From mboxrd@z Thu Jan  1 00:00:00 1970
From: wkt@tuhs.org (Warren Toomey)
Date: Wed, 7 Jan 2015 17:39:35 +1100
Subject: [TUHS] pdp11 UNIX memory allocation.
In-Reply-To: <alpine.BSF.2.11.1501071722340.58880@aneurin.horsfall.org>
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Message-ID: <20150107063935.GA27943@www.oztivo.net>

On Wed, Jan 07, 2015 at 05:29:45PM +1100, Dave Horsfall wrote:
> I heard a story that on sufficiently-early Unices, the header was indeed 
> loaded, hence the "407".
> Any grey-beards here like to comment?

My beard isn't grey (yet), but here's the link to the 1st Edition code
which does exec(2):

http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u2.s

and here is the relevant code:

	mov	$14,u.count
	mov	$u.off,u.fofp
	clr	u.off / set offset in file to be read to zero
	jsr	r0,readi / read in first six words of user's file, starting 
			 / at $core
	mov	sp,r5 / put users stack address in r5
	sub	$core+40.,r5 / subtract $core +40, from r5 (leaves
			     / number of words less 26 available for
			     / program in user core
	mov	r5,u.count /
	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
1:
	jsr	r0,readi / read in rest of file

My memory of PDP-11 assembly is too rusty to interpret this. Anybody else?

Cheers, Warren