Definition Of Exception Processing - Motorola CPU32 Reference Manual

This section discusses system resources related to exception handling, exception pro-
cessing sequence, and specific features of individual exception processing routines

6.1 Definition of 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.
6.1.1 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 proces-
sor, 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 6-1 for information on vector assignment.
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.
All exception vectors, except the reset vector, are located in supervisor data space.
The reset vector is located in supervisor program space. Only the initial reset vector is
fixed in the processor memory map. When initialization is complete, there are no fixed
assignments. Since the VBR stores the vector table base address, the table can be
located anywhere in memory. It can also be dynamically relocated for each task exe-
cuted by an operating system.
CPU32
REFERENCE MANUAL
SECTION 6
EXCEPTION PROCESSING
CAUTION
EXCEPTION PROCESSING MOTOROLA
6-1