3/15/72                                                 A.OUT (V)

NAME            a.out  --   assembler and link editor output

SYNOPSIS        --

DESCRIPTION     a.out is the output file of the assembler as and

                the link editor ld.  In both cases, a.out is exe-

                cutable provided there were no errors and no un-

                resolved external references.

                This file has four sections: a header, the pro-

                gram and data text, a symbol table, and reloca-

                tion bits (in that order).  The last two may be

                empty if the program was loaded with the "-s" op-

                tion of ld or if the symbols and relocation have

                been removed by strip.

                The header always contains 8 words:

                   1  a "br .+20" instruction (407(8))

                   2  The size of the program text segment

                   3  The size of the initialized data segment

                   4  The size of the uninitialized (bss) segment

                   5  The size of the symbol table

                   6  The entry location (always 0 at present)

                   7  The stack size required (0 at present)

                   8  A flag indicating relocation bits have been


                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, and uninitialized segment, in that or-

                der.  The text segment begins at the lowest loca-

                tion in the core image; the header is not loaded.

                The data segment begins immediately after the

                text segment, and the bss segment immediately af-

                ter the data segment.  The bss segment is ini-

                tialized by 0's.  In the future the text segment

                will be write-protected and shared.

                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 reloca-

                tion information is 20+S +S ; the start of the
                                        t  d
                symbol table is 20+2(S +S ) if the relocation in-
                                      t  d
                formation is present, 20+S +S  if not.
                                          t  d

                The symbol table consists of 6-word entries.  The

                first four contain the ASCII name of the symbol,

                null-padded.  The next word is a flag indicating

                the type of symbol.  The following values are


                   00  undefined symbol

                   01  absolute symbol

                   02  text segment symbol

                   03  data segment symbol

                   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 instruc-


                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 inter-

                preted by the loader ld as the name of a common

                region whose size is indicated by the value of

                the symbol.

                If a.out contains no unresolved global refer-

                ences, the text portions are exactly as they will

                appear in core when the file is executed.  If the

                value of a word in the text portion involves a

                reference to an undefined global, the word is re-

                placed by the offset to be added to the symbol's

                value when it becomes defined.

                If relocation information is present, it amounts

                to one word per word of program text or initial-

                ized data.  There is no relocation information if

                the "suppress relocation" flag in the header is


                Bits 3-1 of a relocation word indicate the seg-

                ment referred to by the text or data word associ-

                ated with the relocation word:

                  00  indicates the reference is absolute

                  02  indicates the reference is to the text seg-


                  04  indicates the reference is to the data seg-


                  06  indicates the reference is to the bss seg-


                  10  indicates the reference is to an undefined

                      external symbol.

                Bit 0 of the relocation word indicates if on that

                the reference is relative to the pc (e.g. "clr

                x"); if off, the reference is to the actual sym-

                bol (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 second 1, etc.

FILES           --

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


BUGS            --

OWNER           dmr