Arrange the given steps required for a Direct Memory Access (DMA) transfer in the correct order.
I. Initiate DMA transfer request
II. Transfer data directly between peripheral and memory
III. Processor grants DMA control over the system bus
IV. DMA controller completes data transfer and signals completion
Explanation
Correct Answer: (a) I, III, II, IV
Detailed Chronological Explanation
Direct Memory Access (DMA) is a hardware mechanism that allows an I/O peripheral device to transfer data directly to or from the main system memory without continuous intervention from the CPU. The operational steps occur in a strict sequential chain:
Step I (Initiation): The I/O device or the subsystem sends an initial request to the DMA controller, or the CPU prepares the DMA controller with metadata (memory addresses and word counts) to initiate a DMA transfer request.
Step III (Bus Grant): The DMA controller requests ownership of the system buses from the CPU via a Hold Request (HOLD) signal. The processor grants DMA control over the system bus by returning a Hold Acknowledge (HLDA) signal, temporarily releasing control of its data and address lines.
Step II (Data Transfer): Once the DMA controller has control of the system bus, it executes the high-speed block movement, routing the transfer data directly between the peripheral and memory without cycling through internal CPU registers.
Step IV (Completion): When the structural data word counter reaches zero, the DMA controller completes the data transfer and signals completion back to the CPU via an interrupt line (INT), safely returning bus control to the main processor.
Therefore, the only valid sequence is
I⟶III⟶II⟶IV
, matching option (a).
