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A.OUT(5) File Formats Manual A.OUT(5)

a.out - assembler and link editor output

#include <a.out.h>

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:



struct	exec {	/* a.out header */
	int     	a_magic;	/* magic number */
	unsigned	a_text; 	/* size of text segment */
	unsigned	a_data; 	/* size of initialized data */
	unsigned	a_bss;  	/* size of unitialized data */
	unsigned	a_syms; 	/* size of symbol table */
	unsigned	a_entry; 	/* entry point */
	unsigned	a_unused;	/* not used */
	unsigned	a_flag; 	/* relocation info stripped */
};
#define	A_MAGIC1	0407       	/* normal */
#define	A_MAGIC2	0410       	/* read-only text */
#define	A_MAGIC3	0411       	/* separated I&D */
#define	A_MAGIC4	0405       	/* overlay */
struct	nlist {	/* symbol table entry */
	char    	n_name[8];	/* symbol name */
	int     	n_type;    	/* type flag */
	unsigned	n_value;	/* value */
};
		/* 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
#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 */


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 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
    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 411, 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.


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
    relocation 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
    common 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
    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.


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.


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
    number in the case of external references, and is unused otherwise. The
    first symbol is numbered 0, the second 1, etc.








as(1), ld(1), nm(1)







  UNIX-7