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PCI Functional Description
PCI Cache Mode
SYM53C875/875E Data Manual
3-5
another bus ownership. If the chip is transferring
multiple cache lines it will continue to transfer
until the next cache boundary is reached.
PCI Target Retry
During a Write and Invalidate transfer, if the target
device issues a retry (ST OP with no T RDY, indi-
cating that no data was transferred), the
SYM53C875 will relinquish the bus and immedi-
ately try to finish the transfer on another bus own-
ership. T he chip will issue another Write and
Invalidate command on the next ownership, in
accordance with the PCI specification.
PCI Target Disconnect
During a Write and Invalidate transfer, if the target
device issues a disconnect the SYM53C875 will
relinquish the bus and immediately try to finish the
transfer on another bus ownership. T he chip will
not issue another Write and Invalidate command
on the next ownership unless the address is
aligned.
Memory Read
Line Command
T his command is identical to the Memory Read
command, except that it additionally indicates that
the master intends to fetch a complete cache line.
T his command is intended to be used with bulk
sequential data transfers where the memory system
and the requesting master might gain some perfor-
mance advantage by reading up to a cache line
boundary rather than a single memory cycle.T he
Read Line Mode function that exists in the previ-
ous SYM53C8X X chips has been modified in the
SYM53C875 to reflect the PCI cache line size reg-
ister specifications. T he functionality of the Enable
Read Line bit (bit 3 in DMODE) has been modi-
fied to more resemble the Write and Invalidate
mode in terms of conditions that must be met
before a Read Line command will be issued. How-
ever, the Read Line option will operate exactly like
the previous SYM53C8X X chips when cache
mode has been disabled by a CLSE bit reset or
when certain conditions exist in the chip
(explained below).
T he Read Line mode is enabled by setting bit 3 in
the DMODE register. If cache mode is disabled,
Read Line commands will be issued on every read
data transfer, except op code fetches, as in previ-
ous SYM53C8X X chips.
If cache mode has been enabled, a Read Line com-
mand will be issued on all read cycles, except op
code fetches, when the following conditions have
been met:
1. T he CLSE and Enable Read Line bits must be
set.
2. T he Cache Line Size register must contain a
legal burst size value (2, 4, 8, 16, 32, 64, or
128) AND that value must be less than or
equal to the DMODE burst size.
3. T he number of bytes to be transferred at the
time a cache boundary has been reached must
be equal to or greater than the DMODE burst
size.
4. T he chip must be aligned to a cache line
boundary.
When these conditions have been met, the chip
will issue a Read Line command instead of a
Memory Read during all PCI read cycles. Other-
wise, it will issue a normal Memory Read com-
mand.
Memory Read
Multiple Command
T his command is identical to the Memory Read
command except that it additionally indicates that
the master may intend to fetch more than one
cache line before disconnecting. T he SYM53C875
supports PCI Read Multiple functionality and will
issue Read Multiple commands on the PCI bus
when the Read Multiple Mode is enabled. T his
mode is enabled by setting bit 2 of the DMODE
register (ERMP). If cache mode has been enabled,