Misaligned Operands - Motorola MC68020 User Manual

5.2.2 Misaligned Operands

Since operands may reside at any byte boundary, they may be misaligned. A byte
operand is properly aligned at any address; a word operand is misaligned at an odd
address; a long word is misaligned at an address that is not evenly divisible by four. The
MC68000, MC68008, and MC68010 implementations allow long-word transfers on odd-
word boundaries but force exceptions if word or long-word operand transfers are
attempted at odd-byte addresses. Although the MC68020/EC020 does not enforce any
alignment restrictions for data operands (including PC relative data addresses), some
performance degradation occurs when additional bus cycles are required for long-word or
word operands that are misaligned. For maximum performance, data items should be
aligned on their natural boundaries. All instruction words and extension words must reside
on word boundaries. Attempting to prefetch an instruction word at an odd address causes
an address error exception.
Figure 5-9 shows the transfer (write) of a long-word operand to an odd address in word-
organized memory, which requires three bus cycles. For the first cycle, SIZ1 and SIZ0
specify a long-word transfer, and A2–A0 = 001. Since the port width is 16 bits, only the
first byte of the long word is transferred. The slave device latches the byte and
acknowledges the data transfer, indicating that the port is 16 bits wide. When the
processor starts the second cycle, SIZ1 and SIZ0 specify that three bytes remain to be
transferred with A2–A0 = 010. The next two bytes are transferred during this cycle. The
processor then initiates the third cycle, with SIZ1 and SIZ0 indicating one byte remaining
to be transferred with A2–A0 = 100. The port latches the final byte, and the operation is
complete. Figure 5-10 shows the associated bus transfer signal timing. Figure 5-11 shows
the equivalent operation for a data read cycle.
31
OP0
D31
MSB
XXX
OP1
OP3
Figure 5-9. Misaligned Long-Word Operand Write to Word Port Example
5-14
LONG-WORD OPERAND
OP1
DATA BUS D16
WORD MEMORY
LSB
OP0
OP2
XXX
M68020 USER'S MANUAL
OP2 OP3
MC68020/EC020
SIZ1 SIZ0 A0
A2 A1
0 0 0 0 1
1 1 0 1 0
0
0 1 1 0
0
MEMORY CONTROL
DSACK1 DSACK0
L H
L H
L H
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