Chapter 4
Register Descriptions
© National Instruments Corporation
4-13
VME-MXI-2 User Manual
Extender A16 Window Register (VWR1)
VMEbus A16 Offset:
C (hex)
Attributes:
Read/Write
32, 16, 8-bit accessible
15
14
13
12
11
10
9
8
0
A16EN
A16DIR
1
1
A16SIZE[2]
A16SIZE[1]
A16SIZE[0]
7
6
5
4
3
2
1
0
A16BASE[7]
A16BASE[6]
A16BASE[5]
A16BASE[4]
A16BASE[3]
A16BASE[2]
A16BASE[1]
A16BASE[0]
You can use this register to control the mapping of VMEbus A16 space between the
VMEbus and the MXIbus. Only the lower three quarters of A16 space can be mapped
using this register, because the upper one quarter is VXIbus configuration space, which
must be mapped through VWR0. This register conforms to the VXIbus Mainframe
Extender specification.
This register takes on a different form when the CMODE bit in the VME-MXI-2 Control
Register (VMCR) is set. This different form does not comply with the VXIbus
Mainframe Extender specification, and the CMODE bit should not be set when using a
VXIbus multiframe Resource Manager. For more information on the CMODE bit, refer
to the VMCR register description.
When accessed with a 32-bit cycle, the bits of this register appear on bits 31 to 16 along
with the Extender A24 Window Register (VWR2) on bits 15 to 0. To accommodate 8-bit
masters that write to this register, the window is not enabled until the lower byte of the
register is written. Therefore, 8-bit masters should write the upper byte first, followed by
the lower byte.
Bit
Mnemonic
Description
15
0
Reserved
This bit is reserved and returns 0 when read. This
bit can be written with any value.
14
A16EN
Extender A16 Window Enable
Writing a 1 to this bit enables mapping of
VMEbus A16 space through the Extender A16
Window. When this bit is cleared, no VMEbus
A16 accesses are mapped between the VMEbus
and the MXIbus. This bit is cleared by a hard
reset and is not affected by a soft reset.
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