Motorola PowerQUICC II MPC8280 Series Reference Manual page 304

Address Tenure Operations
Table 8-6. Aligned Data Transfers (continued)
Program Transfer
TSIZ[0–3]
Size
Word 0 1 0 0
0 1 0 0
Double-Word 0 0 0 0
1
OPn: These lanes are read or written during that bus transaction. OP0 is the most-significant byte of a word operand
and OP7 is the least-significant byte.
2
—: These lanes are ignored during reads and driven with undefined data during writes.
The MPC8280 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
MPC8280's 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 MPC8280 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 processor's 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
software attempt to align code and data where possible.
Table 8-7. Unaligned Data Transfer Example (4-Byte Example)
Program Size of
Word (4 bytes)
Aligned
Misaligned—1st access
2nd access
Misaligned—1st access
2nd access
Misaligned—1st access
2nd access
Aligned
8-16
Freescale Semiconductor, Inc.
A[29–31]
B0
0 0 0 OP0
1 0 0 —
0 0 0 OP0
TSIZ[1–3] A[29–31]
1 0 0 0 0 0
0 1 1 0 0 1
0 0 1 1 0 0
0 1 0 0 1 0
0 1 0 1 0 0
0 0 1 0 1 1
0 1 1 1 0 0
1 0 0 1 0 0
MPC8280 PowerQUICC II Family Reference Manual
For More Information On This Product,
Go to: www.freescale.com
Data Bus Byte Lanes
D0... ...D31
B1 B2 B3
OP1 OP2 OP3
— — —
OP1 OP2 OP3
Data Bus Byte Lanes
D0... ...D31
B0 B1 B2 B3
1
A A A A
— A A A
— — — —
— — A A
— — — —
— — — A
— — — —
— — — —
D32... ...D63
B4 B5 B6 B7
— — — —
OP4 OP5 OP6 OP7
OP4 OP5 OP6 OP7
D32... ...D63
B4 B5 B6 B7
2
— — — —
— — — —
A — — —
— — — —
A A — —
— — — —
A A A —
A A A A
MOTOROLA