From mboxrd@z Thu Jan  1 00:00:00 1970
From: jnc@mercury.lcs.mit.edu (Noel Chiappa)
Date: Mon, 20 Nov 2017 11:01:48 -0500 (EST)
Subject: [TUHS] Determining what was on a tape back in the day
Message-ID: <20171120160148.EED1818C091@mercury.lcs.mit.edu>

    > The 0th block does seem to contain some PDP-11 binary - a bootstrap of
    > some sort. I'll look in more detail in a bit.

OK, I had a quick look, and it seems to be a modified version of mboot.s:

  http://minnie.tuhs.org/cgi-bin/utree.pl?file=V6/usr/source/mdec/mboot.s

I had a look through the rest of the likely files in 'mdec', and I didn't find
a better match. I'm too lazy busy to do a complete dis-assembly, and work out
exactly how it's different, though..


A few observations:

  000:    000407 000606 000000 000000 000000 000000 000000 000001 

An a.out header, with the 0407 'magic' here performing its original intended
function - to branch past the header.

  314:	105737 177560 002375

Some console I/O stuff - this two instruction loop waits for the input
ready bit to be set.

  326:	042700 177600 020027 000101 103405 020027 000132 101002

More character processing - the first instruction clears the high bits of R0,
and the next two sets of two instructions compare the contents with two
characters (0101 and 0132), and branch.

  444:    000207 005000 021027 000407 001004 016020 
  460:    000020 020006 103774 012746 137000 005007

This seems like the code that checks to see if the thing is an a.out file
(note the 'cmp *r0, $0407'), but the code is different from that code in
mboot.s; in that, the instruction before the 'clr r0' (at 0446 here) is a
'jsr', whereas in this it's an 'rts pc'. And the code after the 'cmp r0, sp'
and branch is different too. I love the '05007' - not very often you see
_that_ instruction!

  502:    012700 177350 012701 177342 012711 000003 105711 

Clearly the code at 'taper:' (TC11 version).

	Noel