Chapter 6 NI-VXI Configuration Utility
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National Instruments Corporation 6-21 VXI/VME-PCI802 2 for Sola ris
Address Space and Requested Memory
The VXI/VME-MXI-2 requires at least 16KB of address space in A24
space or at least 64KB in A32 space. Use the Address Space control
to se lect w h ether yo u wa nt to use A2 4 s pace o r A3 2 sp ace . Us e the
Requested Memory control to set the amount of memory space that the
VXI/VME-MXI-2 will request. You can select up to 8MB in A24 space
and up to 2GB in A32 space. The default setting uses the minimum
requirement of 16 KB in A24 space.
These controls are necessary if you change the amount of DRAM
installed on the VXI/VME-MXI-2. The amount of memory you set with
the Requested Memory control should match the amount of DRAM
installed on the VXI/VME-MXI-2. If no DRAM is installed, keep the
default setting of 16 KB. Notice that the smallest valid amount in A32
space is 64KB.
Caution: If you install DRAM into the VXI/VME-MXI-2, do not attempt to use the
first 4 KB of memory space. This 4 KB space maps to the registers on the
VXI/VME-MXI-2 and does not access onboard DRAM. Accessing this
region will cause your VXI/VME-MXI-2 to behave incorrectly.
If you do not want to lose 4 KB of DRAM you can get around this
limitation by setting the Requested Memory control to double the
amount that is installed on the VXI/VME-MXI-2, because the DRAM
is aliased throughout the remainder of the requested memory space.
The DRAM should then be accessed in the upper half of the requested
memory space.
A16 and A24/A32 Write Posting
The VXI/VME-MXI-2 can increase performance with its capability to
post write cycles from both the MXIbus and the VXI/VMEbus. Write
cycles should be posted only to devices that cannot return a BERR
signal, because the BERR will not be reported to the originating master.
Click on the checkbox control(s) if you want to use either A16 or
A24/A32 write posting. By default, both options are disabled.
The A16 write posting control affects only write cycles that map
through the A16 window from the VXI/VMEbus to the MXIbus and
vice versa. A16 write cycles in VXI configuration space are never
posted regardless of the setting of this control.
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