MOTOROLA
MC68341 USER’S MANUAL
5- 91
considered. Many instructions have a head cycle count, which can overlap the cycles of
an operand fetch to slower memory started by a previous instruction. In these cases, an
increase in access time has no effect on the total execution time of the pair of instructions.
To trace instruction execution time by monitoring the external bus, note that the order of
operand accesses for a particular instruction sequence is always the same provided bus
speed is unchanged and the interleaving of instruction prefetches with operands within
each sequence is identical.
5.7.1.6 INSTRUCTION EXECUTION TIME CALCULATION. The overall execution time
for an instruction depends on the amount of overlap with previous and subsequent
instructions. To calculate an instruction time estimate, the entire code sequence must be
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 (T1, H2) + C2 min (T2, H3) + C3 min (T3, H4) + .. .. .
where:
CN is the number of cycles listed for instruction N
TN is the tail time for instruction N
HN is the head time for instruction N
min (TN, HM) is the minimum of parameters TN and HM
The number of cycles for the instruction (CN) can include one or two EA calculations in
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 (TEA, HOP) + COP
where:
CEA is the instruction’s EA time
COP is the instruction’s operation time
TEA is the EA’s tail time
HOP is the instruction operation’s head time
min (TN, HM) is the minimum of parameters TN and HM
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:
COP1 min (TOP1, HEA2) + CEA2 min (TEA2, HOP2) + COP2 min (TOP2, HEA3) + . . .
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