2-6 Functional Description
2.1.2.10 Memory Read Multiple Command
This 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. The LSI53C875A suppor ts PCI Memory
Read Multiple functionality and issues Memory Read Multiple command s
on the PCI bus when the Read Multiple Mode is enabled. This mode is
enabled by setting bit 2 (ERMP) of the DMA Mode (DMODE) register. If
cache mode is enabled, a Memory Read Multiple command is issued on
all read cycles, except opcode fetches, when the following conditions are
met:
•The CLSE bit (Cache Line Size Enable, bit 7, DMA Control (DCNTL)
register) and the ERMP bit (Enable Read Multiple, bit 2, DMA Mode
(DMODE) register) are set.
•The CacheLineSizeregisterfor each function contains a legal burst
size value (2, 4, 8, 16, 32, or 64) and that value is less than or equal
to the DMODE burst size.
•The transfer will cross a cache line boundary.
When these conditions are met, the chip issues a Memory Read Multiple
command instead of a Memory Read during all PCI read cy cles.
Burst Size Selection – The Read Multiple command reads in multiple
cache lines of data in a single bus ownership. The number of cache lin es
to read is a multiple of the cache linesize specified in Revision 2.2 of
the PCI specification. The logic selects the largest multiple of the cache
line size based on the amount of data to transfer, with the maximum
allowableburst size dete rmined from the DMA Mode (DMODE) burst size
bits, and the Chip Test Five(CTEST5),bit2.
2.1.2.11 Dual Address Cycle (DAC) Command
The LSI53C875A performs DACs when 64-bit addressing is required.
Refer to the PCI 2.2 specification. If any of the selector registers contain
a nonzero value, a DAC is generated. See 64-bit SCRIPTS Selectors in
Chapter 4, “Registers,” for additional informa tion.
2.1.2.12 Memory Read Line Command
This command is identical to the Memory Read command, except that it
additionally indicates that the master intends to fetch a complete cache