srestha When CPU gets an interrupt signal then it asks the DMA controller to handle the interrupts and transfer data b/w the I/O device and the memory. This requires the control of memory bus which the cpu gives to the DMA for some time. So then you might think what will cpu do now without the bus.. CPU can carry out other instructions which do not require memory like ADD/SUB two register variables. So at this time parallely the DMA works on the transferring b/w I/O and main memory while the CPU handles instructions which do require MM.
But there might arrive a situation where CPU may require the MM for eg to execute LOAD/STORE instructions. But since DMA has the control over the memory bus CPU has to wait till it gets it back. So at this time the cpu activity gets slowed down.