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-
     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 execut-
     ing 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.

 1

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

 2

        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 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
     unused otherwise.  The first symbol is numbered 0, the sec-
     ond 1, etc.

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

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