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5- 4
MC68330 USER'S MANUAL
MOTOROLA
memory addresses of routines that begin execution at the completion of exception
processing. These routines perform a series of operations appropriate for the
corresponding exceptions. Because the exception vectors contain memory addresses,
each consists of one long word, except for the reset vector. The reset vector consists of
two long words: the address used to initialize the SSP and the address used to initialize
the PC.
The address of an interrupt exception vector is derived from an 8-bit vector number and
the VBR. The vector numbers for some exceptions are obtained from an external device;
other numbers are supplied automatically by the processor. The processor multiplies the
vector number by four to calculate the vector offset, which is added to the VBR. The sum
is the memory address of the vector. All exception vectors are located in supervisor data
space, except the reset vector, which is located in supervisor program space. Only the
initial reset vector is fixed in the processor's memory map; once initialization is complete,
there are no fixed assignments. Since the VBR provides the base address of the vector
table, the vector table can be located anywhere in memory; it can even be dynamically
relocated for each task that is executed by an operating system. Refer to 5.6 Exception
Processing for additional details.
31
0
VECTOR BASE REGISTER (VBR)
5.1.5 Exception Handling
The processing of an exception occurs in four steps, with variations for different exception
causes. During the first step, a temporary internal copy of the status register (SR) is made,
and the SR is set for exception processing. During the second step, the exception vector
is determined. During the third step, the current processor context is saved. During the
fourth step, a new context is obtained, and the processor then proceeds with instruction
processing.
Exception processing saves the most volatile portion of the current context by pushing it
on the supervisor stack. This context is organized in a format called the exception stack
frame. This information always includes the SR and PC context of the processor when the
exception occurred. To support generic handlers, the processor places the vector offset in
the exception stack frame. The processor also marks the frame with a frame format. The
format field allows the return-from-exception (RTE) instruction to identify what information
is on the stack so that it may be properly restored.
5.1.6 Addressing Modes
Addressing in the CPU32 is register oriented. Most instructions allow the results of the
specified operation to be placed either in a register or directly in memory; this flexibility
eliminates the need for extra instructions to store register contents in memory.
The seven basic addressing modes are as follows:
Register Direct
Register Indirect
Register Indirect with Index
Program Counter Indirect with Displacement