Operation Theory
25
DMA with Scatter Gathering Capability
In multi-user or multi-tasking OS such as Microsoft Windows,
Linux, etc., it would be difficult to allocate a large continuous
memory block due to memory fragmentation. PLX PCI control-
ler provides scatter /gather or chaining mode to link non-contin-
uous memory blocks into a linked list, so that users can
transfer large amounts of data without being limited by the frag-
ment of memory blocks. Users can configure the linked list for
the input DMA channel and the output DMA channel, individu-
ally.
Figure 4-5 shows the linked list that is constructed by three
DMA de-scriptors. Each descriptor contains a PCI address, a
local address, a transfer size, and the pointer to the next
descriptor. Users can thus collect fragmented memory blocks
and chain their associative DMA descriptors altogether. DAQ/
PXI-2500 SERIES software driver provides users easy ways to
setup scatter/gather functions. Sample programs are also sup-
plied in the all-in-one CD.
Figure 4-5: Scatter/Gather DMA for data transfer