Answer:
It is explained in the Interrupts section of PSoC 3 Datasheet.
1: Interrupt triggered asynchronous to the clock
2: The PEND bit is set on next active clock edge to indicate the interrupt arrival
3: POST bit is set following the PEND bit
4: Interrupt request and the interrupt number sent to CPU core after evaluation priority (Takes 3 clocks)
5: ISR address is posted to CPU core for branching
6: CPU acknowledges the interrupt request
7: ISR address is read by CPU for branching
8, 9: PEND and POST bits are cleared respectively after receiving the IRA from core
10: IRA bit is cleared after completing the current instruction and starting the instruction execution from ISR location (Takes 7 cycles)
11: IRC is set to indicate the completion of ISR, Active int. status is restored with previous status
The total interrupt latency (ISR execution)
= POST + PEND + IRQ + IRA + Completing current instruction and branching
= 1+1+1+2+7 cycles
= 12 cycles