Motorola MPC8260 - Manual (Page 246)

8-14 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part III. The Hardware Interface
Note that the basic coherency size of the bus is 32 bytes for the processor (cache-block
size). Data transfers that cross an aligned 32-byte boundary must present a new address
onto the bus at that boundary for proper snoop operation, or must operate as non-coherent
with respect to the MPC8260.
8.4.3.4 Burst Ordering During Data Transfers
During burst transfers, 32 bytes of data (one cache block) are transferred to or from the
cache. Burst write transfers are performed zero double-word-Þrst. However, because burst
reads are performed critical-double-word-Þrst, a burst-read transfer may not start with the
Þrst double word of the cache block and the cache-block-Þll operation may wrap around
the end of the cache block. Table 8-5 describes MPC8260 burst ordering.
Each data beat is terminated with an assertion of TA.
8.4.3.5 Effect of Alignment on Data Transfers
Table 8-6 lists the aligned transfers that can occur to and from the MPC8260. These are
transfers in which the data is aligned to an address that is an integer multiple of the size of
the data. For example, Table 8-6 shows that 1-byte data is always aligned; however, a 4-byte
word must reside at an address that is a multiple of 4 to be aligned.
In Figure 8-6, Table 8-8, and Table 8-9, OP0 is the most-signiÞcant byte of a word operand
and OP7 is the least-signiÞcant byte.
Negated 0 1 1 1 7 Bytes Extended 7 bytes SDMA (MPC8260 only)
Negated 0 0 0 0 8 Bytes Double-word (maximum data bus size) Core and DMA
Negated 1 0 0 1 16 Bytes Extended double double-word SDMA (MPC8260 only)
Negated 1 0 1 0 24 Bytes Extended triple double-word SDMA (MPC8260 only)
Asserted 0 0 1 0 32 bytes Quad double-word (4 maximum data beats) Core and DMA
Table 8-5. Burst Ordering
Data Transfer
Double-Word Starting Address:
A[27Ð28] = 00 1
1 A[27Ð28] speciÞes the Þrst double word of the 32-byte block being transferred; any subsequent double words must
wrap-around the block. A[29Ð31] are always 0b000 for burst transfers by the MPC8260.
A[27Ð28] = 01 A[27Ð28] = 10 A[27Ð28] = 11
1st data beat DW0 2
2 DWx represents the double word that would be addressed by A[27Ð28] = x if a nonburst transfer were performed.
DW1 DW2 DW3
2nd data beat DW1 DW2 DW3 DW0
3rd data beat DW2 DW3 DW0 DW1
4th data beat DW3 DW0 DW1 DW2
Table 8-4. Transfer Size Signal Encoding (Continued)
TBST TSIZ[0Ð3] Transfer Size Comments Source