[TUHS] Origins of the SGS (System Generation Software) and COFF (Common Object File Format)

Paul Winalski paul.winalski at gmail.com
Mon Feb 27 01:51:12 AEST 2023

On 2/25/23, Brian Walden <tuhs at cuzuco.com> wrote:
> It was originaly 205. See A.OUT(V) (the first page) at
> https://www.bell-labs.com/usr/dmr/www/man51.pdf it was documented as to
> why.
>      The header always contains 6 words:
>           1 "br .+14" instruction (205(8))
>           2 The size of the program text
>           3 The size of the symbol table
>           4 The size of the relocation bits area
>           5 The size of a data area
>           6 A zero word (unused at present)
> I always found this so elegant in it's simplicity. Just load and start
> execution at the start (simplifies exec(2) in the kernel) I always wondered
> if this has done anywhere else before, or invenetd first in unix.

IBM's Basic Program Support (BPS) for System/360 was a set of
stand-alone utilities for developing and running stand-alone programs.
BPS/360 wasn't really an operating system because there wasn't any
resident kernel.  You just IPLed (Initial Program Load; IBM-speak for
"boot") your application directly.  So the executable format for BPS
had a bootstrap loader as the "program header".  Not quite the same
thing as a.out's 205(8) magic number, but similar in concept.

I don't know of any other OS ABI that uses this trick to transfer
control to application programs.

Microsoft uses something similar in PECOFF.  A PECOFF executable for
x86 or X86-64 starts with a bit of code in MS-DOS MZ executable format
that prints the message "This program cannot be run in DOS mode".

-Paul W.

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