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4.1 Overview
4.1.1 Types of Exception Handling and Their Priority
As indicated in table 4-1 (a) and (b), exception handling can be initiated by a reset, address error,
trace, interrupt, or instruction. An instruction initiates exception handling if the instruction is an
invalid instruction, a trap instruction, or a DIVXU instruction with zero divisor. Exception
handling begins with a hardware exception-handling sequence which prepares for the execution of
a user-coded software exception-handling routine.
There is a priority order among the different types of exceptions, as shown in table 4-1 (a). If two
or more exceptions occur simultaneously, they are handled in their order of priority. An
instruction exception cannot occur simultaneously with other types of exceptions.
Table 4-1 (a) Exceptions and Their Priority
Exception
Type
High
Reset
Address error
Trace
Interrupt
Low
Table 4-1 (b) Instruction Exceptions
Exception Type
Invalid instruction
Trap instruction
Zero divide
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Section 4 Exception Handling
Source
Detection Timing
External
RES Low-to-High transition
Internal
Instruction fetch or data read/write
bus cycle
Internal
End of instruction execution, if
T = "1" in status register
External,
End of instruction execution or end
internal
of exception-handling sequence
Start of Exception-Handling Sequence
Attempted execution of instruction with undefined code
Started by execution of trap instruction
Attempted execution of DIVXU instruction with zero divisor
81
Start of Exception-
Handling Sequence
Immediately
End of instruction
execution
End of instruction
execution
End of instruction
execution
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