NEC PD75402A User Manual page 151

(4) Pending interrupt execution
If INT0 and INTCSI are generated simultaneously (during execution of the same instruction), INT0, which
has the higher interrupt priority, is executed first (INTCSI is held pending).
When the INT0 service program is ended by the RETI instruction, the pending INTCSI service program is
started.
140
CHAPTER 6. INTERRUPT FUNCTIONS
<Main Program>
<Main Program>
Reset
EI IECSI
EI IE0
EI
INT0
INTCSI
<INT0 Service Program>
RETI
<INTCSI Service Program>
RETI