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.
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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