2.9BSD/usr/man/man5/a.out.5

.TH A.OUT 5 
.UC
.SH NAME
a.out \- assembler and loader output
.SH SYNOPSIS
.B #include <a.out.h>
.SH DESCRIPTION
.I A.out
is the output file of the assembler
.IR as (1)
and the loader
.IR ld (1).
Both programs make
.I a.out
executable if there were no
errors and no unresolved external references.
Layout information as given in the include file for the PDP11 is:
.PP
.nf
.ta 8n +9n +11n
.PP
#define NOVL	7

#define	A_MAGIC1	0407       	/* normal */
#define	A_MAGIC2	0410       	/* read-only text */
#define	A_MAGIC3	0411       	/* separated I&D */
#define	A_MAGIC4	0405       	/* overlay */
#define	A_MAGIC5	0430       	/* auto-overlay (nonseparate) */
#define	A_MAGIC6	0431       	/* auto-overlay (separate)  */

struct	exec {
	int     	a_magic;	/* magic number */
	unsigned	a_text; 	/* size of text segment */
	unsigned	a_data; 	/* size of initialized data */
	unsigned	a_bss;  	/* size of uninitialized data */
	unsigned	a_syms; 	/* size of symbol table */
	unsigned	a_entry; 	/* entry point */
	unsigned	a_unused;	/* not used */
	unsigned	a_flag; 	/* relocation info stripped */
};

struct	ovlhdr {
	int		max_ovl;	/* maximum ovl size */
	unsigned	ov_siz[NOVL];	/* size of i'th overlay */
};

struct	nlist {
	char    	n_name[8];	/* symbol name */
	int     	n_type;    	/* type flag */
	unsigned	n_value;	/* value */
};

/*
 * Macros which take exec structures as arguments and tell whether
 * the file has a reasonable magic number or offset to text.
 */
#define	N_BADMAG(x) \\
    (((x).a_magic)!=A_MAGIC1 && ((x).a_magic)!=A_MAGIC2 && \\
     ((x).a_magic)!=A_MAGIC3 && ((x).a_magic)!=A_MAGIC4 && \\
     ((x).a_magic)!=A_MAGIC5 && ((x).a_magic)!=A_MAGIC6)

#define	N_TXTOFF(x) \\
	((x).a_magic==A_MAGIC5 || (x).a_magic==A_MAGIC6 ? \\
	 sizeof (struct ovlhdr) + sizeof (struct exec) : sizeof (struct exec))

		/* values for type flag */
#define	N_UNDF	0	/* undefined */
#define	N_ABS	01	/* absolute */
#define	N_TEXT	02	/* text symbol */
#define	N_DATA	03	/* data symbol */
#define	N_BSS	04	/* bss symbol */
#define	N_TYPE	037	/* mask for type flag */
#define	N_REG	024	/* register name */
#define	N_FN	037	/* file name symbol */
#define	N_EXT	040	/* external bit, or'ed in */
#define	FORMAT	"%06o"	/* to print a value */
.fi
.PP
The file has four sections:
a header, the program and data text,
relocation information, and a symbol table
(in that order).
The last two may be empty
if the program was loaded
with the `\-s' option
of
.I ld
or if the symbols and relocation have been
removed by
.IR strip (1).
.PP
In the header the sizes of each section are given in bytes, but are even.
The size of the header is not included in any of the other sizes.
.PP
When an
.I a.out
file is loaded into core for execution, three or four logical segments are
set up: the text segment, a possible text overlay 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 in the header is 0407(8), 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 0410,
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.
If the magic number is 0411,
the text segment is again pure, write-protected, and shared,
and moreover instruction and data space are separated;
the text and data segment both begin at location 0.
If the magic number is 0405, the text segment
is overlaid on an existing (0411 or 0405) text segment
and the existing data segment is preserved.
.PP
If the magic number is 0430, the base text segment is
write-protected and shared and is followed by a text overlay segment.
There are a maximum of 7 overlays, all pure and shared.
The base segment runs from 0 to txtsiz.
At the next 0 mod 8k boundary is the overlay region,
which is as large as the largest overlay.
When running, any one of the 7 overlays can be mapped into this region.
At the following 0 mod 8k byte boundary the data segment begins.
If the magic number is 0431, the situation is the same as for
type 0430 except that instruction and data spaces are separated
and both begin at location 0.
Both 0430 and 0431 executable files have a second header between
the normal a.out header and the start of the text image;
it contains the maximum overlay size and the sizes of each of the
seven overlays.
The text images of the overlays follow the text in the object
file, and are followed by the initialized data.
.PP
The stack segment will occupy the highest possible locations
in the core image: from 0177776(8) and growing downwards.
The stack segment is automatically extended as required.
The data segment is only extended as requested by
.IR brk (2).
.PP
The layout of a symbol table entry and the principal flag values
that distinguish symbol types are given in the include file.
Other flag values may occur if an assembly language program
defines machine instructions.
.PP
If a symbol's type is undefined external,
and the value field is non-zero,
the symbol is interpreted by the loader
.I ld
as
the name of a common region
whose size is indicated by the value of the
symbol.
.PP
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 information
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.
.PP
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 `relocation info stripped'
flag in the header is on.
Automatic-overlay (0430 and 0431) files do not contain relocation
information.
.PP
Bits 3-1 of a relocation word indicate the segment referred
to by the text or data word associated with the relocation
word:
.TP
000
absolute number
.br
.ns
.TP
002
reference to text segment
.br
.ns
.TP
004
reference to initialized data
.br
.ns
.TP
006
reference to uninitialized data (bss)
.br
.ns
.TP
010
reference to undefined external symbol
.PP
Bit 0 of the relocation word indicates, if 1,
that the
reference is relative to the pc (e.g. `clr x');
if 0,
that
the reference is to the actual symbol (e.g.,
`clr *$x').
.PP
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(1), ld(1), nm(1)