MC68340 datasheet(230/441 Pages) FREESCALE

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MOTOROLA

MC68340 USER’S MANUAL

5- 93

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) + 〈CEA〉2 − min (TEA2, HOP2) + COP2 − min (TOP2, HEA3) + . . .

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.

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