
MC68336/376
CENTRAL PROCESSOR UNIT
MOTOROLA
USER’S MANUAL
4-15
4.8.2.3 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 perfor-
mance 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 4-7 shows the
required form of an instruction loop for the processor to enter loop mode.
Figure 4-7 Loop Mode Instruction Sequence
The loop mode is entered when the DBcc instruction is executed, and the loop dis-
placement is –4. Once in loop mode, the processor performs only the data cycles as-
sociated with the instruction and suppresses all instruction fetches. The termination
condition and count are checked after each execution of the data operations of the
looped instruction. The CPU32 automatically exits the loop mode on interrupts or other
exceptions. All single word instructions that do not cause a change of flow can be
looped.
4.9 Exception Processing
An exception is a special condition that preempts normal processing. Exception pro-
cessing is the transition from normal mode program execution to execution of a routine
that deals with an exception.
4.9.1 Exception Vectors
An exception vector is the address of a routine that handles an exception. The vector
base register (VBR) contains the base address of a 1024-byte exception vector table,
which consists of 256 exception vectors. Sixty-four vectors are defined by the
processor, and 192 vectors are reserved for user definition as interrupt vectors. Except
for the reset vector, each vector in the table is one long word in length. The reset vector
is two long words in length. Refer to Table 4-3 for information on vector assignment.
CAUTION
Because there is no protection on the 64 processor-defined vectors,
external devices can access vectors reserved for internal purposes.
This practice is strongly discouraged.
ONE WORD INSTRUCTION
DBCC
DBCC DISPLACEMENT
$FFFC = – 4
1126A
336376UMBook Page 15 Friday, November 15, 1996 2:09 PM