From mboxrd@z Thu Jan  1 00:00:00 1970
From: will.senn@gmail.com (Will Senn)
Date: Mon, 20 Nov 2017 11:37:34 -0600
Subject: [TUHS] Determining what was on a tape back in the day
In-Reply-To: <20171120160148.EED1818C091@mercury.lcs.mit.edu>
References: <20171120160148.EED1818C091@mercury.lcs.mit.edu>
Message-ID: <fb0ad671-8b65-cd37-bb4c-6fceab8568d0@gmail.com>

On 11/20/17 10:01 AM, Noel Chiappa wrote:
>      > 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

Thanks for the insight into how to look into this. I'm off to refreshing 
my pdp-11 assembly language skills...

Will

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