A.OUT(6)                                                 A.OUT(6)

     NAME
          a.out - object file format

     SYNOPSIS
          #include <a.out.h>

     DESCRIPTION
          An executable Plan 9 binary file has up to six sections: a
          header, the program text, the data, a symbol table, a PC/SP
          offset table (MC68020 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 HDR_MAGIC   0x00008000

          #define  _MAGIC(f, b)           ((f)|((((4*(b))+0)*(b))+7))
          #define  A_MAGIC    _MAGIC(0, 8)        /* 68020 */
          #define  I_MAGIC    _MAGIC(0, 11)       /* intel 386 */
          #define  J_MAGIC    _MAGIC(0, 12)       /* intel 960 (retired) */
          #define  K_MAGIC    _MAGIC(0, 13)       /* sparc */
          #define  V_MAGIC    _MAGIC(0, 16)       /* mips 3000 BE */
          #define  X_MAGIC    _MAGIC(0, 17)       /* att dsp 3210 (retired) */
          #define  M_MAGIC    _MAGIC(0, 18)       /* mips 4000 BE */
          #define  D_MAGIC    _MAGIC(0, 19)       /* amd 29000 (retired) */
          #define  E_MAGIC    _MAGIC(0, 20)       /* arm */
          #define  Q_MAGIC    _MAGIC(0, 21)       /* powerpc */
          #define  N_MAGIC    _MAGIC(0, 22)       /* mips 4000 LE */
          #define  L_MAGIC    _MAGIC(0, 23)       /* dec alpha (retired) */
          #define  P_MAGIC    _MAGIC(0, 24)       /* mips 3000 LE */
          #define  U_MAGIC    _MAGIC(0, 25)       /* sparc64 */
          #define  S_MAGIC    _MAGIC(HDR_MAGIC, 26)   /* amd64 */
          #define  T_MAGIC    _MAGIC(HDR_MAGIC, 27)   /* powerpc64 */
          #define  R_MAGIC    _MAGIC(HDR_MAGIC, 28)   /* arm64 */

          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

     A.OUT(6)                                                 A.OUT(6)

          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
          zero.  The stack occupies the highest possible locations in
          the core image, automatically growing downwards.  The bss
          segment may be extended by brk(2).

          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 with the
          high bit set:

               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
          architectures there is a variable offset between the stack
          pointer and the frame pointer.  Following the symbol table,
          MC680X0 executable files contain a spsz-byte table encoding
          the offset of the stack frame pointer as a function of pro-
          gram location; this section is not present for other archi-
          tectures.  The PC/SP table is encoded as a byte stream.  By
          setting the PC to the base of the text segment and the off-
          set to zero and interpreting the stream, the offset can be

     A.OUT(6)                                                 A.OUT(6)

          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 subtracted 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 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

     A.OUT(6)                                                 A.OUT(6)

          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
          db(1), acid(1), 2a(1), 2l(1), nm(1), strip(1), mach(2),
          symbol(2)

     BUGS
          There is no type information in the symbol table; however,
          the -a flags on the compilers will produce symbols for
          acid(1).