# Copyright (C) 2001-2005, The Perl Foundation.
# $Id: pdd13_bytecode.pod 16271 2006-12-27 06:15:08Z rgrjr $

=head1 NAME

docs/pdds/pdd13_bytecode.pod - Parrot Bytecode

=head1 ABSTRACT

This PDD describes the file format for Parrot Bytecode (PBC) files and the
interface through which they may be manipulated programatically.

=head1 VERSION

$Revision: 16271 $

=head1 DESCRIPTION

=over 4

=item - The sequence of instructions making up a Parrot program, a constants
table and debug data are all stored in a binary format called a packfile or
PBC (Parrot Bytecode File).

=item - A PBC file can be read by Parrot on any platform, but may be encoded
more optimally for a particular platform.

=item - It is possible to add arbitrary annotations to the instruction
sequence, for example line numbers in the high level language and other debug
data.

=item - PMCs will be used to represent packfiles and packfile segments to
provide a programming interface to them, both from Parrot programs and the
Parrot internals.

=back


=head1 DEFINITIONS

None.


=head1 IMPLEMENTATION

=head2 Changes From The Current Implementation

A number of things in this proposed PDD differ from what is currently
implemented. This section details these changes and some of the reasoning
behind them.


=head3 Packfile Header

The format of the packfile header has changed completely, based upon a
proposal at
L<http://groups.google.com/group/perl.perl6.internals/browse_thread/thread/1f1af615edec7449/ebfdbb5180a9d813?lnk=gst>
and the requirement to have a UUID. I also observed that the INT field in the
previous header format is used nowhere in Parrot and appears redundant, and
that we were missing storing a patch version number along with the major and
minor, which made the version number less useful. The opcode type is also gone
due to non-use.

The version number now reflects the earliest version of Parrot that is capable
of running the bytecode file, to enable cross-version compatibility that will
be needed in the future.


=head3 Segment Header

Having the type associated with the segment inside the VM is fine, but since
it is in the directory segment anyway it seems odd to duplicate it here. Also
removed the id (did not seem to be used anywhere) and the second size (always
computable by knowing the size of this header, so it appears redundant).


=head3 Fixup Segment

We need to support unicode sub names, so fixup labels should be an index into
the constants table to the relevant string instead of just a C string as they
are now.


=head3 Annotations Segment

This is new and replaces and builds upon the debug segment. See here for some
on-list discussion:

L<http://groups.google.com/group/perl.perl6.internals/browse_thread/thread/b0d36dafb42d96c4/4d6ad2ad2243e677?lnk=gst&rnum=2#4d6ad2ad2243e677>


=head3 Packfile PMCs

This idea will see packfiles and segments within them being represented by
PMCs, easing memory management and providing an interface to packfiles for
Parrot programs.

This part of the proposal is based upon a few previous discussions, mostly on
IRC or in realspace. Here is a mailing list comments that provide one of the
motivations or hints of the proposa.

L<http://groups.google.com/group/perl.perl6.internals/browse_thread/thread/778ea0ac4c8676f7/b249306b543b040a?lnk=gst&q=packfile+PMCs&rnum=2#b249306b543b040a>



=head2 Packfiles

This section of the documentation describes the format of Parrot packfiles.
These contain the bytecode (sequence of instructions), constants table, fixup
table, debug data, annotations and possibly more.

Note that, unless otherwise stated, all offsets and lengths are given in terms
of Parrot opcodes, not bytes. An opcode corresponds to a word size in length.
The word size is specified in the packfile header.


=head3 Packfile Header

PBC files start with a variable length header. All data in this header is
stored as strings or in a single byte so endianness and word size need not be
considered when reading it.

