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DMA
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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CertiGuide to A+ (A+ 4 Real) (http://www.CertiGuide.com/apfr/) on CertiGuide.com
Version 1.0 - Version Date: March 29, 2005
Adapted with permission from a work created by Tcat Houser et al.
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