Skip to content

HTTPS clone URL

Subversion checkout URL

You can clone with
or
.
Download ZIP
branch: master
Fetching contributors…

Cannot retrieve contributors at this time

159 lines (98 sloc) 4.346 kB

NAME

docs/dev/byteorder.pod - Byteorder Conversion Functions

DESCRIPTION

This document addresses the byteorder conversion functions for the Parrot Virtual Machine.

OVERVIEW

The platform byteorder is stored for C code in include/parrot/config.h

  #define PARROT_BYTEORDER        0x1234

for parrot code in

or for perl code via

  use Parrot::Config;
  $PConfig{byteorder};

The byteorder values are analog to perl, see "pack" in perlfunc:

  1234  little-endian 32-bit, 12345678  little-endian 64-bit
  4321  big-endian 32-bit,    87654321  big-endian 64-bit

When reading a pbc stored in a different architecture, the pbc header defines the pbc byteorder for the architecture which stored the pbc, and the src/packfile/pf_items.c functions are used to convert the values to the native endianness, wordsize and ptrsize.

The byteorder code will check the endianness of an INTVAL or an 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 are be placed in the PackFile vtable and are be called when necessary. The Parrot interpreter is be smart enough to avoid calling these functions when converting from and to the same byteorder.

DATA STRUCTURES AND ALGORITHMS

The algorithm to change from one endianness to another is identical and simple to understand. Basically, the size of an INTVAL or 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.

IMPORTANT FUNCTIONS

fetch_iv_le

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

fetch_iv_be

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

fetch_op_le

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

fetch_op_be

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

fetch_buf_le_(4,8,12,16)

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

fetch_buf_be_(4,8,12,16)

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

LOW LEVEL FLOATVAL FETCH AND CONVERT FUNCTIONS

We support two different floattypes, stored in the pbc header as 0 or 1.

  Floattype 0 = IEEE-754 8 byte double
  Floattype 1 = x86 little endian 12 byte long double
cvt_num12_num8

Converts i386 LE 12-byte long double to IEEE 754 8 byte double.

cvt_num12_num8_be

Converts a 12-byte i386 long double into a big-endian IEEE 754 8-byte double. Converting to BE is not yet implemented (throws internal_exception).

cvt_num12_num8_le

Converts a 12-byte i386 long double into a little-endian IEEE 754 8-byte double.

UNIMPLEMENTED FUNCTIONS

endianize_fetch_int

Fetch an INTVAL directly from a bytestream

endianize_put_int

Put an INTVAL directly on a bytestream

HISTORY

Initial version by Melvin on 2002-01-05, more byteorder explanations by Reini Urban 2009-02-02

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 INTVAL's and opcode_t's is made.

REFERENCES

The fetch and transformer functions are implemented in src/packfile/pf_items.c

Jump to Line
Something went wrong with that request. Please try again.