Draw and discuss instruction cycle with interrupt

Interrupt Cycle: In interrupt cycle, an interrupt can occur any time during the program execution. Whenever it is caused, a series of events of events take place so that the instruction fetch execute cycle can again resume after the OS calls the routine to handle the interrupt. This cycle of fetching a new instruction, decoding it and finally executing it continues until the computer is turned off.

Instruction cycle with interrupts

To accommodate interrupts, an interrupt cycle is added to the instruction cycle as shown in figure. In the interrupt cycle, the processor checks to see if any interrupts have occurred, indicated by the presence of an interrupt signal. If no interrupts are pending, the processor proceeds to the fetch cycle and fetches the next instruction of the current program. If an interrupt is pending, the processor does the following:

(i) It suspends execution of the current program being executed and saves its context. This means saving the address of the next instruction to be executed and any other data relevant to the processor's current activity.

(ii) It sets the program counter to the starting address of an interrupt handler routine. The processor now proceeds to the fetch cycle and fetches the first instruction in the interrupt handler program, which will service the interrupt. The interrupt handler program is generally part of the operating system. Typically, this program determines the nature of the interrupt and performs whatever actions are needed.