(^304) PC Hardware: A Beginner’s Guide
On some systems, the CPU must still wait for the DMA data transfer to complete and
the system bus to be released. However, on most systems, this is not the case and the CPU
operates from its cache while the bus is in use.
DMA Channels
A DMA device is assigned to a DMA channel, which is a single-device system resource.
Liketwodevicessharinganysystemresource,twodevicescannottypicallyshareasingle
DMA channel. There are very limited instances where two devices can share a DMA
channel, but like an IRQ, they cannot use the channel at the same time.
There are eight DMA channels, but of these, channels 0 and 4 are reserved by the sys-
tem, and channel 2 is typically assigned to the floppy disk drive; only five channels are
available for assignment to ISA devices. Another DMA channel will be used (either DMA
channel 1 or 3) if the PC includes an ECP (enhanced capabilities port) parallel port.
Table13-5liststheDMAchannelsandthedevicesmostcommonlyassignedtoeach.
Figure 13-16 shows the Device Manager’s Computer Properties window showing the
DMA channel assignments on a typical PC.DMA Modes
IDE/ATA devices, such as a floppy disk drive, use several different DMA modes to
transfer data. These modes are grouped into two sets that are differentiated by how
much data is moved. The first grou pis called single-word DMA modes. A single-wordFigure 13-15. The components of the DMA system