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Appendix
Enabling the DMA Mode
DMA mode - Direct Memory Access - allows certain hardware subsystems within a computer to access
system memory for reading and/or writing independently of the CPU. Many hardware systems use
DMA including disk drive controllers, graphics cards, network cards, and sound cards.
DMA is an essential feature of all modern computers, as it allows devices of different speeds to
communicate without subjecting the CPU to a massive interrupt load. Otherwise, the CPU would have
to copy each piece of data from the source to one of its registers, and then write it back again to the
new location. During this time the CPU would be unavailable for other tasks.
A DMA transfer essentially copies a block of memory from one device to another. While the CPU
initiates the transfer, the transfer itself is performed by the DMA Controller. A typical example is
moving a block of memory from external memory to faster, internal (on-chip) memory. Such an
operation does not stall the processor, which as a result can be scheduled to perform other tasks. It is
essential in providing so-called zero-copy implementations of peripheral device drivers as well as
functionalities such as network packet routing, file copy, streaming video over a network, etc.
To enable the DMA mode you should do the following:
1. Go to the Windows Start menu and select Control Panel ;
2. In the Control Panel window choose the System icon, double-click it to open the system
properties window;
3. In the Hardware tab there is a Device Manager button: