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VMEchip2
2
2-38
Each of the four programmable local bus map decoders has a starting
address, an ending address, an address modifier register with attribute
bits, and an enable bit. The fourth decoder also has address translation
registers. The addresses and bit definitions for these registers are in the
tables below.
A local bus slave map decoder is programmed by loading the starting
address of the segment into the starting address register and the ending
address of the segment into the ending address register. The address
modifier code is programmed into the address modifier register. Because
the local-bus-to-VMEbus interface does not support VMEbus block
transfers, block transfer address modifier codes should not be
programmed.
The address translation register allows a local bus master to view a
portion of the VMEbus that may be hidden by onboard resources or an area
of the VMEbus may be mapped to two local address. For example, some
devices in the I/O map may support write posting while others do not. The
VMEbus area in question may be mapped to two local bus addresses, one
with write posting enabled and one with write posting disabled. The
address translation registers allow local bus address bits A31 through A16
to be modified. The address translation register should be programmed
with the translated address, and the address translation select register
should be programmed to enable the translated address. If address
translation is not desired, then the address translation registers should be
programmed to 0.
The address translation address register and the address translation select
register operate in the following way: If you set a bit in the address
translation select register, then the corresponding bit in the address
translation address register drives the appropriate VMEbus address line. If
you clear the bit in the address translation select register, then the
corresponding local bus address line drives the appropriate VMEbus
address line. The most significant bit of the address translation select
register corresponds to the most significant bit of the address translation
address register and to A32 of the local bus and A32 of the VMEbus.
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