Motorola MC68020 - Read-Modify-Write Cycle Flowchart

5- 40 M68020 USER’S MANUAL MOTOROLA
LOCK BUS
1) ASSERT RMC
ADDRESS DEVICE
1) ASSERT ECS/OCS FOR ONE-HALF CLOCK
2
) SET R/W TO READ
3
) DRIVE ADDRESS ON A31–A0
4
) DRIVE FUNCTION CODES ON FC2–FC0
5
) DRIVE SIZ1, SIZ0
6
) ASSERT AS
7
) ASSERT DS
8
) ASSERT DBEN
ACQUIRE DATA
1) LATCH DATA
2
) NEGATE AS AND DS
3
) NEGATE DBEN
4
) START DATA MODIFICATIO
N
START OUTPUT TRANSFER
1) ASSERT ECS/OCS FOR ONE-HALF CLOCK
2
) DRIVE ADDRESS ON A31–A0 (IF DIFFERENT
)
3
) DRIVE SIZ1, SIZ0
4
) SET R/W TO WRITE
5
) ASSERT AS
6
) ASSERT DBEN
7
) PLACE DATA ON D31–D0
8
) ASSERT DS
TERMINATE OUTPUT TRANSFER
1) NEGATE AS AND DS
2
) REMOVE DATA FROM D31–D0
3
) NEGATE DBEN
UNLOCK BUS
1) NEGATE RMC
START NEXT CYCLE
PRESENT DATA
1) DECODE ADDRESS
2
) PLACE DATA ON D31–D0
3
) ASSERT DSACK1/DSACK0
TERMINATE CYCLE
1) REMOVE DATA FROM D31–D0
2
) NEGATE DSACK1/DSACK0
ACCEPT DATA
1) DECODE ADDRESS
2
) STORE DATA FROM D31–D0
3
) ASSERT DSACK1/DSACK0
TERMINATE CYCLE
A
IF CAS2 INSTRUCTION
A
ND ONLY ONE OPERAN
D
READ, THEN GO TO A
;
IF OPERANDS DO NOT
MATCH, THEN GO TO
C ; ELSE GO TO B
C
B
1) NEGATE DSACK1/DSACK0
IF CAS2 INSTRUCTION
A
ND ONLY ONE OPERAND
WRITTEN, THEN GO TO
D ; ELSE GO TO E
E
D
PROCESSOR
EXTERNAL DEVICE
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This step does not apply to the MC68EC020.
For the MC68EC020, A23–A0.
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Figure 5-29. Read-Modify-Write Cycle Flowchart