ADLINK Technology NuDAQ PCI-9222, Руководство пользователя

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36 Operation Theory
simply need to assign the sampling period and the number of
conversion through their specified counters. After the AD trigger
condition is matched, the data will be transferred to the system
memory by the bus-mastering DMA.
The PCI controller also supports the scatter/gather bus mastering
DMA function that enables transfer of large amounts of data by
linking all the memory blocks into a continuous linked list.
In a multi-user or multitasking OS, like Windows, Linux, etc, it is
difficult to allocate a large continuous memory block to do the
DMA transfer. Therefore, the PCI controller provides the function
of scatter-gather or chaining mode DMA to link the non-continuous
memory blocks into a linked list, allowing transfers of very large
amounts of data without being limited by the fragment of small size
memory. You may configure the linked list for the input DMA
channel or the output DMA channel. Figure 4-8 shows a linked list
that is constructed by three DMA descriptors. Each descriptor
contains a PCI address, a PCI dual-address, a transfer size, and
the pointer to the next descriptor. PCI address and PCI dual-
address support 64-bit addresses which can be mapped into more
than 4 GB of the address space. You can allocate many small size
memory blocks and chain their associative DMA descriptors
altogether by their application programs. The software driver
provides simple settings of the scatter-gather function, and some
sample programs are also provided in the ADLINK All-in-one CD.
Figure 4-8: Scatter-gather DMA for Data Transfer