Microoperations for interrupt cycle.

    At the completion of the execute cycle, a test is made to determine whether any enabled interrupts have occurred. If so, the interrupt cycle occurs. The nature of this cycle varies greatly from one machine to another.
    
    
    t1: MBR<-(PC)
    
    t2: MAR<-Save_Address
    PC<-Routine_Address
    
    t3: Memory<-(MBR)
    

https://imgur.com/a/kvBiWGD

Interrupt Processing works like as shown in the above image.

What needs to be happening is that:

Just save the value of PC somewhere and load PC with Interrupt_Service_Routine address.

However, the book says different(as presented in above microoperations). Please explain