Byte Ordering
MPC885 PowerQUICC Family Reference Manual, Rev. 2
A-2
Freescale Semiconductor
A.3
BE Mode
As shown in
Table A-1
, the MPC885 powers up in BE mode. In BE mode, the caches, internal registers,
the U-bus, and the external bus all use big-endian byte ordering. In BE mode, no address modification nor
data-byte-lane swapping is performed by any of the byte-ordering mechanisms of the MPC885.
The PowerPC architecture defines two bits in the MSR for specifying byte ordering—LE (little-endian
mode) and ILE (exception little-endian mode). In this microprocessor, these bits only control the addresses
generated by the MPC8xx core. The LE bit specifies the endian mode for normal core operation and ILE
specifies the mode to be used when an exception handler is invoked. That is, when an exception occurs,
the ILE bit (as set for the interrupted process) is copied into MSR[LE] to select the endian mode for the
context established by the exception. For both bits, a value of 0 specifies BE mode (or TLE mode,
depending on DC_CST[LES]), and a value of 1 specifies PPC-LE mode.
A.4
TLE Mode
When operating in TLE mode, the external bus uses little-endian byte ordering, so any external agents
should use little-endian byte ordering to access memory. Note however, that internal to the microprocessor,
the caches and internal registers use big-endian byte ordering. The byte-ordering mechanisms for TLE
mode are shown in
Figure A-1
.
Figure A-1. TLE Mode Mechanisms
For TLE mode, MSR[LE] and MSR[ILE] should be cleared as in BE mode. (This disables the 3-bit address
munging used in PPC-LE mode. See
Section A.5, “PPC-LE Mode,”
for more information.)
2-Bit Munge
I-Cache
D-Cache
External bus
MPC8xx Core
MSR[LE]=0
DC_CST[LES]=1
MSR[ILE]=0
DC_CST[LES]=1
SIU
CPM
SDMA
FCR[BO]=1x
U-Bus
2-Bit UnMunge
and Byte Swap
for Accesses
Initiated by the
MPC8xx Core
MPC885
Summary of Contents for PowerQUICC MPC870
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