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Chapter 5
Planning a VXIbus/MXIbus System A16 Address Map
The VXIbus specification does not define a method for dynamically determining the amount of
A16 space each device requires. The specification defines the upper 16 KB of A16 space for
VXIbus device configuration registers. In most cases, the lower 48 KB of A16 space are used
for VMEbus devices installed in the VXIbus system. In a VXIbus/MXIbus system, A16 space is
defined as that lower 48 KB of the A16 address space. As system integrator, you must determine
the A16 address requirements for your VXIbus/MXIbus system and define the A16 space ranges
needed as foreign devices to the system RM.
You should configure the A16 resources for your VMEbus boards in the lower 48 KB (0000
through BFFF hex) of A16 space, so that you do not interfere with VXIbus configuration space.
The logical address window mapping window is then used for mapping configuration space for
VXIbus devices, and the A16 window mapping window is used for mapping configuration space
for VMEbus devices.
When using Base/Size windowing formats, the minimum size of an A16 window is 512 B and
the maximum size is 48 KB (window size = 0). Setting an A16 window address range in the
upper 16 KB of A16 space (A15 = 1, A14 = 1) is not allowed, because it would conflict with the
logical address space. Table 5-3 shows the A16 allocation sizes used for Base/Size systems.
Table 5-3. Amount of A16 Space Allocated for all Size Values
© National Instruments Corporation
Amount of A16 Space Allocated
Size
7
512 B
6
1 KB
5
2 KB
4
4 KB
3
8 KB
2
16 KB
1
32 KB
0
48 KB (All of A16 space)
(in Bytes)
5-21
Programming Considerations
VXI-MXI User Manual
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