Explain the working of Direct Memory Access. Describe the DMA controller and its advantages over other I/O methods.

Working of DMA

The CPU writes the starting memory address, the number of words and the direction into the DMA controller. The device then raises a DMA request. The controller sends a bus request to the CPU. The CPU finishes its current cycle, releases the buses and sends a bus grant. The controller puts the address on the address bus and sends the memory and device control signals, and one word moves directly between the device and memory. The controller increases the address and decreases the count. This repeats until the count is zero. At that point the controller returns the bus to the CPU and sends an interrupt to say that the transfer is complete.

The DMA controller

The controller has these main parts:

• Address register: it holds the address of the next memory word.
• Word count register: it holds the number of words still to be moved.
• Control register: it holds the direction and the mode (burst or cycle stealing).
• Data buffer: it connects the controller to the data bus.
• Bus request and bus grant lines: these connect it to the CPU for taking the buses.
• DMA request and acknowledge lines: these connect it to the I/O device.

The CPU can read and write these registers through the register select input, in the same way as for any other interface.

Advantages over other I/O methods

The main advantages of DMA are that it gives the highest data rate, since words are not routed through the CPU, and it frees the CPU for other tasks. It has far less overhead than interrupts because only one interrupt is used for a complete block. Its disadvantages are the extra hardware and the need to share the memory bus, which can slow the CPU a little during the transfer.