26 Feb
2023
26 Feb
'23
2:14 a.m.
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.
Theres was also a recent discussion of ar(1). That pdf also explains its magic
number a few pages later. It was simply choosen because it seemed unique.
A file produced by ar has a "magic number" at the start,
followed by the constituent files, each preceded by a file
header. The magic number is -147(10), or 177555(8) (it was
chosen to be unlikely to occur anywhere else).
-Brian
On Sat, 25 Feb 2023, Dave Horsfall wrote:
On Thu, 23 Feb 2023, Paul Winalski wrote:
a.out was, as object file formats go, a throwback
to the stone age from
the get-go. Even the most primitive of IBM's link editors for
System/360 supported arbitrary naming of object file sections and the
ability for the programmer to arrange them in whatever order they
wished. a.out's restriction to three sections (.text, .data, .bss) did
manage to get the job done, and even (with ZMAGIC) could support
demand-paged virtual memory, but only just.
That may be so, but those guys didn't exactly have the resources of
IBM behind them...
And I wonder how many people here know the significance of the "407" magic
number?
-- Dave