Note that in this section only, offsets and lengths are in bytes.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 8      | 0xFE 0x50 0x42 0x43 0x0D 0x0A 0x1A 0x0A                |
  |        |        | Parrot "Magic String" to identify a PBC file. In C,    |
  |        |        | this is the string C<\376PBC\r\n\032\n> (ASCII) or     |
  |        |        | C<\xfe\x50\x42\x43\x0d\x0a\x1a\x0a>.                   |
  +--------+--------+--------------------------------------------------------+
  | 8      | 1      | Word size in bytes of words making up the segments of  |
  |        |        | the PBC file. Must be one of:                          |
  |        |        |    0x04 - 4 byte (32-bit) words                        |
  |        |        |    0x08 - 8 byte (64-bit) words                        |
  +--------+--------+--------------------------------------------------------+
  | 9      | 1      | Byte order within the words making up the segments of  |
  |        |        | the PBC file. Must be one of:                          |
  |        |        |    0x00 - Little Endian                                |
  |        |        |    0x01 - Big Endian                                   |
  +--------+--------+--------------------------------------------------------+
  | 10     | 1      | The encoding of floating point numbers in the file.    |
  |        |        | Must be one of:                                        |
  |        |        |    0x00 - IEEE 754 8 byte double                       |
  |        |        |    0x01 - i386 little endian 12 byte long double       |
  +--------+--------+--------------------------------------------------------+
  | 11     | 1      | Major version number of the version of Parrot that     |
  |        |        | wrote this bytecode file. For example, if Parrot 0.9.5 |
  |        |        | wrote it, this byte would have the value 0.            |
  +--------+--------+--------------------------------------------------------+
  | 12     | 1      | Minor version number of the version of Parrot that     |
  |        |        | wrote this bytecode file. For example, if Parrot 0.9.5 |
  |        |        | wrote it, this byte would have the value 9.            |
  +--------+--------+--------------------------------------------------------+
  | 13     | 1      | Patch version number of the version of Parrot that     |
  |        |        | wrote this bytecode file. For example, if Parrot 0.9.5 |
  |        |        | wrote it, this byte would have the value 5.            |
  +--------+--------+--------------------------------------------------------+
  | 14     | 1      | Major version number of the bytecode file format. See  |
  |        |        | the section below on bytecode file format version      |
  |        |        | numbers.                                               |
  +--------+--------+--------------------------------------------------------+
  | 15     | 1      | Minor version number of the bytecode file format. See  |
  |        |        | the section below on bytecode file format version      |
  |        |        | numbers.                                               |
  +--------+--------+--------------------------------------------------------+
  | 16     | 1      | The type of the UUID associated with this packfile.    |
  |        |        | Must be one of:                                        |
  |        |        |    0x00 - No UUID                                      |
  |        |        |    0x01 - MD5                                          |
  +--------+--------+--------------------------------------------------------+
  | 17     | 1      | Length of the UUID associated with this packfile. May  |
  |        |        | be zero if the type of the UUID is 0x00. Maximum       |
  |        |        | value is 255.                                          |
  +--------+--------+--------------------------------------------------------+
  | 18     | u      | A UUID of u bytes in length, where u was specified as  |
  |        |        | the length of the UUID in the previous field. Be sure  |
  |        |        | that UUIDs are stored and read as strings. The UUID is |
  |        |        | computed by applying the hash function specified in    |
  |        |        | the UUID type field over the entire packfile not       |
  |        |        | including this header and its trailing zero padding.   |
  +--------+--------+--------------------------------------------------------+
  | 18 + u | n      | Zero-padding to make the total header length a         |
  |        |        | multiple of 16 bytes in length.                        |
  |        |        |    n = u % 16 == 0 ? 0 : 16 - (u % 16)                 |
  +--------+--------+--------------------------------------------------------+
  
Everything beyond the header is an opcode, with word length and byte ordering
as defined in the header. If the word length and byte ordering of the machine
that is reading the PBC file do not match these, it needs to transform the
words making up the rest of the packfile.

=head4 Bytecode File Version Numbers

The bytecode file version number exists to decouple the format of the bytecode
file from the version of the Parrot implementation that is reading/writing it.
It has a major and a minor part.

The major version number should be incremented whenever there is a change to
the layout of bytecode files. This includes new segments, changes to segment
headers or changes to the format of the data held within a segment.

The minor version number should be incremented in all other cases when a
change is made that means a previous version of Parrot would not be able to
run the program encoded in the packfile. This includes:

=over 4

=item Opcode renumbering

=item Addition of new opcodes and removal of existing ones

=item Addition of new core PMCs and removal of existing ones

=item Changes to the interface (externally visible behaviour) of an opcode or
PMC

=back

A single version of Parrot can support reading and writing of more than one
bytecode file format. In fact, once Parrot is in production use it will be
preferable to write as early a bytecode format as is possible, to allow the
greatest compatibility with previous Parrots.

