Direct Memory Access

Direct Memory Access (DMA)
Definition: A direct memory access (DMA) is an operation in which data is copied (transported) from one resource to another resource in a computer system without the involvement of the CPU. Direct Memory Access is a capability provided by some computer bus architectures that allows data to be sent directly from an attached device (such as a disk drive) to the memory on the computer s motherboard. The microprocessor is freed from involvement with the data transfer, thus speeding up overall computer operation. An alternative to DMA is the Programmed Input/output (PIO) interface in which all data transmitted between devices goes through the processor.
Note//BG: bus grant signal and BR: bus ready signal
Advantages of DMA
-Fast memory transfer of data
-CPU and DMA run concurrently under cache mode
-DMA can trigger an interrupt, which frees the CPU from polling the
channel
Use of this mechanism can greatly increase throughput to and from a
device.
DMA Controller
The DMA controller can issue commands to the memory that behave exactly like the commands issued by the CPU. The DMA controller in a sense is a second processor in the system but is dedicated to an I/O function. The DMA controller as shown below connects one or more I/O ports directly to memory, where the I/O data stream passes through the DMA controller faster and more efficiently than through the processor as the DMA channel is specialized to the data transfer task
??The task of a DMA-controller (DMAC) is to execute the copy operation of data from one resource location to another. The copy of data can be performed from:
- I/O-device to memory
- Memory to I/O-device
- Memory to memory
- I/O-device to I/O-device