[TUHS] Research UNIX PDP 11/45

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue Jul 18 10:49:46 AEST 2023

    > From: Henry Bent

    > there will be a lengthy addendum shortly.

The most useful thing is probably this:


which lists exactly what was there; not only the types, but how many of each
there are. This is from 'nsys', which is slightly before the actual V4, so
it's quite early. 'low.s' is inherently machine-specific; i.e. different
machines would share most kernel files identically, but _not_ this one -
unless they had _absolutely identical_ device sets. So this one is _probably_
the one from the /45 in picture.

It shows:


  1xDP11	(synchronous serial)
  1xDN11	(dial-out asynch control)

  1xDR11C	(parallel port to -11/20)
  2xDC11	(Screw Works voice synthesizer)
  1xDR11A	(voice response unit)
  1xDR11C	(C/A/T typesetter)

(Line printer, card reader and RP11 are commented out; more about the RP11
in a later message.

There's also this:


which is a bit hard to interpret, but I think might list what's in each rack:
the TC11, RK11 (early ones), RF11 and TM11 (early ones) were large custom
wire-wrapped backplanes which bolted into the front or back of a 19 inch
rack; this:


has an image of such an RK11. The "MOS 16-24" is probably a reference to an


which had to mount in the CPU backplane. The "MM" entries are likely core
memory units; probably MM11-K's:


since they seem to be 4KW each. (Maybe MM11-E's or 'F's, though; those are
also 4KW each.) I'm not sure what they "PL"s are - probably Plessey core?
Anyway,it looks like the machine had 104KB total.

This file:


lists all the types of devices on the machine. One oddity is that it lists
two RK11's; but if you look at the RK11 driver:


it's only set up to handle one physical controller. But there is this:

  #define	JRK	1	/* temp */

	if (bp->b_dev.d_major==JRK)
		d = bp->b_dev.d_minor;
		d = bp->b_blkno%3;

so the two different major device entries appear to handle the same disks in
different ways ("d = bp->b_blkno%3" will spread a virtual drive across three
physical drives).

Memory, it would have been hard to say (UNIX even then sized memory at start
up) but then I found that '11-45' file. I also found a copy of the CACM
version of the UNIX paper:


which says the machine had 144KB (so they had added 40KB more at that point).
(I seem to recall someone had scanned the SOSP version; I didn't save the
pointer, but if someone knows where it is, it would be interesting to look,
and see what it says - they seemed to update this paper on a regular basis -
the copy included with V6 talks about the -11/70.)

The system at that point had "a 1M byte fixed-head disk .. four moving-head
disk drives which each provide 2.5M bytes on removable disk cartridges, and
a single moving-head disk drive which uses removable 40M byte disk packs"

The RS11 disks for the RF11 were 512KB, so either they'd added a second one,
or switched to an RS04 (but that's a MASSBUS device). The big disk was an
RP03 so they had added an RP11, which wasn't present earlier.


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