[TUHS] Unix & Memory Management Units (MMU)

Paul Ruizendaal pnr at planet.nl
Fri Dec 9 05:44:13 AEST 2016

Finally took a look at the V1 source.

Referring to http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u2.s
Toward the end of the sysexec function there is:

	cmp	core,$405 / br .+14 is first instruction if file is
			  / standard a.out format
	bne	1f / branch, if not standard format
	mov	core+2,r5 / put 2nd word of users program in r5; number of
			  / bytes in program text
	sub	$14,r5 / subtract 12
	cmp	r5,u.count /
	bgt	1f / branch if r5 greater than u.count
	mov	r5,u.count
	jsr	r0,readi / read in rest of user's program text
	add	core+10,u.nread / add size of user data area to u.nread
	br	2f
	jsr	r0,readi / read in rest of file
	mov	u.nread,u.break / set users program break to end of 
				/ user code
	add	$core+14,u.break / plus data area
	jsr	r0,iclose / does nothing
	br	sysret3 / return to core image at $core

$core is equated to 040000 in another file (u0.s). In V1 apparently the a.out header was 6 words (http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/man/man5/a.out.5), not 8, and hence the magic for a standard executable was 0405. It was already used as magic to distinguish a.out format files from other executables. It also shows that indeed 'exec' jumped to the first word of the header (at location $core).

From this I don't think we will find code for the KS-11 in the V1 source. The file http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u3.s also seems to support a LSX /MX like approach where each process switch equates a swap.

I'd say that the lost V2 is where memory protection first appeared in Unix, i.e. 1972.


PS Sorry for writing 'V0' earlier -- I meant V1 all along.

More information about the TUHS mailing list