Direct Memory Access (DMA) is a technique used in computer systems to facilitate data transfers between memory and I/O devices without the direct involvement of the microprocessor. There are three main types of data transfers in a microcomputer system:
- Data transfer between microprocessor and memory (Ex. using LDA and STA instructions);
- Data transfer between microprocessor and I/O ports (Ex. using IN and OUT instructions);
- Data transfer between memory and I/O ports (DMA data transfer).
Need for DMA
- DMA significantly boosts data transfer efficiency compared to programmed I/O methods.
- It is essential for high-speed I/O devices like hard disks and floppy disks.
- DMA facilitates the handling of large data transfers, even with slower devices such as A/D converters.
- By offloading data transfer tasks, DMA frees up the CPU for other tasks, improving overall system performance.
- DMA ensures data integrity by preventing loss or overwriting during transfers.
Operations of DMA
- Initially, the CPU maintains control over the system buses, managing data transfer and processing tasks.
- When a peripheral device requires data transfer, it sends a DMA request (DRQ) to the DMA controller.
- Upon receiving the DRQ signal, the DMA controller requests control of the buses by sending a Hold Request (HRQ) to the CPU.
- The CPU acknowledges the request by sending a Hold Acknowledge (HLDA) signal to the DMA controller, releasing control of the buses.
- With control over the buses, the DMA controller initiates the data transfer process, loading appropriate control signals onto the buses for the desired operation.
- The DMA controller sends acknowledgment signals (DACK) to the peripheral device, indicating readiness for data transfer.
- The peripheral device transfers data directly to the specified memory location, facilitated by the DMA controller. This bypasses CPU involvement, enhancing system efficiency.
- Upon completing the data transfer, the DMA controller generates an interrupt to notify the CPU of the transfer’s completion.
- The DMA controller relinquishes control of the buses back to the CPU, allowing it to resume normal operation.
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