These versions will be listed in the PBC_COMPAT file, sorted with the latest
version first in the format:

MAJOR.MINOR DATE DESCRIPTION


=head3 Directory Format Header

Packfiles contain a directory that describes the segments that it contains.
This header specifies the format of the directory.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | The format of the directory. Must be:                  |
  |        |        |    0x01 - Directory Format 1                           |
  +--------+--------+--------------------------------------------------------+
  | 1      | 3      | Must be:                                               |
  |        |        |    0x00 0x00 0x00 - Reserved                           |
  +--------+--------+--------------------------------------------------------+

Currently only Format 1 exists. In the future, the format of the directory may
change. A single version of Parrot may then become capable of generating and
reading files of more than one directory format. This header enables Parrot to
detect whether it is able to read the directory segment in the packfile.

This header must be followed immediately by a directory segment.


=head3 Packfile Segment Header

All segments, regardless of type, start with a 1 opcode segment header. All
other segments below are prefixed with this.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | The total size of the segment in opcodes, including    |
  |        |        | this header.                                           |
  +--------+--------+--------------------------------------------------------+


=head3 Segment Padding

All segments must have trailing zero (NULL) values appended so they are a
multiple of 16 bytes in length. (This allows wordsize support of up to 
128 bits.)


=head3 Directory Segment

This segment lists the other segments that make up the packfile and where in
the file they are located. It must occur immediately after the directory
format header. Only one of these segments may occur in a packfile. In the
future, a hierarchy of directories may be allowed.

The directory segment adds one additional header after the standard packfile
header data, which specifies the number of entries in the directory.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | The number of entries in the directory.                |
  |        |        |    n                                                   |
  +--------+--------+--------------------------------------------------------+

Following this are n variable length entries formatted as described in the
following table. Offsets are in words, but are given relative to the start of
an individual entry.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | The type of the segment. Must be one of the following: |
  |        |        |    0x00 - Reserved (Directory Segment)                 |
  |        |        |    0x01 - Default Segment                              |
  |        |        |    0x02 - Fixup Segment                                |
  |        |        |    0x03 - Constant Table Segment                       |
  |        |        |    0x04 - Bytecode Segment                             |
  |        |        |    0x05 - Annotations Segment                          |
  |        |        |    0x06 - PIC Data Segment                             |
  |        |        |    0x07 - Dependencies Segment                         |
  +--------+--------+--------------------------------------------------------+
  | 1      | n      | The name of the segment, as a (NULL terminated) ASCII  |
  |        |        | C string. This must be padded with trailing NULL       |
  |        |        | (zero) values to be a full word in size.               |
  +--------+--------+--------------------------------------------------------+
  | n + 1  | 1      | The offset to the segment, relative to the start of    |
  |        |        | the packfile. Specified as a number of words, where    |
  |        |        | the word size is that specified in the header. (Parrot |
  |        |        | may need to do some computation to transform this to   |
  |        |        | an offset in terms of its own word size.) As segments  |
  |        |        | must always be aligned on 16-byte boundaries, this     |
  |        |        | scheme scales up to 128-bit platforms.                 |
  +--------+--------+--------------------------------------------------------+
  | n + 2  | 1      | The length of the segment, including its header, in    |
  |        |        | words. This must match the length stored at the start  |
  |        |        | of the header of the segment the entry is describing.  |
  +--------+--------+--------------------------------------------------------+


=head3 Default Segment

The default segment has no additional headers. It will, if possible, be memory
mapped. More than one may exist in the packfile, and they are identified by
name. They may be used for storing any data that does not fit into any other
segment, for example the source code from a high level language.


=head3 Bytecode Segment

This segment has no additonal headers. It stores a stream of instructions in
bytecode format. Instructions have variable length. Each instruction starts
with an operation code.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | A valid Parrot operation code, as specified in the     |
  |        |        | operation codes list.                                  |
  +--------+--------+--------------------------------------------------------+

Zero or more operands follow the operation code. Most instructions take a
fixed number of operands but several of them take a variable number, with the
first operand being used to determine the number of additional operands that
follow. This tends to be stored as a PMC constant, meaning that decoding the
instruction stream not only requires knowledge of the operands that each
instruction takes but also the ability to thaw PMCs.

