A.OUT(V)                     9/9/73                      A.OUT(V)


     a.out - assembler and link editor output


     A.out is the output file of the assembler as  and  the  link

     editor  ld.   Both  programs  make a.out executable if there

     were no errors and no unresolved external references.

     This file has four sections: a header, the program and  data

     text,  a  symbol table, and relocation bits (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.

     The header always contains 8 words:

       1   A magic number (407 or 410(8))

       2   The size of the program text segment

       3   The size  of  the  initialized  portion  of  the  data


       4   The size of the uninitialized  (bss)  portion  of  the

          data segment

       5   The size of the symbol table

       6   The entry location (always 0 at present)

       7   Unused

       8   A flag indicating relocation bits have been suppressed

     The sizes of each segment are in bytes but  are  even.   The

     size  of  the  header  is  not  included in any of the other


     When a file produced by the assembler or  loader  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 (word 0) is 407,

     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

     410,  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.

     The stack will occupy the highest possible locations in  the

     core image: from 177776(8) and growing downwards.  The stack

     is automatically extended as required.  The data segment  is

     only extended as requested by the break system call.

     The start of the text segment in  the  file  is  20(8);  the

     start  of  the  data segment is 20+S  (the size of the text)
     the start of the relocation  information  is  20+S +S ;  the
                                                       t  d
     start  of  the symbol table is 20+2(S +S ) if the relocation
                                          t  d
     information is present, 20+S +S  if not.
                                 t  d

     The symbol table consists of 6-word entries.  The first four

     words  contain  the  ASCII  name of the symbol, null-padded.

     The next word is a flag indicating the type of symbol.   The

     following values are possible:

        00  undefined symbol

        01  absolute symbol

        02  text segment symbol

        03  data segment symbol

        37  file name symbol (produced by ld)

        04  bss segment symbol

        40  undefined external (.globl) symbol

        41  absolute external symbol

        42  text segment external symbol

        43  data segment external symbol

        44  bss segment external symbol

     Values other than those given above may occur  if  the  user

     has defined some of his own instructions.

     The last word of a symbol table entry contains the value  of

     the symbol.

     If the 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  bits  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 ``suppress  relocation''  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


        00  indicates the reference is absolute

        02  indicates the reference is to the text segment

        04  indicates the reference is to initialized data

        06  indicates the  reference  is  to  bss  (uninitialized


        10  indicates the reference is to an  undefined  external


     Bit 0 of the  relocation  word  indicates  if  on  that  the

     reference  is  relative  to the pc (e.g. ``clr x''); if off,

     that the reference is to  the  actual  symbol  (e.g.,  ``clr


     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(I), ld(I), strip(I), nm(I)