In microprocessor based system DMA refers to?

Question

Examoogle · Accepted Answer

Direct memory access for the I/O device

Explanation: Direct Memory Access (DMA) is a feature of computer systems that allows certain hardware subsystems to access the main system memory (RAM) independently of the Central Processing Unit (CPU). It enables high-speed data transfer between an I/O device and memory without requiring the CPU to manage every byte of data, thus improving overall system throughput.

Working Principle: When an I/O device requires data transfer, it sends a DMA request (DMR) signal to the DMA controller. The controller then requests control of the system bus from the CPU using a Bus Request (BR) signal. Once the CPU acknowledges with a Bus Grant (BG) signal, the DMA controller takes control of the address, data, and control buses to perform the data transfer directly between the peripheral and memory, freeing the CPU to perform other computational tasks.

Key Formulas: $T_{total} = T_{setup} + T_{transfer}$ — Total time taken for DMA operation.

Key Points: DMA bypasses the CPU for data movement to reduce CPU overhead.; The DMA Controller (DMAC) acts as a temporary bus master.; Used extensively for high-speed peripherals like disk drives and network interface cards.; Supports modes like Burst Mode, Cycle Stealing, and Transparent DMA.

Advantages: Reduces CPU interrupt load significantly.; Increases data transfer rates by using direct bus access.

Disadvantages: Increases hardware complexity by requiring a dedicated DMA controller.; Potential for bus contention between CPU and DMAC.

Applications: Hard disk drive data transfer.; Graphics processing and display frame buffer updates.

Additional Info: In programmed I/O, the CPU is involved in every transfer, which is slower.. Option A and B are incorrect because DMA serves the I/O devices to move data to/from RAM, not as a general service for the microprocessor or user directly.

Answer: C is correct — DMA facilitates data transfer directly between an I/O device and system memory, bypassing the CPU to enhance system performance.

Exam Tip: Remember that while DMA increases efficiency, it requires the system to handle 'bus arbitration' to prevent the CPU and DMA from accessing the bus simultaneously.

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