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Byte Ordering and Bit Coding Convention
Byte Ordering and Bit Coding Convention
The PCI bus uses the Little Endian Byte ordering: byte 0 in a 32-bit word is the Least
Significant Byte (LSB) from an arithmetic point of view and is noted D(7:0). The PowerPC
architecture uses the Big Endian Byte ordering: byte 0 in a 32-bit word is the Most
Significant Byte (MSB) from an arithmetic point of view and is noted D(31:24).
The PowerPC architecture uses the very unusual Little Endian Bit convention, where bit 0
is on the left and is the most significant bit. Unless otherwise noted, this document does not
use this convention. Instead, it uses the classical bit coding convention, where bit 0 (on the
right) is the least significant bit and bit i is the 2
convention. This coding convention applies to data, addresses, and bit fields. In the
following figure, MSB means Most Significant Byte and LSB Least Significant Byte:
The standard C convention is used to identify the numeric format of arithmetical values:
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•
For example 0x12 = 18.
Type Definition
Only a few basic types are used:
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Code Examples
This document provides several algorithm descriptions presented in PowerPC assembly
language and in C language.
Icon Conventions
Icons draw your attention to especially important information:
N
OTE
The Note icon indicates important points of interest related to the current subject.
xii
No prefix for decimal values
0x prefix for a hexadecimal value
byte: unsigned, coded as 8 bits
word: unsigned, coded as two contiguous bytes, most significant first
dword: unsigned, coded as two contiguous words, most significant first
i
weight bit. This is the Big Endian Bit
Interphase Corporation
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