Freescale Semiconductor, Inc.
analyzed. To derive the actual instruction execution times for an instruction sequence, the
instruction times listed in the tables must be adjusted to account for overlap.
The formula for this calculation is as follows:
C1 − min (T , H ) + C − min (T , H ) + C − min (T , H ) + .. .. .
1
2
2
2
3
3
3
4
where:
C is the number of cycles listed for instruction N
N
T is the tail time for instruction N
N
H is the head time for instruction N
N
min (T , H ) is the minimum of parameters T and H
M
N
M
N
The number of cycles for the instruction (C ) can include one or two EA calculations in
N
addition to the raw number in the cycles column. In these cases, calculate overall
instruction time as if it were for multiple instructions, using the following equation:
CEA − min (T , H ) + C
EA OP
OP
where:
CEA is the instruction’s EA time
C
is the instruction’s operation time
OP
EA
T
is the EA’s tail time
H
is the instruction operation’s head time
OP
min (T , H ) is the minimum of parameters T and H
M
N
M
N
The overall head for the instruction is the head for the EA, and the overall tail for the
instruction is the tail for the operation. Therefore, the actual equation for execution time
becomes:
C
− min (T
, H
) + CEA − min (T
, H
) + C
− min (T
, H
) + . . .
OP1
OP1
EA2
2
EA2
OP2
OP2
OP2
EA3
Every instruction must prefetch to replace itself in the instruction pipe. Usually, these
prefetches occur during or after an instruction. A prefetch is permitted to begin in the first
clock of any indexed EA mode operation.
Additionally, a prefetch for an instruction is permitted to begin two clocks before the end of
an instruction provided the bus is not being used. If the bus is being used, then the
prefetch occurs at the next available time when the bus would otherwise be idle.
5.7.1.7 EFFECTS OF NEGATIVE TAILS. When the CPU32 changes instruction flow, the
instruction decode pipeline must begin refilling before instruction execution can resume.
Refilling forces a two-clock idle period at the end of the change-of-flow instruction. This
idle period can be used to prefetch an additional word on the new instruction path.
MOTOROLA
MC68340 USER’S MANUAL
5- 93
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