A.out
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Executable Formats |
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Microsoft |
*nix |
Apple |
A.OUT is the "original" binary format for Unix machines. It is considered obsolete today because of several shortcomings. However, as it is extremely simple and supported by many compilers/assemblers, it may be a good choice if you're willing to develop your own format or have more information than 'raw binary' for your bootloader.
Structure
An a.out binary file consists of up to 7 sections. In order, these sections are:
Section | Description |
---|---|
exec header | Contains parameters used by the kernel to load a binary file into memory and execute it, and by the link editor ld to combine a binary file with other binary files. This section is the only mandatory one. |
text segment | Contains machine code and related data that are loaded into memory when a program executes. Should be loaded read-only. |
data segment | Contains initialized data; always loaded into writable memory. |
text relocations | Contains records used by the link editor to update pointers in the text segment when combining binary files. |
data relocations | Like the text relocation section, but for data segment pointers. |
symbol table | Contains records used by the link editor to cross reference the addresses of named variables and functions (`symbols') between binary files. |
string table | Contains the character strings corresponding to the symbol names. |
The exec header
Every binary file begins with an exec structure:
struct exec {
unsigned long a_midmag;
unsigned long a_text;
unsigned long a_data;
unsigned long a_bss;
unsigned long a_syms;
unsigned long a_entry;
unsigned long a_trsize;
unsigned long a_drsize;
};
The fields have the following functions:
Field | Description |
---|---|
a_midmag | This field is stored in network byte-order so that binaries for machines with alternative byte orders can be distinguished. It has a number of sub-components accessed by the macros N_GETFLAG(), N_GETMID(), and N_GETMAGIC(), and set by the macro N_SETMAGIC(). |
a_text | Contains the size of the text segment in bytes. |
a_data | Contains the size of the data segment in bytes. |
a_bss | Contains the number of bytes in the `bss segment'. The kernel loads the program so that this amount of writable memory appears to follow the data segment and initially reads as zeroes. |
a_syms | Contains the size in bytes of the symbol table section. |
a_entry | Contains the address in memory of the entry point of the program after the kernel has loaded it; the kernel starts the execution of the program from the machine instruction at this address. |
a_trsize | Contains the size in bytes of the text relocation table. |
a_drsize | Contains the size in bytes of the data relocation table. |