Next project: 11/24. Does it need memory?

Noel Chiappa jnc at
Fri Oct 16 13:11:12 CDT 2020

    > From: Chris Zach

    > Next up is the 11/24, this is one of the 5.25 inch rack mounts with a
    > CPU, KT24, and no memory.
    > First question: Will ODT respond at all with no memory on the Unibus?

My _guess_ is probably, since on other KDF11 CPU's, ODT works with no
memory, but I haven't confirmed that.

My PDP-11/24 chassis (a 5-1/4" BA11-L, like yours) I took apart to get the
backplane out, so I could work out how the busses work (see below), and I'd
need to put it back together before I could try my -12/ confirm it. If
you could take some pictures of the insides of your BA11-L, that would be
both a help to the re-assemblhy, and ancentive to do so.

    > Can a 128kw memory board from an 11/34 work in an 11/24?

Yes and no. See:

for more. Note "The top 256 Kbytes of the CPU's address space were devoted to
the UNIBUS". So an MS11-L or similar UNIBUS memory can probably be
read/written by the CPU, but it won't appear at 0 in the CPU's address space.

    > Or do I need one of the 11/44 boards to make it go?

Whether a KDF11-U will work with no memory at physical 0 is unknown: it would
need at least a special bootstrap (since most assume working memory at 0) and
a special OS load (ditto).

Also, I seem to recall that on the -11/45, with mapping enabled, interrupt
vectors are in Kernel D space, so once running, that machine could operate
with no working memory at physical 0, but I don't know if KDF11 CPU's use
mapped Kernel space for vectors. If you're interested, I can run an
test on a KDF11-A and see.


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