Aligned data transfers -15, Xample, table 8-6 sho – Motorola MPC8260 User Manual
Page 247
MOTOROLA
Chapter 8. The 60x Bus
8-15
Part III. The Hardware Interface
The MPC8260 supports misaligned memory operations, although they may degrade
performance substantially. A misaligned memory address is one that is not aligned to the
size of the data being transferred (such as, a word read from an odd byte address). The
MPC8260Õs processor bus interface supports misaligned transfers within a word (32-bit
aligned) boundary, as shown in Table 8-7. Note that the 4-byte transfer in Table 8-7 is only
one example of misalignment. As long as the attempted transfer does not cross a word
boundary, the MPC8260 can transfer the data to the misaligned address within a single bus
transfer (for example, a half-word read from an odd byte-aligned address). It takes two bus
transfers to access data that crosses a word boundary.
Due to the performance degradation, misaligned memory operations should be avoided. In
addition to the double-word straddle boundary condition, the processorÕs address
translation logic can generate substantial exception overhead when the load/store multiple
and load/store string instructions access misaligned data. It is strongly recommended that
Table 8-6. Aligned Data Transfers
Program Transfer
Size
TSIZ[0Ð3]
A[29Ð31]
Data Bus Byte Lanes
D0... ...D31
D32... ...D63
B0
B1
B2
B3
B4
B5
B6
B7
Byte
0 0 0 1
0 0 0
OP0
1
1
OPn: These lanes are read or written during that bus transaction. OP0 is the most-signiÞcant byte of a word
operand and OP7 is the least-signiÞcant byte.
С
2
2
С: These lanes are ignored during reads and driven with undeÞned data during writes.
С
С
С
С
С
С
0 0 0 1
0 0 1
С
OP1
С
С
С
С
С
С
0 0 0 1
0 1 0
С
С
OP2
С
С
С
С
С
0 0 0 1
0 1 1
С
С
С
OP3
С
С
С
С
0 0 0 1
1 0 0
С
С
С
С
OP4
С
С
С
0 0 0 1
1 0 1
С
С
С
С
С
OP5
С
С
0 0 0 1
1 1 0
С
С
С
С
С
С
OP6
С
0 0 0 1
1 1 1
С
С
С
С
С
С
С
OP7
Half-Word
0 0 1 0
0 0 0
OP0
OP1
С
С
С
С
С
С
0 0 1 0
0 1 0
С
С
OP2
OP3
С
С
С
С
0 0 1 0
1 0 0
С
С
С
С
OP4
OP5
С
С
0 0 1 0
1 1 0
С
С
С
С
С
С
OP6
OP7
Word
0 1 0 0
0 0 0
OP0
OP1
OP2
OP3
С
С
С
С
0 1 0 0
1 0 0
С
С
С
С
OP4
OP5
OP6
OP7
Double-Word
0 0 0 0
0 0 0
OP0
OP1
OP2
OP3
OP4
OP5
OP6
OP7