Operating System - Direct Memory Access

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Input/Output operations (I/O) are an important part of an operating system. I/O operations allows communication between the CPU and other devices such as keyboards, mouse, printers, and storage drives. There are various I/O techniques like programmed I/O, interrupt I/O and direct memory access(DMA) out of which DMA does not need CPU continuously.

What is DMA I/O?

Direct Memory Access (DMA) is a technique in which a dedicated hardware (DMA Controller) is used to handle the data transfer between memory and I/O devices. In DMA, CPU initiates the transfer, and configures the DMA controller with required parameters. It makes DMA method independent of CPU. Using DMA, the data can be directly transferred to/from devices to the main memory.

Characteristics of DMA I/O

The key features/characteristics of Direct Memory Access are mentioned below −

• It requires CPU only at the start and at the end.
• It uses a dedicated hardware DMA controller.
• It transfers data directly from I/O device to/from memory.
• It can transfer large amount of data at a time unlike Programmed I/O that transfers a single Byte/character at a time.
• Increase CPU efficiency and reduces CPU overhead as the CPU is required at only beginning and end of the transfer.
• It is used in disk I/O, graphics devices, network packets, audio/video streaming.

How Does Direct Memory Access (DMA) Work?

The working of Direct Memory Access (DMA) is given below in 7 steps −

• In the beginning, the CPU configures the DMA controller and provides parameters such as: memory address, I/O device, and transfer size.
• Then, I/O device sends a DMA request signal to the DMA controller when data is ready to transfer.
• The DMA controller then requests CPU for control of system bus.
• The CPU grants bus control to the DMA controller.
• Then, DMA controller transfers data directly between memory and I/O device.
• When transfer of all the data is completed, then DMA controller gives the access of bus back to the CPU.
• At the end, the DMA controller sends an interrupt signal to the CPU. It represents that the transfer is completed.

Take a look at the following animation. It depicts visually how the DMA works −

DMA Controller

A DMA(Direct Memory Access) controller is a special hardware that transfers data directly between memory and I/O devices without any involvement of CPU. It has four components which are listed below −

• Address Register − It stores the memory address from where the data should be read or written to.
• Data Register − It is used to temporarily store the data being transferred between memory and I/O device.
• Control Logic − It manages the complete transfer process like receiving commands from the CPU, processing signals from devices, and controlling the address and data buses.
• Count Register − It keeps a track of number of bytes that are remaining to transfer.
• Control Register − It specify mode of transfer, direction, and other parameters.

Transfer Modes in DMA

There are three different modes of transfer used by DMA which are as follows −

• Burst Mode − In burst mode, DMA transfers a complete block of data in one go.
• Cycle Stealing Mode − In this mode, DMA transfers one byte/word at a time and then releases the bus.
• Transparent Mode − In the transparent mode, DMA uses bus only when CPU is not using it. It is the slowest transfer mode.

Differences Between Programmed, Interrupt-Initiated and DMA I/O

The differences between Programmed I/O, Interrupt-Initiated I/O and Direct Memory Access(DMA) is mentioned below −

Advantages of DMA

The advantages of Direct Memory Access(DMA) are as follows −

• Data transfer is faster in DMA compared to Programmed and Interrupt-Initiated I/O.
• Less CPU overhead as DMA controller handles the complete task and CPU is required only at the beginning and at the end.
• Improved multitasking as CPU can perform other task and at the same time I/O operation can occur concurrently.
• Best for high-speed devices.
• Low latency since CPU is not involved and everything is managed by a dedicated DMA controller.

Disadvantages of DMA

The disadvantages of Direct Memory Access(DMA) are as follows −

• High cost due to extra hardware (DMA controller).
• CPU operations may slow down as DMA controller has access of Bus.
• More complex to design and implement.
• It is hard to debug.
• It is not suitable for small data transfers.

Applications of Direct Memory Access

Here are some of the real-world application of DMA −

• It is used in hard drives and SSDs to transfer data.
• It is used in Network interface cards(NIC) for moving the packet data between network buffers and main memory.
• It is used in graphics cards for transferring the texture data, frame buffers, and commands between memory and video memory.
• It is used in sound cards to stream audio data continuously, for smooth playback without any latency.

Conclusion

Direct Memory Access (DMA) is a technique in which a dedicated hardware (DMA Controller) is used to handle the data transfer between memory and I/O devices. This chapter provides a detailed explanation of DMA, its characteristics, DMA controller, working, transfer modes, differences from programmed and interrupt-initiated I/O, real-life implementation, advantages, and disadvantages of DMA.