Motorola MPC860 PowerQUICC, User Manual, Page: 1010 / 1106

A-4 MPC860 PowerQUICC UserÕs Manual MOTOROLA
Appendixes
The munged physical address is passed to the internal U-bus, and the speciÞed width of data
is transferred. Only the address is modiÞed, not the byte order. Munging makes it appear to
the core that individual aligned scalars on the U-bus are in little-endian order, when in fact,
they are actually in big-endian order. This allows the core to access data in the inherently
big-endian internal registers with apparent little-endian byte-ordering. However, when
DC_CST[LES] is set, for any access originating from the PowerPC core, the SIU
unmunges the address and swaps the bytes of data within each word at the external
bus/U-bus boundary. The byte swapping is shown in Figure A-2.
Figure A-2. Byte Swapping
The unmunging and byte swapping places all external accesses by the PowerPC core into
true little-endian byte order. Note that the bit ordering remains unchangedÑthat is, bit 0 is
always the msb, and bit 31 is always the lsb.
The communication peripherals (SCCs, SMCs, SPI, I
2
C, PIP, or IDMA) transfer data as
bytes (bytes are received one at a time and transmitted one at a time). Byte transfers have
no inherent endianessÑthey are neither big- nor little-endian. For TLE-mode, the
FCR[BO] parameter of each peripheral should be programmed to 0b1x (that is, either 0b10
or 0b11). Note that the SIU does nothing (no unmunging, no byte-swapping) to accesses
originating from the SDMA controller.
A.4.1 TLE Mode System Examples
The following tables describe how to handle the little-endian program or data in the
little-endian system that is built around the MPC860 for various port sizes.
11 12 13 14
0 7 8 15 16 23 24 31
14 13 12 11
0 7 8 15 16 23 24 31