
MOTOROLA
3-12
SYSTEM RESOURCES
CPU16
REFERENCE MANUAL
Figure 3-3 Operand Byte Order
3.5.5.2 Operand Alignment
Refer to
Table 3-5
for required organization of 8- and 16-bit data ports. A data multi-
plexer establishes the necessary connections for different combinations of address
and data sizes. The multiplexer takes the two bytes of the 16-bit bus and routes them
to their required positions. Positioning of bytes is determined by the size and address
outputs. SIZ1 and SIZ0 indicate the remaining number of bytes to be transferred dur-
ing the current bus cycle. The number of bytes transferred is equal to or less than the
size indicated by SIZ1 and SIZ0, depending on port width.
ADDR0 also affects data multiplexer operation. During an operand transfer, AD-
DR[23:1] indicate the word base address of the portion of the operand to be accessed,
and ADDR0 indicates the byte offset from the base.
Table 3-5
shows the number of
bytes required on the data bus for read cycles. OPn entries are portions of the request-
ed operand that are read or written during a bus cycle and are defined by SIZ1, SIZ0,
and ADDR0 for that bus cycle.
NOTES:
Operands in parentheses are ignored by the CPU16 during read cycles.
*The CPU16 treats misaligned long-word transfers as two misaligned word transfers.
Three-byte transfer cases occur only as a result of a long word to byte transfer.
Operand
Byte Order
16 15
OP1
OP0
31
24 23
8 7
0
Long Word
Three Byte
Word
Byte
OP0
OP2
OP1
OP0
OP3
OP2
OP1
OP0
Table 3-5 Operand Alignment
Transfer Case
SIZ1
SIZ0
ADDR0
DSACK1
DSACK0
DATA
15 8
OP0
OP0
(OP0)
OP0
OP0
OP0
(OP0)
OP0
OP0
OP0
(OP0)
OP0
OP0
OP0
(OP0)
DATA
7 0
(OP0)
(OP0)
OP0
(OP1)
(OP0)
OP1
OP0
(OP1)
(OP0)
OP1
OP0
(OP1)
(OP0)
OP1
OP0
Byte to Byte
Byte to Word (Even)
Byte to Word (Odd)
Word to Byte (Aligned)
Word to Byte (Misaligned)
Word to Word (Aligned)
Word to Word (Misaligned)
3 Byte to Byte (Aligned)
3 Byte to Byte (Misaligned)
3 Byte to Word (Aligned)
3 Byte to Word (Misaligned)
Long Word to Byte (Aligned)
Long Word to Byte (Misaligned)*
Long Word to Word (Aligned)
Long Word to Word (Misaligned)*
0
0
0
1
1
1
1
1
1
1
1
0
1
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
X
0
1
0
1
0
1
0
1
0
1
0
1
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
0
X
X
0
0
X
X
0
0
X
X
0
0
X
X
F
Freescale Semiconductor, Inc.
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Go to: www.freescale.com
n
.