Read-Modify-Write Cycle Flowchart - Motorola MC68020 User Manual

PROCESSOR
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 MODIFICATION
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
*
This step does not apply to the MC68EC020.
**
For the MC68EC020, A23–A0.
5-40
Figure 5-29. Read-Modify-Write Cycle Flowchart
M68020 USER'S MANUAL
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
1) NEGATE DSACK1/DSACK0
EXTERNAL DEVICE
A
IF CAS2 INSTRUCTION
AND ONLY ONE OPERAND
READ, THEN GO TO A ;
IF OPERANDS DO NOT
MATCH, THEN GO TO
C ; ELSE GO TO B
B
D
IF CAS2 INSTRUCTION
AND ONLY ONE OPERAND
WRITTEN, THEN GO TO
D ; ELSE GO TO E
E
C
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