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Format of Object and Executable Files

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0) General

The "a.out" file format is used as format for assembler output

("object files") as well as for linker output ("executable files").

The difference of these two is the size of certain sections of

the file being zero in case of executable files.

The file consists of a header and up to 6 sections:

  - code

  - initialized data

  - code relocation records

  - data relocation records

  - symbol table records

  - symbol string storage

1) Header

The header specifies the sizes of all following sections,

but has itself a fixed length (and is always present):

typedef struct {

  unsigned int magic;           /* must be EXEC_MAGIC */

  unsigned int csize;           /* size of code in bytes */

  unsigned int dsize;           /* size of initialized data in bytes */

  unsigned int bsize;           /* size of uninitialized data in bytes */

  unsigned int crsize;          /* size of code relocation info in bytes */

  unsigned int drsize;          /* size of data relocation info in bytes */

  unsigned int symsize;         /* size of symbol table in bytes */

  unsigned int strsize;         /* size of string space in bytes */

} ExecHeader;

The magic number is used to distinguish executable files from

other file types. This field must have the value EXEC_MAGIC.

The code size is given in bytes, but is always a multiple of 4.

This is the exact size of the code section in the executable file.

For the code size in memory see "executing an executable" below.

The initialized data size is given in bytes, but is always a

multiple of 4. This is the exact size of the initialized data

section in the executable file. For the data size in memory see

"executing an executable" below.

The uninitialized data size is given in bytes, but is always a

multiple of 4. There is no corresponding section to this value

contained within the executable. See "executing an executable"

below for the semantics of this value.

The code relocation info size is given in bytes, but is always

a multiple of sizeof(RelocRecord). These records describe the

changes to be applied to the code section during the link step.

This size is zero if the file is an executable.

The data relocation info size is given in bytes, but is always

a multiple of sizeof(RelocRecord). These records describe the

changes to be applied to the data section during the link step.

This size is zero if the file is an executable.

The size of the symbol table is given in bytes, but is always

a multiple of sizeof(SymbolRecord). The symbol table contains

information about symbols which are exported from or imported

into this object file. It is mainly used during the link step

and may not be present (size = 0) if the file is an executable.

The string space is used to store the names of the symbols in

the symbol table.

2) Code/Initialized Data

These sections contain the instructions and the initialized data

of the program, respectively.

3) Code/Data Relocation Records

The relocation records have the following structure:

typedef struct {

  unsigned int offset;          /* where to relocate */

  int method;                   /* how to relocate */

  int value;                    /* additive part of value */

  int base;                     /* if MSB = 0: segment number */

                                /* if MSB = 1: symbol table index */

} RelocRecord;

The offset gives the position where the relocation has to be done,

in the form of a byte offset from the beginning of the section.

The method determines how the relocation is performed, and must

be one of the following constants:

METHOD_H16      /* write 16 bits with high part of value */

METHOD_L16      /* write 16 bits with low part of value */

METHOD_R16      /* write 16 bits with value relative to PC */

METHOD_R26      /* write 26 bits with value relative to PC */

METHOD_W32      /* write full 32 bit word with value */

"Value" and "base" together are used to compute the final value of

the relocated code or data item. The value is added to the value of

the base. The base is either the start address of a segment in memory,

or the value of an imported symbol. In the former case, which is marked

by an MSB of 0, the base is specified as one of the following constants:

SEGMENT_ABS     /* absolute values */

SEGMENT_CODE    /* code segment */

SEGMENT_DATA    /* initialized data segment */

SEGMENT_BSS     /* uninitialized data segment */

In the latter case, which is marked by an MSB of 1, the remaining bits

specify the index of the symbol in the symbol table.

4) Symbol Table Records

For every symbol which is imported into or exported from the current

object file, there is a corresponding symbol table record:

typedef struct {

  unsigned int name;            /* offset in string space */

  int type;                     /* if MSB = 0: the symbol's segment */

                                /* if MSB = 1: the symbol is undefined */

  int value;                    /* if symbol defined: the symbol's value */

                                /* if symbol not defined: meaningless */

} SymbolRecord;

The name of the symbol is given as an offset into the string space.

If the "type" has an MSB of 0, the symbol is defined here (i.e.,

it is exported), and the "type" specifies the segment (for the

segment constants see above) in which the symbol is defined, while

the "value" holds its value. Otherwise, the symbol is not defined

here (i.e., it is imported), and the "value" has no meaning.

5) Symbol String Storage

The strings are null-terminated and stored without any padding.

6) Executing an Executable

When an executable file is loaded into memory for execution, three

logical segments are set up: the code segment, the data segment (with

initialized data, followed by uninitialized data, which starts off

as all 0), and a stack.

The code segment begins at address 0 in virtual memory and is loaded

with the contents of the code section from the executable file.

The data segment begins at the next page boundary (multiple of 4 KB)

after the code segment. It is loaded with the contents of the data

section from the executable file and is followed immediately by the

"uninitialized data", which must be zeroed by the loader. The data

area is expanded upwards as requested by explicit "brk" system calls.

The stack is located in the highest possible locations in the virtual

address space, which are accessible in user mode, and thus expanding

downwards from (but excluding) the address 0x80000000. It is extended

automatically by the operating system.