Standard UNIX system a.out header

By default, files produced by the link editor for a UNIX system always have a standard UNIX system a.out header in the optional header field. The UNIX system a.out header is 28 bytes. The fields of the optional header are described in Table 7-4, ``Optional header contents''.

Table 7-4 Optional header contents

 Bytes   Declaration   Name         Description
 ------------------------------------------------------------
   0-1   short         magic        Magic number
   2-3   short         vstamp       Version stamp
   4-7   long int      tsize        Size of text in bytes
  8-11   long int      dsize        Size of initialized data
                                    in bytes
 12-15   long int      bsize        Size of uninitialized
                                    data in bytes
 16-19   long int      entry        Entry point
 20-23   long int      text_start   Base address of text
 24-27   long int      data_start   Base address of data

Whereas the magic number in the file header specifies the machine on which the object file runs, the magic number in the optional header supplies information telling the operating system on that machine how that file should be executed. The magic numbers recognized by SCO operating systems are given in Table 7-5, ``UNIX system magic numbers''.

Table 7-5 UNIX system magic numbers

 Value   Meaning
 ------------------------------------------------
 0407    Text segment is not write-protected or
         sharable; data segment is contiguous
         with the text segment.
 0410    Data segment starts at the next segment
         following the text segment and the text
         segment is write-protected.
 0413    Text and data segments are aligned
         within a.out so it can be directly
         paged.
 0443    Defines a.out to be a target shared
         library.