Hitachi F-ZTAT H8/3039 Series Hardware Manual page 120

Method 1
When clearing the IRQaF flag, read ISR as a byte unit instead of using a bit-manipulation
instruction, and write a byte value that clears the IRQaF flag to 0 and sets the other bits to 1.
Example: When a = 0
MOV.B @ISR, R0L
MOV.B #HFE, R0L
MOV.B R0L, @ISR
Method 2
Perform dummy processing within the IRQb interrupt exception handling routine to clear the
IRQbF flag.
Example: When b = 1
IRQB MOV.B #HFD, R0L
MOV.B R0L, @ISR
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