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A.OUT(6)                         Games Manual                         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>, contains 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 */
       #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 three segments
       is set up: the text segment, the data segment, 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 con‐
       sists 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 symbol 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  ap‐
       pear 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 program location; this section is not present for other ar‐
       chitectures.   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  in‐
       terpreting  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 multi‐
       plied 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,  multi‐
       plied 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 exe‐
       cutable; 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 com‐
       piler.  Three symbol types in the main symbol table facilitate  conver‐
       sion of the absolute number to source file and line number:

              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  components,
       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 clustered, one set for each object file in the program, be‐
       fore  any text symbols from that object file.  The set of z symbols 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 sym‐
       bol represents the end of an included file, that is, a pop of the  his‐
       tory 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 num‐
       ber  for  a program location, find the text symbol containing the loca‐
       tion and then the first history 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 lo‐
       cation.  The line number within the file can be found  using  the  line
       numbers in the history stack.  The Z symbols correspond to #line direc‐
       tives 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).



                                                                      A.OUT(6)