DMA is used between_____.

Question

Examoogle · Accepted Answer

Memory and I/O

Explanation: Direct Memory Access (DMA) is a feature that allows peripheral devices (I/O) to transfer data directly to or from the system memory (RAM) without involving the CPU. This mechanism bypasses the microprocessor for data movement, significantly reducing the CPU load during high-speed I/O operations.

Working Principle: When a DMA-enabled device needs to transfer data, it sends a 'Hold Request' (HLDA) to the CPU. The CPU completes its current bus cycle and grants control of the system bus to the DMA controller (DMAC). The DMAC then manages the data transfer directly between the memory address space and the I/O port, acting as a temporary bus master.

Key Formulas: $T_{transfer} = N 	imes T_{cycle}$ — where $T_{transfer}$ is the total time for data transfer, $N$ is the number of data words, and $T_{cycle}$ is the time taken per bus cycle.

Key Points: DMA improves system throughput for high-speed devices like disk drives and network adapters.; The CPU remains idle or performs other tasks while the DMA controller manages the data bus.; A 'Bus Request' and 'Bus Grant' handshake is mandatory for DMA operation.

Advantages: Increased system efficiency by offloading I/O tasks from the CPU.; Higher data transfer rates compared to Programmed I/O or Interrupt-driven I/O.

Disadvantages: Increases hardware complexity by requiring a dedicated DMA controller.; Causes 'cycle stealing' where the CPU may be forced to wait if it needs the bus while the DMA is active.

Applications: Hard disk drives (HDD) and Solid State Drives (SSD) data transfers.; Graphics processing (GPU) memory access.; High-speed network cards (NIC).

Additional Info: Option A is incorrect because while the I/O communicates with the CPU in programmed I/O, DMA specifically targets the path between I/O and Memory.. Option B is incorrect as the CPU-to-Memory path is the primary bus operation, not the definition of DMA.

Answer: C is correct — DMA is a specialized mechanism that facilitates direct data exchange between Memory and I/O devices, bypassing the CPU to enhance efficiency.

Exam Tip: Always remember that DMA 'steals' bus cycles; look for keywords like 'bus master' or 'bus arbitration' in related exam questions.

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