with 128KB of main memory (like Lions') has... 128KB of
> main memory. The 'small memory management model' -11's (like the /40, /60,
> /23, etc) can use at most 64KB of that _at any moment in time_ for user
> processes (i.e. directly accessible by the CPU, in 'user' mode).
> (The kernel on such machines is basically retricted to 56KB at any moment
> in
> time, since one 'segment/page' - the terminology changed over time - has
> to be
> dedicated to the I/O page: the memory management control registers are in
> that, so once the CPU can no longer 'see' them, it's stuck. Long,
> potentially
> interesting digression about, and ways to semi-work around that, elided,
> unless people want to hear it.)
>     > From: Noel Chiappa
>     > 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. 48KB. I know this sounds
>     > incredible, and I'm having a hard time believing it myself, wondering
>     > if my memory is failing with age
> It is:
>   # size /lib/c0
>   13440+2728+10390=26558 (63676)
> ('c1' takes 14848+6950+2088=23886, FWIW.) So 'my' -11/40 must have had more
> than 48KB.
> MINI-UNIX provides, on an -11/05 type machine with the maximum of 56KB of
> addressable main memory (if you plugged in 64KB worth, the /05 CPU couldn't
> 'see' the top 8KB of that), up to 32KB for a user process. So that will
> just hold the stock V6 C compiler.

You made a comment that MINI-UNIX wasn't available outside of Bell... I
meant to say that the AUUG newsletters talk about it. It features a letter
asking for users of it to share patches. There was also an article about
how to get it, though it was an offer to spin a tape for a photocopy of you
Western Electric license...


I'm not now sure how much memory my -11 _did_ have initially, but it's not
> important.
>         Noel
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