AT&T UNIX™PC Unix System V Users Manual - tenox

A.OUT ( 4) A.OUT ( 4)
data segment and the text segment are not writable by the program;
if other processes are executing the same a.out file, they
will share a single text segment.
Magic number 413 (octal) is the same as 410 (octal), except that
413 (octal) permits demand paging. Both the -z and -F options
of the loader ld(l) create a.out files with magic numbers 0413. If �
the -z option is used, both the text and data sections of the file
are on 1024-byte boundaries. If the -F option is used, the text
and data sections of the file are contiguous. Loading a single
4096-byte page into memory requires 4 transfers of 1024 bytes
each for -z, and typically one transfer of 4096 bytes for -F.
Thus a.out files created with -F can load faster and require less
disk space.
The stack begins at the end of memory and grows towards lower
addresses. The stack is automatically extended as required. The
data segment is extended only as requested by the brk (2) system
call.
The value of a word in the text or data portions that is not a
reference to an undefined external symbol is exactly the value that
will appear in memory when the file is executed. If a word in the
text involves a reference to an undefined external symbol, the
storage class of the symbol-table entry for that word will be
marked as an "external symbol," and the section number will be
set to 0. When the file is processed by the link editor and the
external symbol becomes defined, the value of the symbol will be l
added to the word in the file.
File Header
The format of the filehdr header is:
struct filehdr
{
unsigned short f_magic; I* magic number *I
unsigned short f_nscns; I* number of sections *I
long f_timdat; I* time and date stamp *I
long f_symptr; I* file ptr to symtab *I
long f_nsyms; I* # symtab entries *I
unsigned short f_opthdr; I* sizeof(opt hdr) *I
unsigned short f_flags; I* flags *I
};
UNIX System Header
The format of the UNIX system header is:
typedef struct aouthdr
{
short magic; I* magic number *I
short vstamp; I* version stamp *I
long tsize; I* text size in bytes, padded *I
long dsize; I* initialized data (.data) *I
long bsize; I* uninitialized data {. bss) *I
long entry; I* entry point *I
long text_start; I* base of text used for this file *I
long data_start; I* base of data used for this file *I
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