/ 
parrot-0.7.1
/ docs/dev/byteorder.pod

NAME

docs/dev/byteorder.pod - Byteorder Conversion Functions

Overview

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 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.

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.

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 2002/05/01

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