# Copyright (C) 2001-2014, Parrot Foundation.

=head1 NAME

docs/parrotbyte.pod - The Parrot Bytecode (PBC) Format

=head1 DESCRIPTION

This document describes the Parrot bytecode format.

=head1 THE HEADER

The 18-byte header consists of,

  0                                           7
  +----------+----------+----------+----------+
  | Parrot Magic = fe:50:42:43:0d:0a:1a:0a    |
  +----------+----------+----------+----------+

which reads as C<"<fe>PBC\r\n<1a>\n">. The B<Magic> is stored as
endian-less sequence of C<unsigned char magic[18]>. The loader uses
the byteorder header to convert the rest to the host format. More
specifically, ALL words (non-bytes) in the bytecode file are stored in
native order, unless otherwise specified.

  8          9         10         11
  +----------+----------+----------+
  | Wordsize | Byteorder| FloatType|
  +----------+----------+----------+

The B<Wordsize> (or C<opcode_t> size) must be 4 (32-bit) or 8 (64 bit). The
bytecode loader is responsible for transforming the file into the VM native
wordsize on the fly. For performance, a utility F<pbc_dump> is provided to convert
PBCs on disk if they cannot be recompiled. See F<src/pbc_dump.c> for more
information.

B<Byteorder> currently supports two values: (0-Little Endian, 1-Big Endian)

B<FloatType> 0 is IEEE 754 8 byte double, FloatType 1 is
i386 little endian 12 byte long double, FloatType 2 is native
16 byte long double, either powerpc double double, intel 12 byte
padded long double or true IEEE 16 byte long double.

  11        12         13         14         15         16
  +----------+----------+----------+----------+----------+
  |  Major   |  Minor   |  Patch   | BC Major | BC Minor |
  +----------+----------+----------+----------+----------+

B<Major>, B<Minor>, B<Patch> for the version of Parrot that wrote
the bytecode file.

B<BC Major> and B<BC Minor> are for the internal bytecode version.

  16        17         18         19         20         21         22
  +----------+----------+----------+----------+----------+----------+
  | UUID type| UUID size|  *UUID data                               |
  +----------+----------+----------+----------+----------+----------+

After the UUID type and size comes the UUID data pointer.

  22*
  +----------+----------+----------+----------+
  |         dir_format      (1)               |
  +----------+----------+----------+----------+
  |         padding         (0)               |
  +----------+----------+----------+----------+

B<dir_format> has length opcode_t and value 1 for PBC FORMAT 1,
defined in F<packfile.h>

=head1 PBC FORMAT 1

All segments are aligned at a 16 byte boundary. All segments share a common
header and are kept in directories, which itself is a PBC segment. All offsets
and sizes are in native opcodes of the machine that produced the PBC.

After the PBC header, the first PBC directory follows at offset 24*
starting with a:

=head2 Format 1 Segment Header

  +----------+----------+----------+----------+
  | total size in opcodes including this size |
  +----------+----------+----------+----------+
  |         internal type (itype)             |
  +----------+----------+----------+----------+
  |         internal id   (id)                |
  +----------+----------+----------+----------+
  |         size of opcodes following         |
  +----------+----------+----------+----------+

The B<size> entry may be followed by a stream of B<size> opcodes (starting 16
byte aligned), which may of course be no opcode stream at all for size zero.

After this common segment header there can be segment specific data determined
by the segment type. A segment without additional data, like the bytecode
segment, is a B<default> segment. No additional routines are required to unpack
such a segment.

=head2 Directory Segment

  +----------+----------+----------+----------+
  | number of directory entries               |
  +----------+----------+----------+----------+

  +----------+----------+----------+----------+
  | segment type                              |
  +----------+----------+----------+----------+
  | segment name ...                          |
  | ...        0x00       padding             |
  +----------+----------+----------+----------+
  | segment offset                            |
  +----------+----------+----------+----------+
  | segment op_count                          |
  +----------+----------+----------+----------+

The B<op_count> at B<offset> must match the segments B<op_count> and is used to
verify the PBCs integrity.

Currently these segment types are defined:

=over 4

=item Directory segment

=item Unknown segment (conforms to a default segment)

=item Fixup segment

=item Constant table segment

=item Bytecode segment

=item Debug segment

=back

=head2 Segment Names

This is not determined yet.

=head2 Unknown (default) and bytecode segments

These have only the common segment header and the opcode stream appended. The
opcode stream is an F<mmap()>ed memory region, if your operating system
supports this (and if the PBC was read from a disk file). You have therefore to
consider these data as readonly.

=head2 Fixup segment

  +----------+----------+----------+----------+
  | number of fixup entries                   |
  +----------+----------+----------+----------+

  +----------+----------+----------+----------+
  | fixup type   (0)                          |
  +----------+----------+----------+----------+
  | label   name ...                          |
  | ...        0x00       padding             |
  +----------+----------+----------+----------+
  | label offset                              |
  +----------+----------+----------+----------+

The fixup type for constant or ascii strings has a label symbol that is the
name of the "sub" and an offset into the constant table, referencing a
Sub, Closure or Coroutine PMC.

