Q. Explain about DMA and Interrupt Breakpoints?

DMA interface transfers complete block of data one word at a time directly to or from memory without going through processor. When transfer is complete, DMA interface transmits an interrupt signal to processor. So in DMA processor involvement can be restricted at beginning and end of transfer that can be displayed as in figure above. However question is when should DMA take control of bus?

For this we would recall phenomenon of execution of an instruction by processor. Figure below displays five cycles for an instruction execution. Figure also displays five points where a DMA request can be responded to and a point where interrupt request can be responded to. Please note an interrupt request is acknowledged only at one point of an instruction cycle and that is at interrupt cycle.  

Figure: DMA and Interrupt Breakpoints