MC68340 datasheet(140/441 Pages) FREESCALE

140 / 441 page

MOTOROLA

MC68340 USER’S MANUAL

5- 3

CPU32 uses instruction restart, which requires that only a small portion of the internal

machine state be saved. After correcting the page fault, the machine state is restored, and

the instruction is refetched and restarted. This process is completely transparent to the

application program.

BUS CONTROL

INSTRUCTION

PREFETCH

AND

DECODE

EXECUTION

UNIT

SEQUENCER

CONTROL

UNIT

BUS

CONTROL

ADDRESS

BUS

DATA BUS

Figure 5-1. CPU32 Block Diagram

5.1.3 Loop Mode Instruction Execution

The CPU32 has several features that provide efficient execution of program loops. One of

these features is the DBcc looping primitive instruction. To increase the performance of

the CPU32, a loop mode has been added to the processor. The loop mode is used by any

single-word instruction that does not change the program flow. Loop mode is implemented

in conjunction with the DBcc instruction. Figure 5-2 shows the required form of an

instruction loop for the processor to enter loop mode.

ONE-WORD INSTRUCTION

DBcc

DBcc DISPLACEMENT

$FFFC = 4

Figure 5-2. Loop Mode Instruction Sequence

The loop mode is entered when the DBcc instruction is executed and the loop

displacement is –4. Once in loop mode, the processor performs only the data cycles

associated with the instruction and suppresses all instruction fetches. The termination

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