Motorola MC68020 User Manual

MOTOROLA M68020 USER’S MANUAL 8- 5
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CLOCK
BUS
ACTIVIT
Y
PREFETCH
WRITE
READ
PREFETCH
WRITE
PREFETCH
B
YTES n +
8
BUS
C
ONTROLLE
R
WRITE TO (A1)+
PREFETCH
BYTES n + 12
WRITE TO –(A2)
READ FROM (A1)
IDLE
PERFORM
MOVE #1
IDLE
PERFORM
ADD #2
CALCULATE
S
OURCE E
A
MOVE #3
CALCULATE
D
ESTINATIO
N
EA
MOVE #3
IDLE
PERFORM
MOVE #3
PERFORM
ADD #4
MOVE.L D4,(A1)+
MOVE.L (A1),–(A2)
ADD.L
D5,D
6
SEQUENCER
INSTRUCTION
E
XECUTION TIM
E
(6)
(9)
(1)
CLOCK
C
OUN
T
LEGEND:
1) MOVE.L D4,(A1)+
2) ADD.L D4,D5
3) MOVE.L (A1),–(A2)
4) ADD.L D5,D6
Figure 8-3. Processor Activity for Example 1
For the first three clocks of this example, the bus controller and sequencer are both
performing tasks associated with the MOVE #1 instruction. The next three clocks (clocks
4, 5, and 6) demonstrate instruction overlap. The bus controller is performing a write to
memory as part of the MOVE #1 instruction. The sequencer, on the other hand, is
performing the ADD #2 instruction for two clocks (clocks 4 and 5) and beginning source
effective address (EA) calculations for the MOVE #3 instruction. The bus controller activity
completely overlaps the execution of the ADD #2 instruction, causing the ADD #2
attributed execution time to be zero clocks. The overlap also shortens the effective
execution time of the MOVE #3 instruction by one clock because the bus controller
completes the MOVE #1 write operation while the sequencer begins the MOVE #3 EA
calculation.
The sequencer continues the source EA calculation for one more clock period (clock 7)
while the bus controller begins a read for MOVE #3. When counting instruction execution
time in bus clocks, the MOVE #1 completes at the end of clock 6, and the execution of
MOVE #3 begins on clock 7.
Both the sequencer and bus controller continue with MOVE #3 until the end of clock 14,
when the sequencer begins to perform ADD #4. Timing for MOVE #3 continues because
the bus controller is still performing the write to the destination of MOVE #3. The bus
activity for MOVE #3 completes at the end of clock 15. The effective execution time for
MOVE #3 is nine clocks.
The one clock cycle (clock 15) when the sequencer is performing ADD #4 and the bus
controller is writing to the destination of MOVE #3 is absorbed by the execution time of
MOVE #3. This overlap shortens the effective execution time of ADD #4 by one clock,
giving it an attributed execution time of one clock.

Motorola MC68020 Specifications

General

Brand	Motorola
Model	MC68020
Category	Computer Hardware
Language	English