Some expansion boards, such as CD-ROM cards, are capable of accessing the computer's DMA channel. When you install the board, you must specify which DMA channel is to be used, which sometimes involves setting a jumper or DIP switch.

PC system resources reference guide 
Covers IRQs, DMAs, I/O, and memory addresses, as well as system configuration, resource conflicts, and plug and play.

Direct Memory Access (DMA) Channels
Describes DMA channel function and operation, with details on each of the 8 DMA channels. This page is from "The PC Guide".