Running UNIX V6 on an -11/23

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It is possible to run UNIX Sixth Edition on a PDP-11/23, although this is not a configuration supported 'out of the box' with the Version 6 distribution.

There are three issues which have to be addressed before V6 will run on a /23:

All of them are fairly simple to deal with, but they require minor modifications to the system, so you will need a running V6 to do these; bringing it under a simulator, e.g. Installing UNIX Sixth Edition on Ersatz-11, is therefore required.

Use the 'm40.s' machine language file, as the memory management supported by the /23 is the same 'simple' kind (i.e. without split I+D) as that on the PDP-11/40.

Switch register

V6 expects to see a switch register, and refers to it during booting to see whether to come up single-user or not.

The best general approach is to provide a 'software switch register'; a location in low memory whose contents are used as the switch register.

The following can be used to do so:

  • insert four lines into l.s (in the appropriate location in the source):
. = 50^.
.globl  _SW
_SW:    memcsw
memcsw: 173030 
  • change the last line of param.h to read:
extern	int	*SW;
  • add the following three lines to main.c (last two go just after

the check for the LTC)

#define SWREG	0177570

if (fuiword(SWREG) != -1)
  • then recompile sys4.c and prf.c, plus the two files above.


V6 requires either a KW11-L or KW11-P clock, and will 'panic' unless one or the other is found. The -11/23 does have a 60Hz clock, but... there is no control register. There are two ways to deal with this:

  1. Install a BDV11 card, which has a simulation of the KW11-L on it, or
  2. Modify the OS

To do the latter, in main.c, change:

			panic("no clock");


			lks = 0;
			printf("No clock?\n");
			printf("On an 11/23, turn on clock now.\n");

and change the:

	*lks = 0115;


	if (lks != 0)
		*lks = 0115;

NOTE: It is critical to turn off the system's built-in clock (usually possible with a switch on the console) before and during booting, as a clock interrupt before UNIX is ready for it can trash the system (and has been observed to trash the disk, if you are really unlucky).

Larger main memory

All -11/23's after the (very rare) model A can support more than 256KB of main memory. If you put more than 256KB on an -11/23 and boot V6, it will ignore the extra memory above 256KB (technically, above 248KB).

That is because 22-bit operation must be explicitly enabled, using SSR3. So, make a copy of m40.s (m23.s seems the obvious name), and add the following:

	mov     $EN22BIT, *$SSR3	/ allow > 256KB

just before the

	inc	SSR0

You will also need to add:

SSR3	= 172516
EN22BIT	= 20

somewhere (probably at the end, with the other register definitions).

Finally, the PAR contents to reach the I/0 page are different in a 22-bit system from an 18-bit system, so change:

IO	= 7600


IO	= 177600

You might also want to change this line:

	printf("mem = %l\n", maxmem*5/16);

in main.c to this:

	printf("mem = %l\n", (maxmem/16)*5);

to prevent overflow issues in machines with a lot of memory.

Edit the 'run' file that builds the system to use m23.o instead of m40.o, and that should do it.