Direct Memory Access and ISA DMA channel assignments

Direct Memory Access (DMA) is a means by which devices can exchange data with memory or with each other without requiring intervention by the processor. Standard DMA allows a device to exchange data with memory, but not with another device. Bus Mastering DMA allows two devices to communicate directly with each other. The advantage of using DMA is that it reduces the load on the processor, allowing it to perform other tasks. There are even fewer DMA channels than IRQs—eight versus 16—but DMA channels are much less in demand than IRQs, so DMA channel availability is almost never an issue.The table below lists DMA channels and the uses to which they are typically put.

DMA 2 is used by nearly all systems for the floppy disk drive controller. Excluding DMA 4, which is a dedicated cascade channel (used to access the secondary DMA controller), the other DMA channels are available for use with expansion cards. DMA 0 is almost never used because, although it appears only in 16-bit slots, it supports only 8-bit transfers. Most ISA sound cards require two DMA channels, with 8-bit sound using DMA 1 and 16-bit sound using DMA 5. Note that these DMA channels pertain only to ISA cards. PCI devices do not require one of these DMA channels to use DMA. For example, if you enable DMA transfer mode on one or both of the embedded PCI IDE controllers, you will find that they operate in DMA mode without occupying ISA DMA channels.

The only time DMA conflicts are likely to arise is if you install an ISA sound card and an ISA SCSI host adapter. Nearly all ISA sound cards use both DMA 1 and DMA 5, and some ISA SCSI cards are configured by default to use DMA 5, which causes a conflict. The easy answer is to configure the SCSI host adapter to use DMA 6 or DMA 7. The better answer, as usual, is to avoid ISA cards whenever possible.