From mboxrd@z Thu Jan  1 00:00:00 1970
From: jnc@mercury.lcs.mit.edu (Noel Chiappa)
Date: Wed,  9 Dec 2015 12:50:24 -0500 (EST)
Subject: [TUHS] v6tar from v7 on v6, too large?
Message-ID: <20151209175024.5180118C0C7@mercury.lcs.mit.edu>

    > From: Will Senn

    > my now handy-dandy PDP11/40 processor handbook

That's good for the instruction set, but for the memory management hardware,
etc you'll really want one of the {/44, /45, /70, /73, etc} group, since only
those models support split I+D.

    > the 18 bits holding the word 000407

You mean '16 bits', right? :-)

    > This means that branches are to 9th, 10th, 11th and 7th words,
    > respectively. It'll be a while before I really understand what the
    > ramifications are.

Only the '407' is functional. (IIRC, in early UNIX versions, the OS didn't
strip the header on loading a new program into memory, so the 407 was actually
executed.)  The others are just magic numbers, inspired by the '407' - the
code always starts at byte 020 in the file.

    > Oh and by the way, jumping between octal and decimal is weird, but
    > convenient once you get the hang of it - 512 is 1000, which is nifty
    > and makes finding buffer boundaries in an octal dump easy :).

The _real_ reason octal is optimal for PDP-11 is that when looking at core,
most instructions are more easily understood in octal, because the PDP-11 has
8 registers (3 bits), and 3 bits worth of mode modifier, and the fields are
aligned to show up in octal.

I.e. in the double-op instruction '0awxyz', the 'a' octit gives the opcode,
'w' is the mode for the first operand, 'x' is the register for the first
operand, and 'y' and 'z' similarly for the second operand. So '12700' is
'MOV (PC)+, R0' - AKA 'MOV #bbb, R0', where 'bbb' is the contents of the word
after the '12700'.

	Noel