MC68340 datasheet(178/441 Pages) FREESCALE

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MOTOROLA

MC68340 USER’S MANUAL

5- 41

Finally, the processor prepares to resume normal execution of instructions. The exception

vector offset is determined by multiplying the vector number by 4, and the offset is added

to the contents of the VBR to determine displacement into the exception vector table. The

exception vector is loaded into the PC. If no other exception is pending, the processor will

resume normal execution at the new address in the PC.

5.5.1.3 EXCEPTION STACK FRAME. During exception processing, the most volatile

portion of the current context is saved on the top of the supervisor stack. This context is

organized in a format called the exception stack frame.

The exception stack frame always includes the contents of SR and PC at the time the

exception occurred. To support generic handlers, the processor also places the vector

offset in the exception stack frame and marks the frame with a format code. The format

field allows an RTE instruction to identify stack information so that it can be properly

restored.

The general form of the exception stack frame is illustrated in Figure 5-10. Although some

formats are peculiar to a particular M68000 Family processor, format 0000 is always legal

and always indicates that only the first four words of a frame are present. See 5.5.4

CPU32 Stack Frames for a complete discussion of exception stack frames.

STATUS REGISTER

PROGRAM COUNTER LOW

FORMAT

VECTOR OFFSET

OTHER PROCESSOR STATE INFORMATION,

DEPENDING ON EXCEPTION

(0, 2, OR 8 WORDS)

PROGRAM COUNTER HIGH

Figure 5-10. Exception Stack Frame

5.5.1.4 MULTIPLE EXCEPTIONS. Each exception has been assigned a priority based on

its relative importance to system operation. Priority assignments are shown in Table 5-17.

Group 0 exceptions have the highest priorities; group 4 exceptions have the lowest

priorities. Exception processing for exceptions that occur simultaneously is done by

priority, from highest to lowest.

It is important to be aware of the difference between exception processing mode and

execution of an exception handler. Each exception has an assigned vector that points to

an associated handler routine. Exception processing includes steps described in 5.5.1.2

Exception Processing Sequence, but does not include execution of handler routines,

which is done in normal mode.

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