# Copyright: 2001-2004 The Perl Foundation.  All Rights Reserved.
# $Id: byteorder.dev 5463 2004-02-28 00:30:43Z mikescott $

=head1 NAME

docs/dev/byteorder.dev - Byteorder Conversion Functions

=head1 Overview

The byteorder code will check the endianness of an C<INTVAL> or
an C<opcode_t> value and swap from little to big, or big to little
when appropriate.  Functions also exist to convert a 4, 8, 12,
or 16 byte character buffer to big or little endian.
The functions will be placed in the PackFile vtable and will be called 
when necessary. The Parrot interpreter should be smart enough to avoid
calling these functions when converting from and to the same byteorder.

=head1 Data Structures and Algorithms

The algorithm to change from one endianness to another is
identical and simple to understand.  Basically,
the size of an C<INTVAL> or C<opcode_t> is used to
determine at compile time how many bits should
be shifted around.  Then the correct bits are shifted
by the correct amounts (please look at source code for
exact amounts).  The buffer change functions are implemented
by a straight forward algorithm that assigns swaps all
of the bytes.

=head1 Important Functions

=over 4

=item C<fetch_iv_le> 

This function will convert an C<INTVAL> into
little endian format.  It is a no-op if the native
format is already little endian.

=item C<fetch_iv_be>

This function will convert an C<INTVAL> into
big endian format. It is a no-op if the native
format is already big endian.

=item C<fetch_op_le>

This function will convert an C<opcode_t> into
little endian format. It is a no-op if the native
format is already little endian.

=item C<fetch_op_be>

This function will convert an C<opcode_t> into
big endian format. It is a no-op if the native
format is already big endian.

=item C<fetch_buf_le_>(4,8,12,16)

This set of functions
will convert an unsigned character buffer into
little endian format.  Only a C<memcpy> is performed
if the native format is already little endian.

=item C<fetch_buf_be_>(4,8,12,16)

This set of functions
will convert an unsigned character buffer into
big endian format.  Only a C<memcpy> is performed
if the native format is already big endian.

=back

=head1 Unimplemented Functions

=over 4

=item C<endianize_fetch_int>

Fetch an C<INTVAL> directly from a bytestream

=item C<endianize_put_int>

Put an C<INTVAL> directly on a bytestream

=back

=head1 History

Initial version by Melvin on 2002/05/01

=head1 Notes

This assumes big or little endianness...other, more
esoteric forms (such as middle endian) are not supported.
Also, an assumption of 4 or 8 byte C<INTVAL>'s
and C<opcode_t>'s is made.

=head1 References
