Direct Memory Access (DMA)
Many computers avoid burdening the main CPU with programmed I/O by offloading some of this work to a special purpose processor. This type of processor is called, a Direct Memory Access(DMA) controller. A special control unit is used to transfer block of data directly between an external device and the main memory, without intervention by the processor. This approach is called Direct Memory Access(DMA).
DMA can be used with either polling or interrupt software. DMA is particularly useful on devices like disks, where many bytes of information can be transferred in single I/O operations. When used with an interrupt, the CPU is notified only after the entire block of data has been transferred. For each byte or word transferred, it must provide the memory address and all the bus signals controlling the data transfer. Interaction with a device controller is managed through a device driver.
Handshaking is a process between the DMA controller and the device controller. It is performed via wires using terms DMA request and DMA acknowledge.
| Step | Description |
|---|---|
| 1 | Device driver is instructed to transfer disk data to a buffer address X. |
| 2 | Device driver then instruct disk controller to transfer data to buffer. |
| 3 | Disk controller starts DMA transfer. |
| 4 | Disk controller sends each byte to DMA controller. |
| 5 | DMA controller transfers bytes to buffer, increases the memory address, decreases the counter C until C becomes zero. |
| 6 | When C becomes zero, DMA interrupts CPU to signal transfer completion. |