MOTOROLA
Appendix A. Byte Ordering
A-7
Appendixes
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 modiÞed (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 Ô3-bit
munging.Õ
The munged physical address is passed to the cache or to external memory, and the
speciÞed width of data is transferred (in big-endian orderÑthat is, MSB at the lowest
address, LSB at the highest address). In PPC-LE mode, only the address is modiÞed, 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
Table A-6. PPC-LE 3-bit Munging
Data Width (Bytes)
EA ModiÞcation
4
XOR with 0b100
2
XOR with 0b110
1
XOR with 0b111
Note that the MPC860 does not support 8-byte scalars
3-Bit Munge
I-Cache
External bus
D-Cache
PowerPC 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
MPC860
Содержание MPC860 PowerQUICC
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