Scientific Operations



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



NAME
     a.out - assembler and link editor output

DESCRIPTION
     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, 410, or 411(8))
       2  The size of the program text segment
       3  The size of the initialized portion of the data segment
       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
     sizes.

     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-pro-
     tected and shared, so the data segment is  immediately  con-
     tiguous  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.  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.   See the 11/45 handbook for restrictions which
     apply to this situation.

     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


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


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

     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 exe-
     cuted.  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 ed-
     itor  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
     word:

        00 indicates the reference is absolute
        02 indicates the reference is to the text segment
        04 indicates the reference is to initialized data


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


        06 indicates the reference is to bss (uninitialized data)
        10 indicates the reference is to  an  undefined  external
           symbol.

     Bit 0 of the relocation word indicates if on that the refer-
     ence is relative to the pc (e.g. ``clr x'');  if  off,  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 un-
     used  otherwise.  The first symbol is numbered 0, the second
     1, etc.

SEE ALSO
     as(I), ld(I), strip(I), nm(I)











































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