Motorola MVME172 - Manual (Page 84)

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VMEchip2
2
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.
When write posting is enabled, the VMEchip2 stores the local bus address
and data and then acknowledges the local bus master. The local bus is then
free to perform other operations while the VMEbus master requests the
VMEbus and performs the requested operation.
The write post buffer stores one byte, two-byte, four-byte, or one cache
line (four four-bytes). Write posting should only be enabled when bus
errors are not expected. If a bus error is returned on a write posted cycle,
the local processor is interrupted, if the interrupt is enabled. The address of
the error is not saved. Normal memory never returns a bus error on a write
cycle. However, some VMEbus ECC memory cards perform a read-
modify-write operation and therefore may return a bus error if there is an
error on the read portion of a read-modify-write. Write posting should not
be enabled when this type of memory card is used. Also, memory should
not be sized using write operations if write posting is enabled. I/O areas
that have holes should not be write posted if software may access non-
existent memory. Using the programmable map decoders, write posting
can be enabled for “safe” areas and disabled for areas which are not “safe”.
Block transfer is not supported because the MC68060 block transfer
capability is not compatible with the VMEbus.
The VMEbus master supports dynamic bus sizing. When a local device
initiates a quad-byte access to a VMEbus slave that only has the D16 data
transfer capability, the chip executes two double-byte cycles on the
VMEbus, acknowledging the local device after all requested four-bytes