V4/man/man5/a.out.5

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.th A.OUT V 9/9/73
.sh NAME
a.out \*- assembler and link editor output
.sh DESCRIPTION
.it A.out
is the output file of the assembler
.it as
and the link editor
.it ld.
Both programs make
.it a.out
executable if there were no
errors and no unresolved external references.
.s3
This file has four sections:
a header, the program and data text, a symbol table, and relocation bits
(in that order).
The last two may be empty
if the program was loaded
with the ``\*-s'' option
of
.it ld
or if the symbols and relocation have been
removed by
.it strip.
.s3
The header always contains 8 words:
.s3
.lp +5 3
1	A magic number (407 or 410(8))
.lp +5 3
2	The size of the program text segment
.lp +5 3
3	The size of the initialized portion of the data segment
.lp +5 3
4	The size of the uninitialized (bss) portion of the data segment
.lp +5 3
5	The size of the symbol table
.lp +5 3
6	The entry location (always 0 at present)
.lp +5 3
7	Unused
.lp +5 3
8	A flag indicating relocation bits have been suppressed
.s3
.i0
The sizes of each segment are in bytes but are even.
The size of the header is not included in any of the other sizes.
.s3
When a file produced by the assembler or loader is
loaded into core for execution, three logical segments are
set up: the text segment, the data segment
(with uninitialized data, which starts off as all 0, following
initialized),
and a stack.
The text segment begins at 0
in the core image; the header is not loaded.
If the magic number (word 0) is 407, it indicates that the text
segment is not to be write-protected and shared,
so the data segment is immediately contiguous
with the text segment.
If the magic number is 410,
the data segment begins at the first 0 mod 8K byte
boundary following the text segment,
and the text segment is not writable by the program;
if other processes are executing the same file,
they will share the text segment.
.s3
The stack will occupy the highest possible locations
in the core image: from 177776(8) and growing downwards.
The stack is automatically extended as required.
The data segment is only extended as requested by
the
.it break
system call.
.s3
The start of the text segment in the file is 20(8);
the start of the data segment is 20+S\s6\dt\u\s10 (the size of the text)
the start of the relocation information is
20+S\s6\dt\u\s10+S\s6\dd\u\s10;
the start of the symbol table is
20+2(S\s6\dt\u\s10+S\s6\dd\u\s10)
if the
relocation information is present,
20+S\s6\dt\u\s10+S\s6\dd\u\s10
if not.
.s3
The symbol table consists of 6-word entries.  The first
four words contain the ASCII name of the symbol, null-padded.
The next word is a flag indicating the type of symbol.
The following values are possible:
.s3
.lp +6 3
00	undefined symbol
.lp +6 3
01	absolute symbol
.lp +6 3
02	text segment symbol
.lp +6 3
03	data segment symbol
.lp +6 3
37	file name symbol (produced by ld)
.lp +6 3
04	bss segment symbol
.lp +6 3
40	undefined external (.globl) symbol
.lp +6 3
41	absolute external symbol
.lp +6 3
42	text segment external symbol
.lp +6 3
43	data segment external symbol
.lp +6 3
44	bss segment external symbol
.i0
.s3
Values other than those given above may
occur if the user has defined some of his own instructions.
.s3
The last word of a symbol table entry contains the value of the symbol.
.s3
If the symbol's type is undefined external,
and the value field is non-zero,
the symbol is interpreted by the loader
.it ld
as
the name of a common region
whose size is indicated by the value of the
symbol.
.s3
The value of a word in the text or data portions which is not
a reference to an undefined external symbol
is exactly that value which will appear in core
when the file is executed.
If a word in the text or data portion
involves a reference to an undefined external symbol,
as indicated by the relocation bits
for that word,
then the value of the word as stored in the file
is an offset from the associated external symbol.
When the file is processed by the
link editor and the external symbol becomes
defined, the value of the symbol will
be added into the word in the file.
.s3
If relocation
information is present, it amounts to one word per
word of program text or initialized data.
There is no relocation information if the ``suppress relocation''
flag in the header is on.
.s3
Bits 3-1 of a relocation word indicate the segment referred
to by the text or data word associated with the relocation
word:
.s3
.lp +6 3
00	indicates the reference is absolute
.lp +6 3
02	indicates the reference is to the text segment
.lp +6 3
04	indicates the reference is to initialized data
.lp +6 3
06	indicates the reference is to bss (uninitialized data)
.lp +6 3
10	indicates the reference is to an undefined external symbol.
.i0
.s3
Bit 0 of the relocation word indicates if
.it on
that the
reference is relative to the pc (e.g. ``clr x'');
if
.it off,
that
the reference is to the actual symbol (e.g.,
``clr *$x'').
.s3
The remainder of the relocation word (bits 15-4)
contains a symbol number in the case of external
references, and is unused otherwise.
The first symbol is numbered 0, the second 1, etc.
.sh "SEE ALSO"
as(I), ld(I), strip(I), nm(I)