NEC switch - Manual (Page 175)

CHAPTER 11 INTERRUPT FUNCTIONS
User’s Manual U12978EJ3V0UD
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11.4.3 Multiplexed interrupt servicing
Servicing in which another interrupt is acknowledged while an interrupt is being processed is called multiplexed
interrupt servicing.
Multiplexed interrupt is not performed unless interrupt requests are enabled (IE = 1) (except the non-maskable
interrupt request). Other interrupt requests are disabled (IE = 0) as soon as an interrupt request is acknowledged.
Therefore, it is necessary to set (1) the IE flag to realize the interrupt enable state using the EI instruction during
interrupt request servicing in order to enable multiplexed interrupt servicing.