National Instruments NI-VXI User Manual
Page 67
Chapter 3 Software Overview
© National Instruments Corporation
3-29
NI-VXI User Manual
can have multiple pointers in the same process or over multiple
processes to access a particular window simultaneously, while still
guaranteeing that the hardware context does not change between
accesses. The call succeeds under either of the following conditions:
•
No processes are mapped for the window (first caller for
Access-Only Privilege for this window). The hardware context is
set as requested in the call. The call returns a successful status and a
valid C pointer and windowId for Access-Only Privilege.
•
No process currently has Owner Privilege to the required window.
There are processes with Access-Only Privilege, but they are using
the same hardware context (privilege state, byte order, address
range) for their accesses to the window. Because the hardware
context is compatible, it does not need to be changed. The call
returns a successful status and a valid C pointer and windowId for
Access-Only Privilege.
The successful call returns a valid pointer and a non-negative return
value. The 32-bit window number signifies that the access privileges to
the window are Access-Only Privilege.
With Access-Only Privilege, you cannot use the
SetPrivilege
,
SetByteOrder
, and
SetContext
calls in your application to
dynamically change the hardware context. No Access-Only accessor
can change the state of the window. The initial Access-Only call sets
the hardware context for the window, which cannot be changed until
all Access-Only accessors have called
UnMapVXIAddress
to free the
window. The functions
GetPrivilege
,
GetByteOrder
, and
GetContext
will succeed regardless of whether the caller has Owner
Privilege or Access-Only Privilege.
The following paragraphs describe the low-level VXIbus access
functions. The descriptions are presented at a functional level
describing the operation of each of the functions. The functions are
grouped by area of functionality.
Note:
On MITE-based platforms,
MapVXIAddress
cannot be called while the
CPU is in interrupt context. For this reason, it is recommended that you
not use the
SaveContext
and
RestoreContext
functions. Due to the
multiple window support of the MITE, you should not need these
functions.