Figure 3-8. Read-Modify-Write Cycle Flowchart - Motorola MC68306 User Manual

that uses the read-modify-write cycle) only operates on bytes. Thus, all read-modify-write
cycles are byte operations. The read-modify-write flowchart is shown in Figure 3-8 and the
timing diagram is shown in Figure 3-9.
BUS MASTER
ADDRESS THE DEVICE
1) SET R/W TO READ
2) PLACE FUNCTION CODE ON FC2–FC0
3) PLACE ADDRESS ON ADDRESS BUS
4) ASSERT ADDRESS STROBE (AS)
5) ASSERT UPPER DATA STROBE (UDS)
OR LOWER DATA STROBE (LDS)
ACQUIRE THE DATA
1) LATCH DATA
1) NEGATE UDS AND LDS
2) START DATA MODIFICATION
START OUTPUT TRANSFER
1) SET R/W TO WRITE
2) PLACE DATA ON D7–D0 OR D15–D8
3) ASSERT UPPER DATA STROBE (UDS)
OR LOWER DATA STROBE (LDS)
TERMINATE OUTPUT TRANSFER
1) NEGATE UDS OR LDS
2) NEGATE AS
3) REMOVE DATA FROM D7–D0 OR
D15–D8
4) SET R/W TO READ
START NEXT CYCLE
3-8

Figure 3-8. Read-Modify-Write Cycle Flowchart

MC68306 USER'S MANUAL
SLAVE
OUTPUT THE DATA
1) DECODE ADDRESS
2) PLACE DATA ON D7–D0 OR D15–D0
3) ASSERT DATA TRANSFER
ACKNOWLEDGE (DTACK)
TERMINATE THE CYCLE
1) REMOVE DATA FROM D7–D0
OR D15–D8
2) NEGATE DTACK
INPUT THE DATA
1) STORE DATA ON D7–D0 OR D15–D8
2) ASSERT DATA TRANSFER
ACKNOWLEDGE (DTACK)
TERMINATE THE CYCLE
1) NEGATE DTACK
MOTOROLA