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Table Of Contents  CertiGuide to A+ (Core Hardware)
 9  Chapter 0001:  Power Supplies - System Board
      9  XV  Demanding Attention Digitally

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XVI  DMA and Bus Mastering
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DMA, the PC and the AT

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.

[spacer]DMA Details

DMA channels 0-4 originally were 8-bit (only) and capable of a maximum of 64KB per transfer. In the AT and above, 0, 5, 6 and 7 are 16-bit channels and capable of 128KB per transfer. Only one transfer can occur at a time.

DMA must reside in conventional memory.

Not many ISA based cards were designed to take advantage of the new DMA channels available in the AT.

DMA channel 3 presents the greatest opportunity for a DMA conflict, which will result in very erratic behavior.


Keep in mind that while DMA channels became standardized, PCI does not use standardized DMA the way other expansion bus slots use them. Some refer to DMA as 'third-party DMA. The first and second to the party are the two sub-systems, with DMA being the third party.


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XVI  DMA and Bus Mastering
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