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Why DMA Channels Were Invented for Data Transfer
As you know, the processor is the "brain" of the machine, and in many ways it can also be likened to the conductor of an orchestra. In early machines the processor really did almost everything. In addition to running programs it was also responsible for transferring data to and from peripherals. Unfortunately, having the processor perform these transfers is very inefficient, because it then is unable to do anything else.
The invention of DMA enabled the devices to cut out the "middle man", allowing the processor to do other work and the peripherals to transfer data themselves, leading to increased performance. Special channels were created, along with circuitry to control them, that allowed the transfer of information without the processor controlling every aspect of the transfer. This circuitry is normally part of the system chipset on the motherboard.
Note that DMA channels are only on the ISA bus (and EISA and VLB, since they are derivatives of it). PCI devices do not use standard DMA channels at all.