3 pci cache mode, Pci cache mode – LSI 53C875A User Manual
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PCI Functional Description
2-9
After each data transfer, the chip re-evaluates the burst size based on
the amount of remaining data to transfer and again selects the highest
possible multiple of the cache line size, and no larger than the
burst size. The most likely scenario of this scheme is
that the chip selects the
burst size after alignment,
and issues bursts of this size. The burst size is, in effect, throttled down
toward the end of a long Memory Move or Block Move transfer until only
the cache line size burst size is left. The chip finishes the transfer with
this burst size.
Latency – In accordance with the PCI specification, the latency timer is
ignored when issuing a Memory Write and Invalidate command such that
when a latency time-out occurs, the LSI53C875A continues to transfer
up to a cache line boundary. At that point, the chip relinquishes the bus,
and finishes the transfer at a later time using another bus ownership. If
the chip is transferring multiple cache lines it continues to transfer until
the next cache boundary is reached.
PCI Target Retry – During a Memory Write and Invalidate transfer, if the
target device issues a retry (STOP with no TRDY/, indicating that no data
was transferred), the chip relinquishes the bus and immediately tries to
finish the transfer on another bus ownership. The chip issues another
Memory Write and Invalidate command on the next ownership, in
accordance with the PCI specification.
PCI Target Disconnect – During a Memory Write and Invalidate
transfer, if the target device issues a disconnect the LSI53C875A
relinquishes the bus and immediately tries to finish the transfer on
another bus ownership. The chip does not issue another Memory Write
and Invalidate command on the next ownership unless the address is
aligned.
2.1.3 PCI Cache Mode
The LSI53C875A supports the PCI specification for an 8-bit
register located in the PCI configuration space. The
register provides the ability to sense and react to nonaligned
addresses corresponding to cache line boundaries. In conjunction with
the
register, the PCI commands Memory Read Line,
Memory Read Multiple, and Memory Write and Invalidate are each