A.OUT(5)A.OUT(5)NAMEa.out - assembler and link editor output
SYNOPSIS
#include <a.out.h>
DESCRIPTION
A.out is the output file of the assembler as(1) and the link editor
ld(1). Both programs make 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:
#ifndef __A_OUT_GNU_H__
#define __A_OUT_GNU_H__
#include <bits/a.out.h>
#define __GNU_EXEC_MACROS__
struct exec
{
unsigned long a_info; /* Use macros N_MAGIC, etc for access. */
unsigned int a_text; /* Length of text, in bytes. */
unsigned int a_data; /* Length of data, in bytes. */
unsigned int a_bss; /* Length of uninitialized data area for file, in bytes. */
unsigned int a_syms; /* Length of symbol table data in file, in bytes. */
unsigned int a_entry; /* Start address. */
unsigned int a_trsize;/* Length of relocation info for text, in bytes. */
unsigned int a_drsize;/* Length of relocation info for data, in bytes. */
};
enum machine_type
{
M_OLDSUN2 = 0,
M_68010 = 1,
M_68020 = 2,
M_SPARC = 3,
M_386 = 100,
M_MIPS1 = 151,
M_MIPS2 = 152
};
#define N_MAGIC(exec) ((exec).a_info & 0xffff)
#define N_MACHTYPE(exec) ((enum machine_type)(((exec).a_info >> 16) & 0xff))
#define N_FLAGS(exec) (((exec).a_info >> 24) & 0xff)
#define N_SET_INFO(exec, magic, type, flags) ((exec).a_info = ((magic) & 0xffff) | (((int)(type) & 0xff) << 16) | (((flags) & 0xff) << 24))
#define N_SET_MAGIC(exec, magic) ((exec).a_info = ((exec).a_info & 0xffff0000) | ((magic) & 0xffff))
#define N_SET_MACHTYPE(exec, machtype) ((exec).a_info = ((exec).a_info&0xff00ffff) | ((((int)(machtype))&0xff) << 16))
#define N_SET_FLAGS(exec, flags) ((exec).a_info = ((exec).a_info&0x00ffffff) | (((flags) & 0xff) << 24))
/* Code indicating object file or impure executable. */
#define OMAGIC 0407
/* Code indicating pure executable. */
#define NMAGIC 0410
/* Code indicating demand-paged executable. */
#define ZMAGIC 0413
/* This indicates a demand-paged executable with the header in the text.
The first page is unmapped to help trap NULL pointer references. */
#define QMAGIC 0314
/* Code indicating core file. */
#define CMAGIC 0421
#define N_TRSIZE(a) ((a).a_trsize)
#define N_DRSIZE(a) ((a).a_drsize)
#define N_SYMSIZE(a) ((a).a_syms)
#define N_BADMAG(x) (N_MAGIC(x) != OMAGIC&& N_MAGIC(x) != NMAGIC && N_MAGIC(x) != ZMAGIC && N_MAGIC(x) != QMAGIC)
#define _N_HDROFF(x) (1024 - sizeof (struct exec))
#define N_TXTOFF(x) (N_MAGIC(x) == ZMAGIC ? _N_HDROFF((x)) + sizeof (struct exec) : (N_MAGIC(x) == QMAGIC ? 0 : sizeof (struct exec)))
#define N_DATOFF(x) (N_TXTOFF(x) + (x).a_text)
#define N_TRELOFF(x) (N_DATOFF(x) + (x).a_data)
#define N_DRELOFF(x) (N_TRELOFF(x) + N_TRSIZE(x))
#define N_SYMOFF(x) (N_DRELOFF(x) + N_DRSIZE(x))
#define N_STROFF(x) (N_SYMOFF(x) + N_SYMSIZE(x))
/* Address of text segment in memory after it is loaded. */
#define N_TXTADDR(x) (N_MAGIC(x) == QMAGIC ? 4096 : 0)
/* Address of data segment in memory after it is loaded. */
#define SEGMENT_SIZE 1024
#define _N_SEGMENT_ROUND(x) (((x) + SEGMENT_SIZE - 1) & ~(SEGMENT_SIZE - 1))
#define _N_TXTENDADDR(x) (N_TXTADDR(x)+(x).a_text)
#define N_DATADDR(x) (N_MAGIC(x)==OMAGIC? (_N_TXTENDADDR(x)) : (_N_SEGMENT_ROUND (_N_TXTENDADDR(x))))
#define N_BSSADDR(x) (N_DATADDR(x) + (x).a_data)
#if !defined (N_NLIST_DECLARED)
struct nlist
{
union
{
char *n_name;
struct nlist *n_next;
long n_strx;
} n_un;
unsigned char n_type;
char n_other;
short n_desc;
unsigned long n_value;
};
