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Computer organization Notes

Interrupt Cycle

The way that the interrupt is handled by the computer can be explained by means of the flowchart of Figure below. An interrupt flip-flop R is included in the computer. When R = 0, the computer goes through an instruction cycle. During the execute phase of the instruction cycle IEN is checked by the control. If it is 0, it indicates that the programmer does not want to use the interrupt, so control continues with the next instruction cycle. If IEN is 1, control checks the flag bits. If both flags are 0, it indicates that neither the input nor the output registers are ready for transfer of information. In this case, control continues with the next instruction cycle. If either flag is set to 1 while IEN = 1, flip-flop R is set to 1. At the end of the execute phase, control checks the value of R, and if it is equal to 1, it goes to an interrupt cycle instead of an instruction cycle.
The interrupt cycle is a HW implementation of a branch and save return address operation. The return address available in PC is stored in a specific location where it can be found later when the program returns to the instruction at which it was interrupted. This location may be a processor register, a memory stack, o r a specific memory location. Here we choose the memory location at address 0 as the place for storing the return address. Control then inserts address 1 into PC and clears IEN and R so that no more interruptions can occur until the interrupt request from the flag has been serviced.

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