An individual operand is always one word in length and may be of one of the
following forms.

  +------------------+-------------------------------------------------------+
  | Operand Type     | Description                                           |
  +------------------+-------------------------------------------------------+
  | Register         | An integer specifying a register number.              |
  +------------------+-------------------------------------------------------+
  | Integer Constant | An integer that is the constant itself. That is, the  |
  |                  | constant is stored directly in the instruction        |
  |                  | stream. Storing integer constants of length greater   |
  |                  | than 32 bits has undefined behaviour and should be    |
  |                  | considered unportable.                                |
  +------------------+-------------------------------------------------------+
  | Number Constant  | An index into the constants table.                    |
  +------------------+-------------------------------------------------------+
  | String Constant  | An index into the constants table.                    |
  +------------------+-------------------------------------------------------+
  | PMC Constant     | An index into the constants table.                    |
  +------------------+-------------------------------------------------------+


=head3 Constants Segment

This segment stores number, string and PMC constants. It adds one extra field
to its header.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 2      | 1      | The number of constants in the table.                  |
  |        |        |    n                                                   |
  +--------+--------+--------------------------------------------------------+

Following this are n constants, each with a single word header specifying the
type of constant that follows.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | The type of the constant. Must be one of:              |
  |        |        |    0x00 - No constant                                  |
  |        |        |    0x6E - Number constant (ASCII 'n')                  |
  |        |        |    0x73 - String constant (ASCII 's')                  |
  |        |        |    0x70 - PMC constant (ASCII 'p')                     |
  |        |        |    0x6B - Key constant (ASCII 'k')                     |
  +--------+--------+--------------------------------------------------------+

All constants that are not a multiple of the word size in length must be
padded with trailing zero bytes up to a word size boundary.

=head4 Number Constants

The number is stored in the format defined in the Packfile header. Any padding
that is needed will follow.

=head4 String Constants

String constants are stored in the following format, with offsets relative to
the start of the constant including its type.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | Flags, copied from the string structure.               |
  +--------+--------+--------------------------------------------------------+
  | 2      | 1      | Character set; either the index of a built-in one or a |
  |        |        | dynamically loaded one whose index is in a range given |
  |        |        | in the dependencies table.                             |
  +--------+--------+--------------------------------------------------------+
  | 3      | 1      | Encoding, either the index of a built-in one or a      |
  |        |        | dynamically loaded one whose index is in a range given |
  |        |        | in the dependencies table.                             |
  +--------+--------+--------------------------------------------------------+
  | 4      | 1      | Length of the string data in bytes.                    |
  +--------+--------+--------------------------------------------------------+
  | 5      | n      | String data with trailing zero padding as required.    |
  +--------+--------+--------------------------------------------------------+

=head4 PMC Constants

PMCs that can be saved in packfiles as constants implement the freeze and thaw
v-table methods. Their frozen data is placed in a string, stored in the same
format as a string constant.

=head4 Key Constants

Key constants are made up a number of components, where one component is a
"dimension" in the key. The number of components in the key is stored at the
start of the constant.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | Number of key components that follow.                  |
  |        |        |    n                                                   |
  +--------+--------+--------------------------------------------------------+

Following this are n entries of two words each that specify the key's type and
value. The key value may be a register or another constant, but not another key
constant. All constants other than integer constants are indexes into the
constants table.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | Type of the key. Must be one of:                       |
  |        |        |    0x00 - Integer register                             |
  |        |        |    0x01 - String register                              |
  |        |        |    0x02 - PMC register                                 |
  |        |        |    0x03 - Number register                              |
  |        |        |    0x10 - Integer constant                             |
  |        |        |    0x11 - String constant (constant table index)       |
  |        |        |    0x12 - PMC constant (constant table index)          |
  |        |        |    0x13 - Number constant (constant table index)       |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | Value of the key.                                      |
  +--------+--------+--------------------------------------------------------+

{{ TODO: Figure out slice bits and document them here. }}


=head3 Fixup Segment

The fixup segment maps names of subs to offsets in the bytecode stream. It
adds one extra field to its header.

