Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!samsung!spool.mu.edu!uunet!rbj
From: rbj@uunet.UU.NET (Root Boy Jim)
Newsgroups: comp.unix.questions
Subject: Re: why different swapping modes on executables?
Message-ID: <126505@uunet.UU.NET>
Date: 26 Mar 91 06:04:55 GMT
References: <1991Mar12.012401.557@nowhere.uucp> <1991Mar12.104352.23097@kithrup.COM> <573@adpplz.UUCP>
Organization: UUNET Communications Services, Falls Church, VA
Lines: 21

In article <573@adpplz.UUCP> martin@adpplz.UUCP (Martin Golding) writes:
>
>In the very bad old days, PDP 11's had two ways to map memory: one 
>combined the data and program in a single 64k segment, and one had 
>separate (64k) data and program spaces. Given the addressing modes, 
>the difference between data and program accesses were a study in themselves.

Actually, they had three. The third used separate text and data
segments within the same virtual 64K.

>My impression is that the (original) version of the Magic Number was
>the actual value to load into the PDP 11 MMU control register, to select
>one or the other model.

HAHAHAHA! It's even worse than that! The Magic Number was a JUMP
(actually a branch) across the a.out header. A 407 executable
had a seven word (14 byte) header, a 410, eight words, 411 nine...
Sneaky, huh?
-- 
		[rbj@uunet 1] stty sane
		unknown mode: sane