Bus Operation
MOTOROLA MC68030 USER’S MANUAL 7-23
The table shows that the MC68030 transfers the number of bytes specified by the size
signals to or from the specified address unless the operand is misaligned or the number of
bytes is greater than the port width. In these cases, the device transfers the greatest number
of bytes possible for the port. For example, if the size is four bytes and the address offset
(A1:A0) is 01, a 32-bit slave can only receive three bytes in the current bus cycle. A 16- or
8-bit[lz slave can only receive one byte. The table defines the byte enables for all port sizes.
Byte data strobes can be obtained by combining the enable signals with the data strobe
signal. Devices residing on 8-bit ports can use the data strobe by itself since there is only
one valid byte for every transfer. These enable or strobe signals select only the bytes
required for write cycles or for noncachable read cycles. The other bytes are not selected,
which prevents incorrect accesses in sensitive areas such as I/O.
Figure 7-18 shows a logic diagram for one method for generating byte data enable signals
for 16- and 32-bit ports from the size and address encodings and the read/write signal.
Table 7-7. Data Bus Write Enable Signals for
Byte, Word, and Long-Word Ports
Transfer
Size
SIZ1 SIZ0 A1 A0
Data Bus Active Sections
Byte (B) - Word (W) - Long-Word (L) Ports
D31:D24 D23:D16 D15:D8 D7:D0
Byte 0
0
0
0
1
1
1
1
0
0
1
1
0
1
0
1
BWL
B
BW
B
—
WL
—
W
—
—
L
—
—
—
—
L
Word 1
1
1
1
0
0
0
0
0
0
1
1
0
1
0
1
BWL
B
BW
B
WL
WL
W
W
—
L
L
—
—
—
L
L
3 Byte 1
1
1
1
1
1
1
1
0
0
1
1
0
1
0
1
BWL
B
BW
B
WL
WL
W
W
L
L
L
—
—
L
L
L
Long Word 0
0
0
0
0
0
0
0
0
0
1
1
0
1
0
1
BWL
B
BW
B
WL
WL
W
W
L
L
L
—
L
L
L
L
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