{{ TODO: I think label fixups are no longer used. Check if that is so. }}

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | Number of fixup table entries that follow.             |
  |        |        |    n                                                   |
  +--------+--------+--------------------------------------------------------+

This is followed by n fixup table entries, of variable length, that take the
following form.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | Type of the fixup. Must be:                            |
  |        |        |    0x01 - Subroutine fixup                             |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | The label that is being fixed up. A string constant,   |
  |        |        | stored as an index into the constants table.           |
  +--------+--------+--------------------------------------------------------+
  | 2      | 1      | For subroutine fixups, this is an index into the       |
  |        |        | constants table for the sub PMC corresponding to the   |
  |        |        | label.                                                 |
  +--------+--------+--------------------------------------------------------+


=head3 Annotations Segment

Annotations allow any instruction in the bytecode stream to have zero or more
key/value pairs associated with it. These can be retrived at runtime. High
level languages can use annotations to store file names, line numbers, column
numbers and any other data, for debug purposes or otherwise, that they need.

The segment comes in three parts:

=over 4

=item A list of annotation keys (for example, "line" and "file").

=item An annotation groups table, used to group together annotations for a
particular HLL source file (an annotation group starting clears all active
annotations, so they will not spill over between source files; it also
allows for faster lookup of annotations).

{{ TODO: Does it clear all annotations, or all annotation groups? }}

=item A list of indexes into the bytecode stream and key/value pairings (for
example, starting at instruction 235, the annotation "line" has value "42").

=back

The first word in the segment supplies the number of keys.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | Number of annotation key entries that follow.          |
  |        |        |    n                                                   |
  +--------+--------+--------------------------------------------------------+

Following this are n annotation key entries. There is one entry per key (such
as "line" or "file"), but the bytecode may be annotated many times with that
key. Key entries take the following format.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | Index into the constants table of a string containing  |
  |        |        | the name of the key.                                   |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | The type of value that is stored with the key.         |
  |        |        |    0x00 - Integer                                      |
  |        |        |    0x01 - String Constant                              |
  |        |        |    0x02 - Number Constant                              |
  |        |        |    0x03 - PMC Constant                                 |
  +--------+--------+--------------------------------------------------------+

The annotation groups table comes next. This starts with a single integer to
specify the number of entries in the table.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | Number of annotation group entries that follow.        |
  |        |        |    n                                                   |
  +--------+--------+--------------------------------------------------------+

A group entry maps an offset in the bytecode segment to an offset in the list
of annotations (that is, offset 0 refers to the first word following this
table). The list of offsets into the bytecode segment (and by the definition
of this segment, the offsets into the annotations list) must be in ascending
order.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | Offset into the bytecode segment where the             |
  |        |        | instructions for a particular high level source file   |
  |        |        | start.                                                 |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | Offset into the annotations list specifying where the  |
  |        |        | annotations for the given instruction start.           |
  +--------+--------+--------------------------------------------------------+

The rest of the segment is made up of a sequence of instructions to key and
value mappings, taking the following format.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | Offset into the bytecode segment, in words, of the     |
  |        |        | instruction being annotated. At runtime, this will     |
  |        |        | correspond to the program counter.                     |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | The key of the annotation, specified as an index into  |
  |        |        | the zero-based list of keys specified in the first     |
  |        |        | part of the segment. That is, if key "line" was the    |
  |        |        | first entry and "file" the second, they would have     |
  |        |        | indices 0 and 1 respectively.                          |
  +--------+--------+--------------------------------------------------------+
  | 2      | 2      | The value of the annotation. If the annotation type    |
  |        |        | (specified with the key) is an integer, the value is   |
  |        |        | placed directly into this word. Otherwise, an index    |
  |        |        | into the constants table is used.                      |
  +--------+--------+--------------------------------------------------------+

Note that the value of an annotation with a particular key is taken to apply
to all following instructions up to the point of a new value being specified
for that key with another annotation. This means that if 20 instructions make
up the compiled form of a single line of code, only one line annotation is
required. Note that this also implies that annotations must be placed in
the same order as the instructions.


=head3 Dependencies Segment

This segment holds a table of external and possibly dynamically loaded items
that are needed for this packfile to run. This includes:

=over 4

=item Dynamic PMC libraries (.loadlib)

=item Dynamic opcode libraries (.loadlib)

=item Dynamically loaded string encoding

=item Dynamically loaded character set

=back

The segment starts with the number of entries in the table.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 1      | 1      | Number of entries in the dependencies table.           |
  |        |        |    n                                                   |
  +--------+--------+--------------------------------------------------------+

Following this are n entries of variable length, taking the following format.

