Freescale Semiconductor MCF5480 - Manual (Page 447)

Functional Description
MCF548x Reference Manual, Rev. 3
Freescale Semiconductor 17-31
Figure 17-34. Longword Write Burst to 8-Bit Port 4-1-1-1 (Address Setup and Hold)
17.6.7 Misaligned Operands
Because operands, unlike opcodes, can reside at any byte boundary, they are allowed to be misaligned. A
byte operand is properly aligned at any address, a word operand is misaligned at an odd address, and a
longword is misaligned at an address not a multiple of four. Although the MCF548x enforces no alignment
restrictions for data operands (including program counter (PC) relative data addressing), additional bus
cycles are required for misaligned operands.
Instruction words and extension words (opcodes) must reside on word boundaries. Attempting to prefetch
a misaligned instruction word causes an address error exception.
The MCF548x converts misaligned, cache-inhibited operand accesses to multiple aligned accesses.
Figure 17-35 shows the transfer of a longword operand from a byte address to a 32-bit port. First a byte is
transferred at an offset of 0x1. The slave device supplies the byte and acknowledges the data transfer.
When the MCF548x starts the second cycle, a word is transferred with a byte offset of 0x2. The next two
bytes are transferred in this cycle. In the third cycle, byte 3 is transferred. The byte offset is now 0x0, the
port supplies the final byte, and the operation is complete.
Figure 17-35. Example of a Misaligned Longword Transfer (32-Bit Port)
If an operand is cacheable and is misaligned across a cache-line boundary, both lines are loaded into the
cache. The example in Figure 17-36 differs from the one in Figure 17-35 because the operand is
word-sized and the transfer takes only two bus cycles.
CLK
AD[23:0]
AD[31:24]
R/W
ALE
TSIZ[1:0]
TA
TBST
S0 AS S1 S2 S2 S2 S2 S3 AH
OE
ADDR[23:0]
A[31:24] DATA DATA DATA
FBCSn, BE/BWEn
11
DATA
–– Byte 0 –– –– Transfer 1
–– –– Byte 1 Byte 2
Byte 3 –– –– ––
Transfer 2
Transfer 3
001
010
100
16 15 31 0 24 23 7 8 A[2:0]