Technical Note 	: SLE0004

Author	: Scott Evans

Created/Modified	: 30/10/97

Description	: An explanation of byte ordering

The term byte ordering refers to the order in which a computer stores
bytes in memory. This document describes the little endian and big
endian byte ordering methods. The R3000 (Yaroze CPU) uses the little
endian method.

Big Endian

The hexadecimal number 0x1234 will occupy two bytes of memory. Each
hexadecimal digit can be represented by 4 bits (a nibble). So 0x12 will
be stored in one byte and 0x34 will be stored in another byte.

A big endian computer would store the number 0x1234 in memory as
follows, where A is a pointer to a byte in memory.

Starting with the leftmost byte of the number and working towards the
right. 0x12 will be stored at A and the last byte 0x34 will be stored at
A+1. In memory this would look like.

A	A+1

0x12	0x34



The same applies for larger numbers. A 32 bit number (4 bytes)
0x12345678 will be stored at address A as follows. The leftmost byte
0x12 will be stored at A, the next byte 0x34 at A+1, the next 0x56 and
A+2 and the last byte 0x78 at A+3.

A	A+1	A+2	A+3

0x12	0x34	0x56	0x78



Little Endian

Little endian computers on the other hand use the opposite method to big
endian computers. The number 0x1234 would be stored at location A as
follows.

This time we start with the rightmost byte of the number 0x34 and place
this at address A. The next byte (working left) 0x12 is then placed at
A+1.

A	A+1

0x34	0x12



It has the same byte ordering as a big endian computer storing the
number 0x3412.

The number 0x12345678 will be stored as the number 0x78563412 on a big
endian computer. So the rightmost byte 0x78 is stored at A, the next
byte 0x56 is stored at A+1, the next byte 0x34 is stored at A+2 and the
last byte 0x12 is stored at A+3.

A	A+1	A+2	A+3

0x78	0x56	0x34	0x12