  +--------+--------+--------------------------------------------------------+
  | Offset | Length | Description                                            |
  +--------+--------+--------------------------------------------------------+
  | 0      | 1      | Number of entries in the dependencies table.           |
  |        |        |    0x00 - Dynamic PMC Library                          |
  |        |        |    0x01 - Dynamic Opcode Library                       |
  |        |        |    0x02 - Dynamically Loaded String Encoding           |
  |        |        |    0x03 - Dynamically Loaded Character Set             |
  +--------+--------+--------------------------------------------------------+
  | 1      | n      | A hint for finding and loading the resource; usually   |
  |        |        | the name of the dynamic library, but possibly a full   |
  |        |        | path too. Given as an ASCII NULL-terminated string,    |
  |        |        | zero-padded to a full word.                            |
  +--------+--------+--------------------------------------------------------+
  | n + 1  | 1      | The lowest index for the given type of resource that   |
  |        |        | is contained in this dependency. For example, if this  |
  |        |        | entry was for a dynamic opcode library containing ops  |
  |        |        | numbered 5000 through 5042, this entry would be 5000.  |
  +--------+--------+--------------------------------------------------------+
  | n + 2  | 1      | The highest index for the given type of resource that  |
  |        |        | is contained in this dependency. For example, if this  |
  |        |        | entry was for a dynamic opcode library containing ops  |
  |        |        | numbered 5000 through 5042, this entry would be 5042.  |
  +--------+--------+--------------------------------------------------------+



=head2 Packfile PMCs

A packfile will be represented in memory by Parrot as a tree of PMCs. These
will provide a programatic way to construct and walk packfiles, both for the
Parrot internals and from programs running on the Parrot VM.

{{ TODO... QUESTION: Will the CStruct PMC make it into Parrot? If so, we
may want to change the interface of these PMCs to take advantage of it. 
ANSWER: Yes it will (most likely with a different name), but it needs to
be prototyped first. Do you want to hold off on implementing the
bytecode changes until it's available? I say go ahead and implement now,
then revise when the new PMC is available. }}


=head3 Packfile.pmc

This PMC represents the packfile overall. It will be constructed by the VM
when reading a packfile. It implements the following methods.

=head4 get_string (v-table)

Serializes this packfile data structure into a bytestream ready to be written
to disk (that is, maps from PMCs to on-disk representation).

=head4 set_string_native (v-table)

Takes a string containing an entire packfile in the on-disk format, attempts
to unpack it into a tree of Packfile PMCs and sets this Packfile PMC to 
represent the top of that tree (that is, maps from on-disk representation to a
tree of PMCs).

=head4 get_integer_keyed_str (v-table)

Used to get data about fields in the header that have an integer value. Valid
keys are:

=over 4

=item wordsize

=item byteorder

=item fptype

=item version_major

=item version_minor

=item version_patch

=item bytecode_major

=item bytecode_minor

=item uuid_type

=item uuid_length

=back

=head4 get_string_keyed_str (v-table)

Used to get data about fields in the header that have a string value. Valid
keys are:

=over 4

=item uuid

=back

=head4 set_integer_keyed_str (v-table)

Used to set fields in the packfile header. Some fields are not allowed to be
written since they are determined by the VM when serializing the packfile for
storage on disk. The fields that may be set are:

=over 4

=item version_major

=item version_minor

=item version_patch

=item uuid_type

=back

Be very careful when setting a version number; you should usually trust the VM
to do the right thing with this.

Setting the uuid_type will not result in immediate re-computation of the UUID,
but rather will only cause it to be computed using the selected algorithm when
the packfile is serialized (by calling the get_string v-table method). Setting
an invalid uuid_type value will cause an exception to be thrown immediately.

=head4 get_directory()

Returns the PackfileDirectory PMC that represents the directory segment at the
start of the packfile.


=head3 PackfileSegment.pmc

An abstract PMC that is the base class for all other segments. It has two
abstract methods, which are to be implemented by all subclasses. They will not
be listed under the method list for other segment PMCs to save space.

=head4 STRING* pack()

Packs the segment into the on-disk format and returns a string holding it.