=head2 Debug Segment

The opcode stream will contain one line number per bytecode instruction. No
information as to what file that line is from will be stored in this stream.

The header will start with a count of the number of source file to bytecode
position mappings that are in the header.

  0 (relative)
  +----------+----------+----------+----------+
  |   number of source => bytecode mappings   |
  +----------+----------+----------+----------+

A source to bytecode position mapping simply states that the bytecode that
starts from the specified offset up until the offset in the next mapping, or
if there is none up until the end of the bytecode, has its source in
location X.

A mapping always starts with the offset in the bytecode, followed by the
type of the mapping.

  0 (relative)
  +----------+----------+----------+----------+
  |              bytecode offset              |
  +----------+----------+----------+----------+

  4
  +----------+----------+----------+----------+
  |                mapping type               |
  +----------+----------+----------+----------+

There are 3 mapping types.

Type B<0> means there is no source available for the bytecode starting at the
given offset. No further data is stored with this type of mapping; the next
mapping continues immediately after it.

Type B<1> means the source is available in a file. An index into the constants
table follows, which will point to a string containing the filename.

Type B<2> means the source is available in a source segment. Another integer
follows, which will specify which source file in the source segment to use.

Note that the ordering of the offsets into the bytecode must be sequential;
a mapping for offset 100 cannot follow a mapping for offset 200, for
example.

=head2 CONSTANT TABLE SEGMENT

  0 (relative)
  +----------+----------+----------+----------+
  |            Constant Count (N)             |
  +----------+----------+----------+----------+

For each constant:

  +----------+----------+----------+----------+
  |             Constant Type (T)             |
  +----------+----------+----------+----------+
  |                                           |
  |        S bytes of constant content        |
  :       appropriate for representing        :
  |              a value of type T            |
  |                                           |
  +----------+----------+----------+----------+


=head2 CONSTANTS

For integer constants:

  << integer constants are represented as manifest constants in
     the bytecode stream currently, limiting them to 32 bit values. >>

For number constants (S is constant, and is equal to C<sizeof(FLOATVAL)>):

  +----------+----------+----------+----------+
  |                                           |
  |             S' bytes of Data              |
  |                                           |
  +----------+----------+----------+----------+

where

  S' = S + (S % 4) ? (4 - (S % 4)) : 0

If S' E<gt> S, then the extra bytes are filled with zeros.


For string constants (S varies, and is the size of the particular string):

  4, 4 + (16 + S'0), 4 + (16 + S'0) + (16 + S'1)
  +----------+----------+----------+----------+
  |                   Flags                   |
  +----------+----------+----------+----------+
  |                  Encoding                 |
  +----------+----------+----------+----------+
  |                   Type                    |
  +----------+----------+----------+----------+
  |                  Size (S)                 |
  +----------+----------+----------+----------+
  |                                           |
  :             S' bytes of Data              :
  |                                           |
  +----------+----------+----------+----------+

where

  S' = S + (S % 4) ? (4 - (S % 4)) : 0

If S' E<gt> S, then the extra bytes are filled with zeros.


=head2 BYTE CODE SEGMENT

The pieces that can be found in the bytecode segment are as follows:

  +----------+----------+----------+----------+
  |              Operation Code               |
  +----------+----------+----------+----------+

  +----------+----------+----------+----------+
  |             Register Argument             |
  +----------+----------+----------+----------+

  +----------+----------+----------+----------+
  |    Integer Argument (Manifest Constant)   |
  +----------+----------+----------+----------+

  +----------+----------+----------+----------+
  |   String Argument (Constant Table Index)  |
  +----------+----------+----------+----------+

  +----------+----------+----------+----------+
  |   Number Argument (Constant Table Index)  |
  +----------+----------+----------+----------+

  +----------+----------+----------+----------+
  |    PMC Argument (Constant Table Index)    |
  +----------+----------+----------+----------+

The number of arguments and the type of each argument can usually be determined
by consulting Parrot::Opcode, or programmatically by obtaining the op_info_t
structure for the opcode in question.

There are currently 4 opcodes that can take a variable number of arguments:
set_args, get_params, set_returns and get_results. These ops always have one
required argument, which is a PMC constant. Calling the elements VTABLE
function on this PMC will give the number of extra variable arguments that
follow.


=head2 SOURCE CODE SEGMENT

Currently there are no utilities that use this segment, even though it is
mentioned in some of the early Parrot documents.

=head1 SEE ALSO

F<packfile.c>, F<packfile.h>, F<packdump.c>, F<pf/*.c>, and the
B<pbc_dump> utility F<pbc_dump.c>.

=cut
