MC68340 datasheet(231/441 Pages) FREESCALE

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5- 94

MC68340 USER’S MANUAL

MOTOROLA

Because of the stipulation that each instruction must prefetch to replace itself, the concept

of negative tails has been introduced to account for these free clocks on the bus.

On a two-clock bus, it is not necessary to adjust instruction timing to account for the

potential extra prefetch. The cycle times of the microsequencer and bus are matched, and

no additional benefit or penalty is obtained. In the instruction execution time equations, a

zero should be used instead of a negative number.

Negative tails are used to adjust for slower fetches on slower buses. Normally, increasing

the length of prefetch bus cycles directly affects the cycle count and tail values found in

the tables.

In the following equations, negative tail values are used to negate the effects of a slower

bus. The equations are generalized, however, so that they may be used on any speed bus

with any tail value.

NEW_TAIL

= OLD_TAIL + (NEW_CLOCK – 2)

IF ((NEW_CLOCK – 4) >0) THEN

NEW_CYCLE = OLD_CYCLE

+ (NEW_CLOCK -2) + (NEW_CLOCK – 4)

ELSE

NEW_CYCLE = OLD_CYCLE

+ (NEW _CLOCK – 2)

where:

NEW_TAIL/NEW_CYCLE

is the adjusted tail/cycle at the slower speed

OLD_TAIL/OLD_CYCLE

is the value listed in the instruction timing tables

NEW_CLOCK

is the number of clocks per cycle at the slower speed

Note that many instructions listed as having negative tails are change-of-flow instructions

and that the bus speed used in the calculation is that of the new instruction stream.

5.7.2 Instruction Stream Timing Examples

The following programming examples provide a detailed examination of timing effects. In

all examples, the memory access is from external synchronous memory, the bus is idle,

and the instruction pipeline is full at the start.

5.7.2.1 TIMING EXAMPLE 1—EXECUTION OVERLAP. Figure 5-33 illustrates execution

overlap caused by the bus controller's completion of bus cycles while the sequencer is

calculating the next EA. One clock is saved between instructions since that is the

minimum time of the individual head and tail numbers.

Instructions

MOVE.W

A1, (A0)

ADDQ.W

#1, (A0)

CLR.W

$30 (A1)

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