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VMEchip2
2
2-6
Using programmable map decoders with programmable attribute
bits, the local bus to VMEbus interface can be configured to provide
the following VMEbus capabilities:
Addressing capabilities: A16, A24, A32
Data transfer capabilities: D08, D16, D32
The local bus slave includes six local bus map decoders for accessing
the VMEbus. The first four map decoders are general purpose
programmable decoders, while the other two are fixed and are
dedicated for I/O decoding.
The first four map decoders compare local bus address lines A31
through A16 with a 16-bit start address and a 16-bit end address.
When an address in the selected range is detected, a VMEbus select
is generated to the VMEbus master. Each map decoder also has
eight attribute bits and an enable bit. The attribute bits are for
VMEbus AM codes, D16 enable, and write post (WP) enable.
The fourth map decoder also includes a 16-bit alternate address
register and a 16-bit alternate address select register. This allows
any or all of the upper 16 address bits from the local bus to be
replaced by bits from the alternate address register. The feature
allows the local bus master to access any VMEbus address.
Using the four programmable map decoders, separate VMEbus
maps can be created, each with its own attributes. For example, one
map can be configured as A32, D32 with write posting enabled
while a second map can be A24, D16 with write posting disabled.
The first I/O map decoder decodes local bus addresses $FFFF0000
through $FFFFFFFF as the short I/O A16/D16 or A16/D32 area,
and the other provides an A24/D16 space at $F0000000 to
$F0FFFFFF and an A32/D16 space at $F1000000 to $FF7FFFFF.
Supervisor/non-privileged and program/data space is determined
by attribute bits. Write posting may be enabled or disabled for each
decoder I/O space and this map decoder may be enabled or
disabled.
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