ADLINK PCI-9846 - Manual (Page 53)

Operation Theory 43
3.6 Data Transfers
Since the maximum data throughput on the PCI/PXI-9846 (40MS/
s * 4 channels *2 Bytes/channel = 320MB/s) is much higher than
the 32bit/33MHz PCI-bus bandwidth, samples are acquired into
the onboard SDRAM memory before being transferred to the host
computer. Since the number of stored samples per acquisition is
limited by the amount of on-board memory, the PCI/PXI-9816/26/
46 supports maximum 512MB in order to meet application require-
ments.
Once all the data has been stored in the on-board memory, the
data will be transferred to the host computer’s memory through
bus-mastering DMA.
In a multi-user or multi-tasking OS, like Microsoft Windows, Linux,
and so on, it is difficult to allocate a large continuous memory
block to do the DMA transfer. Therefore, the PCI/PXI-9816/26/46
provides the function of scatter-gather DMA to link the non-contin-
uous memory blocks into a linked list so that you can transfer very
large amounts of data without being limited by the fragment of
small size memory, as illustrated in Figure 3-17.
Figure 3-17: Scatter-Gather DMA for Data Transfer
PC
I B
us
Local
Memory
512MB
PXI-9816/PXI-9826
/PXI-9846
First PCI Address
First Local Address
Transfer Size
Next Descriptor
PCI Address
Local Address
Transfer Size
Next Descriptor
PCI Address
Local Address
Transfer Size
Next Descriptor
System Memory