DIRECT MEMORY ACCESS CHANNELS (DMAS)


DMA is a memory controller with a straight path to memory. DMA channels, unlike IRQs, allow DMA devices to access memory directly, without interrupting the CPU. This allows the CPU to carry out more important functions and the DMA devices themselves to process requests faster.

DMA controller chips are integrated into the motherboard and control the DMA channels. DMA devices, such as ISA (non-plug-and-play) cards and IDE/ATA controller interfaces, have access to DMA services. DMA does not support PCI or AGP technology.

Early computing systems, such as the AT-class computer, provided only one DMA controller with only four DMA channels, which supported 8-bit and 16-bit cards. Today’s computers are equipped with two DMA controllers and eight DMA channels, DMA 0 to DMA 7. Table 18.4 shows standard DMA channel device assignments. Refer to Figure 18.3 for a Windows 2000 display of DMA channels. You need to be familiar with DMA channel assignments 2 and 4 for the test. You may also be asked to identify which DMA channels are available by default for use with peripheral devices.

Table 18.4: DMA Channel Device Assignments

DMA Channel

Device Attached

0

Available

1

Available

2

Floppy drive (possible tape drive)

3

Available

4

Second DMA controller (cascades to DMA channels 0-3)

5

Available

6

Available

7

Available

click to expand
Figure 18.3: Windows 2000 direct memory access (DMA) assignments.

As with IRQs, there can only be one active device using a DMA channel at a time. If more than one device attempts to access a DMA channel already in use, a DMA conflict will occur.

Different forms of DMA technology are available. Third-party DMA is the original implementation of the DMA. The DMA controller chip resides on the motherboard and is designed for supporting ISA devices. Newer, first-party DMA is very popular. With first-party DMA, the DMA controller resides on the peripheral device. This allows the peripheral to actually take control of the system bus to handle the transfer of data in and out of memory. This process is referred to as bus mastering. Newer DMA modes are available, such as ultra DMA, offering even faster transfer rates of data.




The A+ Certification & PC Repair Handbook
The A+ Certification & PC Repair Handbook (Charles River Media Networking/Security)
ISBN: 1584503726
EAN: 2147483647
Year: 2003
Pages: 390

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