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Section 4 Exception Handling
4.1 Overview
4.1.1 Exception Handling Types and Priority
As table 4-1 indicates, exception handling may be caused by a reset, trap instruction, or interrupt.
Exception handling is prioritized as shown in table 4-1. If two or more exceptions occur
simultaneously, they are accepted and processed in priority order. Trap instruction exceptions are
accepted at all times in the program execution state.
Table 4-1 Exception Types and Priority
Priority Exception Type
High
Reset
Interrupt
Low
Trap instruction (TRAPA) Started by execution of a trap instruction (TRAPA)
4.1.2 Exception Handling Operation
Exceptions originate from various sources. Trap instructions and interrupts are handled as follows.
1.
The program counter (PC) and condition code register (CCR) are pushed onto the stack.
2.
The CCR interrupt mask bit is set to 1.
3.
A vector address corresponding to the exception source is generated, and program execution
starts from the address indicated in the vector address.
For a reset exception, steps 2 and 3 above are carried out.
Start of Exception Handling
Starts immediately after a low-to-high transition at the RES pin
Interrupt requests are handled when execution of the current
instruction or handling of the current exception is completed
65
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