Instruction Execution Overlap - Motorola CPU32 Reference Manual

8.1.4 Instruction Execution Overlap

Overlap is the time, measured in clock cycles, that an instruction executes concurrent-
ly with the previous instruction. As shown in Figure 8-2, portions of instructions A and
B execute simultaneously, so that total execution time is reduced. Because portions of
instructions B and C also overlap, overall execution time for all three instructions is
also reduced.
Each instruction contributes to the total overlap time. The portion of execution time at
the end of instruction A that can overlap the beginning of instruction B is called the tail
of instruction A. The portion of execution time at the beginning of instruction B that can
overlap the end of instruction A is called the head of instruction B. The total overlap
time between instructions A and B is the smaller tail of A and the head of B.
INSTRUCTION A
Figure 8-2 Simultaneous Instruction Execution
The execution time attributed to instructions A, B, and C after considering the overlap
is illustrated in Figure 8-3. The overlap time is attributed to the execution time of the
completing instruction. The following equation shows the method for calculating the
overlap time:
INSTRUCTION A
MOTOROLA
8-4
INSTRUCTION B
OVERLAP
Overlap = min (Tail
INSTRUCTION B
OVERLAP
PERIOD
(ABSORBED BY
INSTRUCTION A)
Figure 8–3 Attributed Instruction Times
INSTRUCTION EXECUTION TIMING
INSTRUCTION C
OVERLAP
, Head )
N N+1
INSTRUCTION C
OVERLAP
PERIOD
(ABSORBED BY
INSTRUCTION B)
CPU32
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