What is Direct Memory Access?
Some computer bus architectures incorporate a functionality called Direct Memory Access whereby data can be transferred directly from an attached device like a disk drive to the system’s main memory in the motherboard without necessarily passing through the CPU. Data transfer frees the microprocessor from operations that include data transfer leading to a faster computer work rate.
DMA is basically about allowing disk drives, external memory, graphic cards, network cards, and sound cards among other devices to read and write data into the memory of the computer. This is achieved without interfering with the operations of the other sections like the CPU.
The process of data management avoids overloading the computer’s CPU with many requests for external devices, which prevents the performing of other operations by the computer. A user can operate a CPU to commence a data transfer on DMA with another attached equipment while other works are being executed in parallel. Data movement between devices and the computer is possible at a reduced cost for the CPU through DMA.
Instead of DMA, we have the Ultra DMA delivering up to a transfer rate of 33 MBps. Hard disks with ultra DMA/33 operate in PIO-1, PIO-3, PIO-4, and multiword DMA-2 mode at 16.8 MBs.
The CPU sends a signal to the DMA controller to start transferring data, and it sets up the addressing of the system memory as well as the read/write modes in the process.
Direct Memory Access in OS
During transferring the data process, involves processors, memory, and input-output devices. The transfer starts when the processor initiates it and ends upon storage of the data. Therefore, the processor’s load increases and should be constant to keep its efficiency high. While a station master assists with increasing the pace of data transmission between I/O devices and memory, the DMA controller functions like the station master. To reduce their work, the DMA controller tries to minimize the role of the processor in this whole operation.
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