MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
A-1
Appendix A
Byte Ordering
This microprocessor supports three byte-ordering conventions—big-endian (BE), true little-endian (TLE),
and modified little-endian (MOD-LE). This chapter describes each of the three endian modes. Chapter 3,
“Operand Conventions,” in Programming Environments Manual for 32-bit Implementations of the
PowerPC Architecture, provides a general overview of byte ordering.
A.1
Byte Ordering Overview
For big-endian byte ordering, the most-significant byte (MSB) is stored at the lowest address while the
least-significant byte (LSB) is stored at the highest address. This is called big-endian because the big end
of the scalar comes first in memory.
For true little-endian byte ordering, the LSB is stored at the lowest address while the MSB is stored at the
highest address. This is called true little-endian because the little end of the scalar comes first in memory.
For modified little-endian byte ordering (also referred to as ‘munged little-endian’), the address of data is
modified so that the memory structure appears little-endian to the executing processor, when in fact, the
byte ordering is big-endian. The address modification is called ‘munging’. Note that the term ‘munging’
is not defined or used in the PowerPC architecture specification.
A.2
Byte-Ordering Mechanisms
There are several byte-ordering mechanisms that are controlled by programmable parameters. The
MSR[LE] and MSR[ILE] bits control a 3-bit address modifier in the MPC8xx core. The DC_CST[LES]
bit controls a 2-bit address modifier in the core and a 2-bit address modifier and byte lane swapper in the
SIU. The FCR[BO] field of each peripheral (SCCs, SMCs, SPI, I
2
C, PIP, or IDMA) controls a 3-bit
address modifier in the SDMA.
Table A-1
correlates the programmable parameters with the byte-ordering
modes of operation.
Table A-1. Byte-Ordering Parameters
Byte-Ordering Mode
Parameter
MSR[LE] or MSR[ILE]
DC_CST[LES]
FCR[BO]
BE
0
0
1x
TLE
0
1
1x
PPC-LE
1
0
01
Note:
The MPC885 powers up in BE mode.
Содержание PowerQUICC MPC870
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