LSI LSI53C895A, Technical Manual

Page: 36 / 364

2-10 Functional Description
addresses corresponding to cache line boundaries. In conjunction with
the
Cache Line Size register, the PCI commands Memory Read Line,
Memory Read Multiple, Memory Write and Invalidate are each software
enabled or disabled to allow the user full flexibility in using these
commands.
2.1.3.1 Enabling Cache Mode
In order to enable the cache logic to issue PCI cache commands
(Memory Read Line, Memory Read Multiple, and Memory Write and
Invalidate) on any given PCI master operation the following conditions
must be met:
•
The Cache Line Size Enable bit in DMA Control (DCNTL) register
must be set.
•
The PCI Cache Line Size register must contain a valid binary cache
size, i.e. 2, 4, 8, 16, 32, 64, or 128 Dwords. Only these values are
considered valid cache sizes.
•
The programmed burst size (in Dwords) must be equal to or greater
than the
Cache Line Size register. The DMA Mode (DMODE) register
bits [7:6] and Chip Test Five (CTEST5) bit 2 are the burst length bits.
•
The part must be doing a PCI Master transfer. The following PCI
Master transactions do not utilize the PCI cache logic and thus no
PCI cache command is issued during these types of cycles: a
nonprefetch SCRIPTS fetch, a Load/Store data transfer, a data flush
operation. All other types of PCI Master transactions will utilize the
PCI cache logic.
The above four conditions must be met for the cache logic to control the
type of PCI cache command that is issued, along with any alignment that
may be necessary during write operations. If these conditions are not
met for any given PCI Master transaction, a Memory Read or Memory
Write is issued and no cache write alignment is done.
2.1.3.2 Issuing Cache Commands
In order to issue each type of PCI cache command, the corresponding
enable bit must be set (2 bits in the case of Memory Write and
Invalidate). These bits are detailed below: