User's Manual

Chapter 2 Function Reference — Align_DMA_Buffer
NI-DAQ FRM for PC Compatibles 2-24
© National Instruments Corporation
bufferSize ≥ count + 1. If neither form of alignment is possible, the function returns an error.
If
Align_DMA_Buffer partially aligned the data, the function returns a memPageError
warning indicating that a page boundary is still in the data.
Note Physical DMA page boundaries do not exist on EISA bus computers. However,
page boundaries can be introduced on these computers as a side effect of Windows
386 Enhanced mode and the Windows NT virtual memory management system.
This happens when a buffer is locked into physical memory in preparation for a
DAQ operation. If the memory manager cannot find a contiguous space large
enough, it fragments the buffer, placing pieces of it here and there in physical
memory. This type of page boundary only affects the performance on an AT bus
computer. NI-DAQ uses the DMA chaining feature available on EISA computers
to chain across page boundaries, thus avoiding the delay involved in DMA
programming.
Call
Align_DMA_Buffer after your application has loaded buffer with the data samples
(for waveform generation or digital output) and before calling
WFM_Op, WFM_Load,
DIG_Block_In, or DIG_Block_Out. You should pass the aligned buffer to the waveform
generation and pattern generation functions the same way you would an unaligned buffer. The
count parameter in the waveform generation or pattern generation function call should be the
same as the count parameter passed to
Align_DMA_Buffer, not bufferSize.
If you want to access the data in buffer after calling
Align_DMA_Buffer, access the data
starting at buffer[alignIndex] (if your array is zero based).
After using an aligned buffer for waveform generation or pattern generation, NI-DAQ
unaligns the data. After the buffer has been unaligned, the first data sample is at offset zero
of the buffer again. If you want to use the buffer for waveform generation or pattern
generation again after it has been unaligned, you must make another call to
Align_DMA_Buffer before calling WFM_Op, WFM_Load, DIG_Block_In, or
DIG_Block_Out.
See Waveform Generation Application Hints and Digital I/O Application Hints in Chapter 3,
Software Overview, of the NI-DAQ User Manual for PC Compatibles for more information
on the use of
Align_DMA_Buffer. See Chapter 4, DMA and Programmed I/O Performance
Limitations, of the NI-DAQ User Manual for PC Compatibles for a discussion of DMA page
boundaries and special run-time considerations.