Direct Memory Access (DMA) channels are used by communications devices that must send and receive information at high speeds. A serial or parallel port does not use a DMA channel, but a sound card or SCSI adapter often does. DMA channels sometimes can be shared if the devices are not the type that would need them simultaneously.

For example, you can have a network adapter and a tape backup adapter sharing DMA channel 1, but you can't back up while the network is running. To back up during network operation, you must ensure that each adapter uses a unique DMA channel.

8-Bit ISA Bus DMA Channels

In the 8-bit ISA bus, four DMA channels support high-speed data transfers between I/O devices and memory. Three of the channels are available to the expansion slots. Table 1 shows the typical uses of these DMA channels.

DMA	Standard Function	Bus Slot
0	Dynamic RAM Refresh	No
1	Available	Yes (8-bit)
2	Floppy disk controller	Yes (8-bit)
3	Hard disk controller	Yes (8-bit)

Because most systems typically have both a floppy and hard disk drive, only one DMA channel is available in 8-bit ISA systems.

16-Bit ISA DMA Channels

Since the introduction of the 286 CPU, the ISA bus has supported eight DMA channels, with seven channels available to the expansion slots. Similar to the expanded IRQ lines described earlier in this chapter, the added DMA channels were created by cascading a second DMA controller to the first one.

DMA channel 4 is used to cascade channels 0–3 to the microprocessor. Channels 0–3 are available for 8-bit transfers, and channels 5–7 are for 16-bit transfers only. Table 2 shows the typical uses for the DMA channels.

DMA	Standard Function	Bus Slot	Card Type	Transfer	Recommended Use
0	Available	Yes	16-bit	8-bit	Sound
1	Available	Yes	8/16-bit	8-bit	Sound
2	Floppy Disk Controller	Yes	8/16-bit	8-bit	Floppy Controller
3	Available	Yes	8/16-bit	8-bit	LPT1: in ECP Mode
4	1st DMA Controller Cascade	No	—	16-bit	—
5	Available	Yes	16-bit	16-bit	Sound
6	Available	Yes	16-bit	16-bit	Available
7	Available	Yes	16-bit	16-bit	Available

Note that PCI adapters don't use these ISA DMA channels; these are only for ISA cards. However, some PCI cards emulate the use of these DMA channels (such as sound cards) to work with older software. The only standard DMA channel used in all systems is DMA 2, which is universally used by the floppy controller.

DMA 4 is not usable and does not appear in the bus slots. DMA channels 1 and 5 are most commonly used by ISA sound cards, such as the Sound Blaster 16, or by newer PCI sound cards that emulate an older one for backwards compatibility. These cards use both an 8-bit and a 16-bit DMA channel for high-speed transfers.