Freescale Semiconductor, Inc.
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.
SEQUENCER
CONTROL
UNIT
INSTRUCTION
PREFETCH
AND
DECODE
DATA BUS
16
32
BUS
CONTROL
BUS CONTROL
EXECUTION
UNIT
ADDRESS
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
MOTOROLA
MC68340 USER’S MANUAL
5- 3
For More Information On This Product,
Go to: www.freescale.com
FREESCALE [ Freescale ]