=head4 unpack(STRING*)

Takes the packed representation for a segment of the given type and then
unpacks it, setting this PMC to represent that segment as a result of the
unpacking. If an error occurs during the unpacking process, an exception will
be thrown.


=head3 PackfileDirectory.pmc (isa PackfileSegment)

This PMC represents a directory segment. Essentially it is an array of
PackfileSegment PMCs. When indexed using an integer key, it gets the segment
at that positiion in the segments table. When indexed using a string key, it
looks for a segment of that name. It implements the following methods.

=head4 elements (v-table)

Gets the number of segments listed in the directory.

=head4 get_pmc_keyed_int (v-table)

Gets a PackfileSegment PMC or an appropriate subclass of it representing the
segment at the specified index in the directory segment.

=head4 get_string_keyed_int (v-table)

Gets a string containing the name of the segment at the specified index in the
directory segment.

=head4 get_pmc_keyed_str (v-table)

Searches the directory for a segment with the given name and, if one exists,
returns a PackfileSegment PMC (or one of its subclasses) representing it.

=head4 set_pmc_keyed_str (v-table)

Adds a PackfileSegment PMC (or a subclass of it) to the directory with the
name specified by the key. This is the only way to add another segment to the
directory. If a segment of the given name already exists in the directory, it
will be replaced with the supplied PMC.


=head3 RawSegment.pmc (isa PackfileSegment)

This PMC presents a segment of a packfile as an array of integers. This is the
lowest possible level of access to a segment, and covers both the default and
bytecode segment types. It implements the following methods.

=head4 get_integer_keyed_int (v-table)

Reads the integer at the specified offset into the segment, excluding the data
in the common segment header but including the data making up additional
fields in the header for a specific type of segment.

=head4 set_integer_keyed_int (v-table)

Stores an integer at the specified offset into the segment. Will throw an
exception if the segment is memory mapped.

=head4 elements (v-table)

Gets the length of the segment in words, excluding the length of the common
segment but including the data making up additional fields in the header for a
specific type of segment.


=head3 PackfileConstantTable.pmc (isa PackfileSegment)

This PMC represents a constants table. It provides access to constants through
the keyed integer interface (the interpreter may choose to access underlying
structures directly to improve performance, however). 

The table of constants can be added to using the keyed set methods; it will
grow automatically.

The PMC implements the following methods.

=head4 elements (v-table)

Gets the number of constants contained in the table.

=head4 get_number_keyed_int (v-table)

Gets the value of the number constant at the specified index in the constants
table. If the constant at that position in the table is not a number, an
exception will be thrown.

=head4 get_string_keyed_int (v-table)

Gets the value of the string constant at the specified index in the constants
table. If the constant at that position in the table is not a string, an
exception will be thrown.

=head4 get_pmc_keyed_int (v-table)

Gets the value of the PMC or key constant at the specified index in the
constants table. If the constant at that position in the table is not a PMC
or key, an exception will be thrown.

=head4 set_number_keyed_int (v-table)

Sets the value of the number constant at the specified index in the constants
table. If the constant at that position in the table is not already a number
constant, an exception will be thrown. If it does not exist, the table will be
extended.

=head4 set_string_keyed_int (v-table)

Sets the value of the string constant at the specified index in the constants
table. If the constant at that position in the table is not already a string
constant, an exception will be thrown. If it does not exist, the table will be
extended.

=head4 set_pmc_keyed_int (v-table)

Sets the value of the PMC or key constant at the specified index in the
constants table. If the constant at that position in the table is not already
a PMC or key constant, an exception will be thrown. If it does not exist, the
table will be extended.

=head4 int get_type(int)

Returns an integer value denoting the type of the constant at the specified
index. Possible values are:

  +--------+-----------------------------------------------------------------+
  | Value  | Constant Type                                                   |
  +--------+-----------------------------------------------------------------+
  | 0x00   | No Constant                                                     |
  +--------+-----------------------------------------------------------------+
  | 0x6E   | Number Constant                                                 |
  +--------+-----------------------------------------------------------------+
  | 0x73   | String Constant                                                 |
  +--------+-----------------------------------------------------------------+
  | 0x70   | PMC Constant                                                    |
  +--------+-----------------------------------------------------------------+
  | 0x6B   | Key Constant                                                    |
  +--------+-----------------------------------------------------------------+


=head3 PackfileFixupTable.pmc (isa PackfileSegment)

This PMC provides a keyed integer interface to the fixup table. Each entry in
the table is represented by a PackfileFixupEntry PMC. It implements the
following methods.