#endif /* no N_NLIST_DECLARED. */
#define N_UNDF 0
#define N_ABS 2
#define N_TEXT 4
#define N_DATA 6
#define N_BSS 8
#define N_FN 15
#define N_EXT 1
#define N_TYPE 036
#define N_STAB 0340
#define N_INDR 0xa
#define N_SETA 0x14 /* Absolute set element symbol. */
#define N_SETT 0x16 /* Text set element symbol. */
#define N_SETD 0x18 /* Data set element symbol. */
#define N_SETB 0x1A /* Bss set element symbol. */
#define N_SETV 0x1C /* Pointer to set vector in data area. */
#if !defined (N_RELOCATION_INFO_DECLARED)
/* This structure describes a single relocation to be performed.
The text-relocation section of the file is a vector of these structures,
all of which apply to the text section.
Likewise, the data-relocation section applies to the data section. */
struct relocation_info
{
int r_address;
unsigned int r_symbolnum:24;
unsigned int r_pcrel:1;
unsigned int r_length:2;
unsigned int r_extern:1;
unsigned int r_pad:4;
};
#endif /* no N_RELOCATION_INFO_DECLARED. */
#endif /* __A_OUT_GNU_H__ */
The file has four sections: a header, the program and data text, relo‐
cation information, and a symbol table (in that order). The last two
may be empty if the program was loaded with the `-s' option of ld or if
the symbols and relocation have been removed by strip(1).
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.
When an a.out file is loaded into core for execution, three logical
segments are set up: the text segment, the data segment (with unini‐
tialized 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 411,
the text segment is again pure, write-protected, and shared, and more‐
over instruction and data space are separated; the text and data seg‐
ment 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.
The stack will occupy the highest possible locations in the core image:
from 0177776(8) and growing downwards. The stack is automatically
extended as required. The data segment is only extended as requested
by brk(2).
The start of the text segment in the file is 020(8); the start of the
data segment is 020+St (the size of the text) the start of the reloca‐
tion information is 020+St+Sd; the start of the symbol table is
020+2(St+Sd) if the relocation information is present, 020+St+Sd if
not.
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.
If a symbol's type is undefined external, and the value field is non-
zero, the symbol is interpreted by the loader ld as the name of a com‐
mon region whose size is indicated by the value of the symbol.
The value of a word in the text or data portions which is not a refer‐
ence 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.
If relocation information is present, it amounts to one word per word
of program text or initialized data. There is no relocation informa‐
tion if the `relocation info stripped' flag in the header is on.
Bits 3-1 of a relocation word indicate the segment referred to by the
text or data word associated with the relocation word:
000 absolute number
002 reference to text segment
004 reference to initialized data
006 reference to uninitialized data (bss)
010 reference to undefined external symbol
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').
The remainder of the relocation word (bits 15-4) contains a symbol num‐
ber in the case of external references, and is unused otherwise. The
first symbol is numbered 0, the second 1, etc.
SEE ALSOas(1), ld(1), nm(1)A.OUT(5)
