User`s guide

128 Agilent VISA User’s Guide
4 Programming via GPIB and VXI
Using VXI-Specific Attributes
VXI- specific attributes can be useful to determine the state
of your VXI system. Attributes are read- only and read/write.
Read- only attributes specify things such as the logical
address of the VXI device and information about where your
VXI device is mapped. This section shows how you might
use some of the VXI- specific attributes. See VISA Resource
Classes in the VISA Online Help for information on VISA
attributes.
Using the Map Address as a Pointer
The VI_ATTR_WIN_ACCESS read- only attribute specifies how
a window can be accessed. You can access a mapped
window with the VISA low- level memory functions or with a
C pointer if the address is de- referenced. To determine how
to access the window, read the VI_ATTR_WIN_ACCESS
attribute.
VI_ATTR_WIN_ACCESS Settings
The VI_ATTR_WIN_ACCESS read- only attribute can be set to
one of the following:
Table 31 Settings for the VI_ATTR_WIN_ACCESS Attribute
Setting Description
VI_NMAPPED Specifies that the window is not mapped.
VI_USE_OPERS Specifies that the window is mapped and you can only use
the low-level memory functions to access the data.
VI_DEREF_ADDR Specifies that the window is mapped and has a
de-referenced address. In this case you can use the
low-level memory functions to access the data, or you can
use a C pointer. Using a de-referenced C pointer will allow
faster access to data.