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DMA Channels and the System Bus
All of the DMA channels except channel 4 are accessible to devices on the ISA system bus. Channel 4 is used to cascade the two DMA controllers together. PCI devices do not use standard system DMA channels.
As was the case with IRQs, the second DMA controller was added when the ISA bus was expanded to 16 bits with the creation of the AT. The lines to access these extra DMA channels were placed on the second part of the AT slot that is used by 16-bit cards. This means that only 16-bit cards can access DMA channels 5, 6 or 7. Unfortunately, many devices even today are still only 8-bit cards. You can tell by looking at them and seeing that they only use the first part of the two-part ISA bus connector on the motherboard.