Question

Why is the interrupt flag checked at the end of each instruction cycle? Why couldn't this be done at a different point (for example, after the instruction operand decoding)?

Answer 1

An instruction cycle can be divided into 4 stages.

a) Fetch cycle

b) Instruction decode and register fetch

c) Execute

d) Memory Access

e) Register Write Back

In fetch cycle the following micro operation happens

a) Copying the next instruction address to be fetched from PC to Memory Address Register(MAR)

b) Copying the data from Memory to Memory Buffer Register(MBR) and increment PC to point to next instruction to be executed

c) Copying the instruction to the Instruction Register from MBR.

So if in case interrupt flag is checked between the instruction cycle phases, the content of PC which points to the next instruction to be executed will be stored in memory and then the interrupt routine is executed .After executing the interrupt routine , the PC will be loaded with the address of the next instruction which was stored in memory before Interrupt. This will result in the failure to complete the current instruction which was executing before interrupt call. Because of this reason interrupt flag is checked and interrupt routine is called only after completing each instruction cycle not between a cycle.