Byte Ordering
MPC885 PowerQUICC Family Reference Manual, Rev. 2
A-6
Freescale Semiconductor
Figure A-3. PPC-LE Mode Mechanisms
For PPC-LE mode, MSR[LE] and MSR[ILE] should be set. When the MSR[LE] is set, the effective
address is modified (munged) by the core before being used to access the caches (or memory for
caching-inhibited accesses). The three low-order address bits of the effective address are exclusive-ORed
(XOR) with a three-bit value that depends on the length of the operand (1, 2, or 4 bytes), as shown in
Table A-6. This process is called ‘3-bit munging.’
The munged physical address is passed to the cache or to external memory, and the specified width of data
is transferred (in big-endian order—that is, MSB at the lowest address, LSB at the highest address). In
PPC-LE mode, only the address is modified, not the byte order. Munging makes it appear to the core that
individual aligned scalars are stored in little-endian order, when in fact, they are stored in big-endian order,
but at different byte addresses within double words. Note that the instruction and data caches operate less
efficiently when address munging is performed on cache accesses. Some performance degradation should
be expected when the MPC885 is operating in PPC-LE mode.
Table A-6. PPC-LE 3-bit Munging
Data Width (Bytes)
EA Modification
4
XOR with 0b100
2
XOR with 0b110
1
XOR with 0b111
Note that the MPC885 does not support 8-byte scalars.
3-Bit Munge
I-Cache
External bus
D-Cache
MPC8xx Core
MSR[LE]=1
DC_CST[LES]=0
MSR[ILE]=1
DC_CST[LES]=0
SIU
3-Bit Munge
CPM
SDMA
FCR[BD]=01
U-Bus
MPC885
Содержание PowerQUICC MPC870
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Страница 158: ...The MPC8xx Core MPC885 PowerQUICC Family Reference Manual Rev 2 3 18 Freescale Semiconductor ...
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Страница 606: ...SDMA Channels and IDMA Emulation MPC885 PowerQUICC Family Reference Manual Rev 2 19 20 Freescale Semiconductor ...
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Страница 780: ...SCC Ethernet Mode MPC885 PowerQUICC Family Reference Manual Rev 2 27 24 Freescale Semiconductor ...
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Страница 944: ...Parallel I O Ports MPC885 PowerQUICC Family Reference Manual Rev 2 34 26 Freescale Semiconductor ...
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Страница 1004: ...Buffer Descriptors and Connection Tables MPC885 PowerQUICC Family Reference Manual Rev 2 37 30 Freescale Semiconductor ...
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Страница 1068: ...ATM Pace Control MPC885 PowerQUICC Family Reference Manual Rev 2 40 22 Freescale Semiconductor ...
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Страница 1120: ...AAL2 Implementation MPC885 PowerQUICC Family Reference Manual Rev 2 44 30 Freescale Semiconductor ...
Страница 1162: ...Fast Ethernet Controller FEC MPC885 PowerQUICC Family Reference Manual Rev 2 45 40 Freescale Semiconductor ...
Страница 1172: ...SEC Lite Overview MPC885 PowerQUICC Family Reference Manual Rev 2 46 8 Freescale Semiconductor ...
Страница 1176: ...SEC Lite Address Map MPC885 PowerQUICC Family Reference Manual Rev 2 47 4 Freescale Semiconductor ...
Страница 1214: ...SEC Lite Execution Units MPC885 PowerQUICC Family Reference Manual Rev 2 48 38 Freescale Semiconductor ...
Страница 1312: ...Byte Ordering MPC885 PowerQUICC Family Reference Manual Rev 2 A 8 Freescale Semiconductor ...
Страница 1313: ...MPC885 PowerQUICC Family Reference Manual Rev 2 Freescale Semiconductor B 1 Appendix B Serial Communications Performance TBD ...
Страница 1314: ...Serial Communications Performance MPC885 PowerQUICC Family Reference Manual Rev 2 B 2 Freescale Semiconductor ...
Страница 1320: ...Register Quick Reference Guide MPC885 PowerQUICC Family Reference Manual Rev 2 C 6 Freescale Semiconductor ...
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