
4-11
ML63326 User's Manual
Chapter 4 Interrupt (INT326)
4
4.3.2 Return from an Interrupt Routine
Return from a watchdog timer interrupt routine is performed with an "RTNMI" instruction.
Return from all other interrupt routines is performed with an "RTI" instruction.
Execution of "RTI" and "RTNMI" instructions both require 1 machine cycle.
When returning from an interrupt routine, the CPU performs the following processes.
(1) The call stack pointer (SP) is decremented by 1. (SP
¨
SP–1)
(2) MIE is set to "1" (when an "RTNMI" instruction is used, MIE is restored to its state prior
to the interrupt).
(3) 1 is added to the call stack contents and that value is loaded into the program counter (PC).
!
Notes:
•
While the MIE flag is "0" (interrupt disabled state), if a watchdog timer interrupt is
processed and an "RTI" instruction is executed, the MIE flag will be set to "1" and interrupts
enabled.
•
Use "RTNMI" instructions to return from watchdog timer interrupts only. Use "RTI"
instructions for normal interrupts.
4.3.3 Interrupt Hold Instructions
Interrupt requests are not received during execution of the following instructions. These
instructions are processed with priority, and interrupt processing is delayed until completion
of the instruction.
•
ROM table reference instructions
•
External memory transfer instruction
•
Stack operation instructions
•
Jump instructions
•
Conditional branch instructions
•
Call/return instructions
•
"EI" (set MIE flag) instructions, "DI" (clear MIE flag) instructions and "MSA cadr15" (start
melody output) instructions within control instructions
!
Note:
If the above instructions are used consecutively, even if an interrupt is generated, that
interrupt may be put on hold for a considerable amount of time before the interrupt routine
begins.
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