<P> Scatter - gather or vectored I / O DMA allows the transfer of data to and from multiple memory areas in a single DMA transaction . It is equivalent to the chaining together of multiple simple DMA requests . The motivation is to off - load multiple input / output interrupt and data copy tasks from the CPU . </P> <P> DRQ stands for Data request; DACK for Data acknowledge . These symbols, seen on hardware schematics of computer systems with DMA functionality, represent electronic signaling lines between the CPU and DMA controller . Each DMA channel has one Request and one Acknowledge line . A device that uses DMA must be configured to use both lines of the assigned DMA channel . </P> <P> Standard ISA DMA assignments: </P> <Ol> <Li> DRAM Refresh (obsolete), </Li> <Li> User hardware, usually sound card 8 - bit DMA </Li> <Li> Floppy disk controller, </Li> <Li> Hard disk (obsoleted by PIO modes, and replaced by UDMA modes), Parallel Port (ECP capable port), certain SoundBlaster Clones like the OPTi 928 . </Li> <Li> Cascade from XT DMA controller, </Li> <Li> Hard Disk (PS / 2 only), user hardware for all others, usually sound card 16 - bit DMA </Li> <Li> User hardware . </Li> <Li> User hardware . </Li> </Ol>

The primary purpose of channel programs is to transfer data using dma between