DMA stands for stands for Direct Memory Access and is a method for some but not all computer hardware to transfer data to and from RAM completely bypassing the CPU. This not only makes it faster for a device to manipulate data but is conserves CPU time making other processes move faster as well.
The original IBM PC and AT had four DMA channels, labeled 0 to 3. Channel 0 was used to update (refresh) the memory on the PC. DMA channel 1 quickly became popular for use with the ISA based Sound Blaster sound card. DMA channel 2 was (and still is) reserved for the floppy drive. Channel 3 is most often used to extend the capabilities of the parallel port in the Enhanced Capability Port (ECP) mode. That leaves channel 4, and the same sort of cramming that occurred with IRQs happened with DMA.
So, when IBM came out with the AT, they added four more DMA channels. However, for reasons beyond the scope of this book, the designers had to give up DMA 4 to address DMA channels 5, 6, and 7.
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