=head4 elements (v-table)

Gets the number of entries in the fixup table.

=head4 get_pmc_keyed_int (v-table)

Gets a PackfileFixupEntry PMC for the fixup entry at the position given in
the key. If the index is out of range, an exception will be thrown.

=head4 set_pmc_keyed_int (v-table)

Used to add a PackfileFixupEntry PMC to the fixups table or to replace an
existing one. If the PMC that is supplied is not of type PackfileFixupEntry,
an exception will thrown.


=head3 PackfileFixupEntry.pmc

This PMC represents an entry in the fixup table. It implements the following
methods.

=head4 get_string (v-table)

Gets the label field of the fixup entry.

=head4 set_string_native (v-table)

Sets the label field of the fixup entry.

=head4 get_integer (v-table)

Gets the offset field of the fixup entry.

=head4 set_integer_native (v-table)

Sets the offset field of the fixup entry.

=head4 int get_type()

Gets the type of the fixup entry. See the entries table for possible fixup
types.

=head4 set_type(int)

Sets the type of the fixup entry. See the entries table for possible fixup
types. Specifying an invalid type will result in an exception.


=head3 PackfileAnnotations.pmc (isa PackfileSegment)

This PMC represents the bytecode annotations table. The key ID to key name and
key type mappings are stored in a separate PackfileAnnotationKeys PMC. Each 
(offset, key, value) entry is represented by a PackfileAnnotation PMC. The
following methods are implemented.

=head4 PMC* get_key_list()

Returns a PackfileAnnotationKeys PMC containing the names and types of the
annotation keys. Fetch and add to this to create a new annotation key.

=head4 elements (v-table)

Gets the number of annotations in the table.

=head4 get_pmc_keyed_int (v-table)

Gets the annotation at the specified index. If there is no annotation at that
index, an exception will be thrown. The PMC that is returned will always be a
PackfileAnnotation PMC.

=head4 set_pmc_keyed_int (v-table)

Sets the annotation at the specified index. If there is no annotation at that
index, it is added to the list of annotations. An exception will be thrown
unless all of the following conditions are met:

=over 4

=item The type of the PMC passed is PackfileAnnotation

=item The entry at the previous index is defined

=item The offset of the previous entry is less than this entry

=item The offset of the next entry, if it exists, is greater than this entry

=item The key ID references a valid annotation key

=back


=head3 PackfileAnnotationKeys.pmc

This PMC represents the table of keys and the type of value that is stored
against that key. It implements the following methods.

=head4 get_string_keyed_int (v-table)

Gets the name of the annotation key specified by the index. An exception will
be thrown if the index is out of range.

=head4 set_string_keyed_int (v-table)

Sets the name of the annotation key specified by the index. If there is no key
with that index currently, a key at that position in the table will be added.

=head4 get_integer_keyed_int (v-table)

Gets an integer representing the type of the value that is stored with the key
at the specified index. An exception will be thrown if the index is out of
range.

=head4 set_integer_keyed_int (v-table)

Sets the type of the value this is stored with the key at the specified index.
If there is no key with that index currently, a key at that position in the
table will be added.


=head3 PackfileAnnotation.pmc

This PMC represents an individual bytecode annotation entry in the annotations
segment. It implements the following methods.

=head4 int get_offset()

Gets the offset into the bytecode of the instruction that is being annotated.

=head4 set_offset(int)

Sets the offset into the bytecode of the instruction that is being annotated.

=head4 int get_key_id()

Gets the ID of the key of the annotation.

=head4 int set_key_id()

Sets the ID of the key of the annotation.

=head4 get_integer (v-table)

Gets the value of the annotation. This may be, depending upon the type of the
annotation, an integer annotation or an index into the constants table.

=head4 set_integer (v-table)

Sets the value of the annotation. This may be, depending upon the type of the
annotation, an integer annotation or an index into the constants table.


=head1 LANGUAGE NOTES

None.


=head1 ATTACHMENTS

None.


=head1 FOOTNOTES

None.


=head1 REFERENCES

None.


=cut

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