A.OUT(10.6)                                           A.OUT(10.6)

     NAME
          a.out - native kernel object file format

     SYNOPSIS
          #include <a.out.h>

     DESCRIPTION
          An executable native binary file has up to six sections: a
          header, the program text, the data, a symbol table, a PC/SP
          offset table (MC680x0 only), and finally a PC/line number
          table.  The header, given by a structure in <a.out.h>, con-
          tains 4-byte integers in big-endian order:

          typedef struct Exec {
              long    magic;  /* magic number */
              long    text;   /* size of text segment */
              long    data;   /* size of initialized data */
              long    bss;    /* size of uninitialized data */
              long    syms;   /* size of symbol table */
              long    entry;  /* entry point */
              long    spsz;   /* size of pc/sp offset table */
              long    pcsz;   /* size of pc/line number table */
          } Exec;
          #define _MAGIC(b)   ((((4*b)+0)*b)+7)
          #define A_MAGIC     _MAGIC(8)   /* 68020 */
          #define I_MAGIC     _MAGIC(11)  /* intel 386 */
          #define J_MAGIC     _MAGIC(12)  /* intel 960 */
          #define K_MAGIC     _MAGIC(13)  /* sparc */
          #define V_MAGIC     _MAGIC(16)  /* mips 3000 */
          #define X_MAGIC     _MAGIC(17)  /* att dsp 3210 */
          #define M_MAGIC     _MAGIC(18)  /* mips 4000 */
          #define D_MAGIC     _MAGIC(19)  /* amd 29000 */
          #define E_MAGIC     _MAGIC(20)  /* arm 7-something */
          #define Q_MAGIC     _MAGIC(21)  /* powerpc */
          #define N_MAGIC     _MAGIC(22)  /* mips 4000-le */
          #define L_MAGIC     _MAGIC(23)  /* dec alpha */

          Sizes are expressed in bytes.  The size of the header is not
          included in any of the other sizes.

          When a Plan 9 binary file is executed, a memory image of
          three segments is set up: the text segment, the data seg-
          ment, and the stack.  The text segment begins at a virtual
          address which is a multiple of the machine-dependent page
          size.  The text segment consists of the header and the first
          text bytes of the binary file.  The entry field gives the
          virtual address of the entry point of the program.  The data
          segment starts at the first page-rounded virtual address
          after the text segment.  It consists of the next data bytes
          of the binary file, followed by bss bytes initialized to

     A.OUT(10.6)                                           A.OUT(10.6)

          zero.  The stack occupies the highest possible locations in
          the core image, automatically growing downwards.

          The next syms (possibly zero) bytes of the file contain sym-
          bol table entries, each laid out as:

               uchar value[4];
               char  type;
               char  name[n];   /* NUL-terminated */

          The value is in big-endian order and the size of the name
          field is not pre-defined: it is a zero-terminated array of
          variable length.

          The type field is one of the following characters:

               T    text segment symbol
               t    static text segment symbol
               L    leaf function text segment symbol
               l    static leaf function text segment symbol
               D    data segment symbol
               d    static data segment symbol
               B    bss segment symbol
               b    static bss segment symbol
               a    automatic (local) variable symbol
               p    function parameter symbol

          A few others are described below.  The symbols in the symbol
          table appear in the same order as the program components
          they describe.

          The Plan 9 compilers implement a virtual stack frame pointer
          rather than dedicating a register; moreover, on the MC680x0
          there is a variable offset between the stack pointer and the
          frame pointer.  Following the symbol table, MC680x0 exe-
          cutable files contain a spsz-byte table encoding the offset
          of the stack frame pointer as a function of program loca-
          tion; this section is not present for other architectures.
          The PC/SP table is encoded as a byte stream.  By setting the
          PC to the base of the text segment and the offset to zero
          and interpreting the stream, the offset can be computed for
          any PC.  A byte value of 0 is followed by four bytes that
          hold, in big-endian order, a constant to be added to the
          offset.  A byte value of 1 to 64 is multiplied by four and
          added, without sign extension, to the offset.  A byte value
          of 65 to 128 is reduced by 64, multiplied by four, and sub-
          tracted from the offset.  A byte value of 129 to 255 is
          reduced by 129, multiplied by the quantum of instruction
          size (e.g. two on the MC680x0), and added to the current PC
          without changing the offset.  After any of these operations,
          the instruction quantum is added to the PC.

     A.OUT(10.6)                                           A.OUT(10.6)

          A similar table, occupying pcsz-bytes, is the next section
          in an executable; it is present for all architectures.  The
          same algorithm may be run using this table to recover the
          absolute source line number from a given program location.
          The absolute line number (starting from zero) counts the
          newlines in the C-preprocessed source seen by the compiler.
          Three symbol types in the main symbol table facilitate con-
          version of the absolute number to source file and line num-
          ber:

               f    source file name components

               z    source file name

               Z    source file line offset

          The f symbol associates an integer (the value field of the
          `symbol') with a unique file path name component (the name
          of the `symbol').  These path components are used by the z
          symbol to represent a file name: the first byte of the name
          field is always 0; the remaining bytes hold a zero-
          terminated array of 16-bit values (in big-endian order) that
          represent file name components from f symbols.  These compo-
          nents, when separated by slashes, form a file name.  The
          initial slash of a file name is recorded in the symbol table
          by an f symbol; when forming file names from z symbols an
          initial slash is not to be assumed.  The z symbols are clus-
          tered, one set for each object file in the program, before
          any text symbols from that object file.  The set of z sym-
          bols for an object file form a history stack of the included
          source files from which the object file was compiled.  The
          value associated with each z symbol is the absolute line
          number at which that file was included in the source; if the
          name associated with the z symbol is null, the symbol repre-
          sents the end of an included file, that is, a pop of the
          history stack.  If the value of the z symbol is 1 (one), it
          represents the start of a new history stack.  To recover the
          source file and line number for a program location, find the
          text symbol containing the location and then the first his-
          tory stack preceding the text symbol in the symbol table.
          Next, interpret the PC/line offset table to discover the
          absolute line number for the program location.  Using the
          line number, scan the history stack to find the set of
          source files open at that location.  The line number within
          the file can be found using the line numbers in the history
          stack.  The Z symbols correspond to #line directives in the
          source; they specify an adjustment to the line number to be
          printed by the above algorithm.  The offset is associated
          with the first previous z symbol in the symbol table.

     SEE ALSO
          acid(10.1), 2a(10.1), 2l(10.1), inm(10.1)

     A.OUT(10.6)                                           A.OUT(10.6)

     BUGS
          There is no type information in the symbol table.