Question

In this chapter, we assumed that the interrupt service always starts at the beginning of the instruction cycle. It may not be desirable to wait until the end of current instruction cycle during complex instructions (such as multiple moves). What are the implications of interrupting an instruction cycle in progress? How would you restart the interrupted instruction?

Answer 1

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ANSWER

The implications of interrupting an instruction cycle in progress are -

1) The memory address where PC (Program Counter) is pointing may be lost.

2) The values of certain variables stored in memory locations which are required for computation may get corrupted.

3) Since pointer to memory address gets changed so address of next instruction cannot be determined when resumed. Hence complete program will be affected.

Restarting the interrupted instruction

In some instructions, the interrupted instructions need to re-compute the values stored in the register or memory locations. This increases the latency of the instruction. So they restart from the last memory location accessed. However in some micro-code instructions they have mechanisms for being interrupted without having to restart from scratch. So, computaion can resume from where it was interrupted.

Example - rep movsb can leave RSI, RDI, and RCX updated to part-way through a copy so it will finish the copy on restart.