= time required for bus grant + 1 bus cycle to transfer data + time required to release the bus, it will be So, for 1 byte of transfer of data, time taken by using cycle stealing mode (T). The transfer won’t depend upon the transfer rate of device. Release the control of the bus back to CPU.īefore moving on transfer next byte of data, device performs step 1 again so that bus isn’t tied up and.Transfer the byte (at system bus speed).Inform the CPU that the device has 1 byte to transfer (i.e.The CPU delays its operation only for one memory cycle to allow the direct memory I/O transfer to “steal” one memory cycle. Y µsec =memory cycle time or cycle time or transfer time (words/block)Īn alternative method in which DMA controller transfers one word at a time after which it must return the control of the buses to the CPU. X µsec =data transfer time or preparation time (words/block) = Bus grant request time + (N) * (memory transfer rate) + Bus release control time. So, total time taken to transfer the N bytes Release the control of the bus back to CPU.Speed at which the data can be transferred to CPU. Transfer the entire block of data at transfer rate of device because the device is usually slow than the.When the DMAC operates in burst mode, the CPU is halted for the duration of the data A register is used as a byte count,īeing decremented for each byte transfer, and upon the byte count reaching zero, the DMAC will Types of DMA transfer using DMA controller:ĭMA returns the bus after complete data transfer. This transfer can take place in many ways. Once the DMA has taken the control of the buses it transfers the data. The DMA controller takes over the buses to manage the transfer directly between the I/O devices and the memory unit.īus Request : It is used by the DMA controller to request the CPU to relinquish the control of the buses.īus Grant : It is activated by the CPU to Inform the external DMA controller that the buses are in high impedance state and the requesting DMA can take control of the buses. During DMA the CPU is idle and it has no control over the memory buses. This type of data transfer technique is known as DMA or direct memory access. Thus we can allow the peripherals directly communicate with each other using the memory buses, removing the intervention of the CPU. Direct Memory Access: The data transfer between a fast storage media such as magnetic disk and memory unit is limited by the speed of the CPU.The processor is tied up in managing an I/O transfer a number of instructions must be executed.The I/O transfer rate is limited by the speed with which the processor can test and service a.Thus both these forms of I/O suffer from two inherent drawbacks. Processor to transfer data between memory and the I/O module, and any data transfer must transverseĪ path through the processor.
Note: Both the methods programmed I/O and Interrupt-driven I/O require the active intervention of the Upon detection of an external interrupt signal the CPU stops momentarily the task that it was already performing, branches to the service program to process the I/O transfer, and then return to the task it was originally performing. Whenever it is determined that the device is ready for data transfer it initiates an interrupt request signal to the computer. The interface meanwhile keeps monitoring the device. In the meantime the CPU can proceed for any other program execution. By using interrupt facility and special commands to inform the interface to issue an interrupt request signal whenever data is available from any device. This situation can very well be avoided by using an interrupt driven method for data transfer.
Interrupt- initiated I/O: Since in the above case we saw the CPU is kept busy unnecessarily.This situation can be avoided by using an interrupt facility.
This is a time consuming process since it needlessly keeps the CPU busy. In programmed I/O, the CPU stays in the program loop until the I/O unit indicates that it is ready for data transfer. A transfer from I/O device to memory requires the execution of several instructions by the CPU, including an input instruction to transfer the data from device to the CPU and store instruction to transfer the data from CPU to memory. In this case it requires constant monitoring by the CPU of the peripheral devices.Įxample of Programmed I/O: In this case, the I/O device does not have direct access to the memory unit. Usually the transfer is from a CPU register and memory. Each data item transfer is initiated by an instruction in the program.