line. This command is intended for use with bulk sequential data transfers where the memory system and the requesting master might gain some performance advantage by reading to a cache line boundary rather than a single memory cycle. The Read Line function in the LSI53C875A takes advantage of the PCI 2.2 specification regarding issuing this command.

If the cache mode is disabled, Read Line commands are not issued.

If the cache mode is enabled, a Read Line command is issued on all read cycles, except nonprefetch opcode fetches, when the following conditions are met:

The CLSE (Cache Line Size Enable, bit 7, DMA Control (DCNTL) register) and ERL (Enable Read Line, bit 3, DMA Mode (DMODE) register) bits are set.

The Cache Line Size register must contain a legal burst size value in Dwords (2, 4, 8, 16, 32, 64, or 128) and that value is less than or equal to the DMA Mode (DMODE) burst size.

The transfer will cross a Dword boundary but not a cache line boundary.

When these conditions are met, the chip issues a Read Line command instead of a Memory Read during all PCI read cycles. Otherwise, it issues a normal Memory Read command.

Read Multiple with Read Line Enabled – When both the Read Multiple and Read Line modes are enabled, the Read Line command is not issued if the above conditions are met. Instead, a Read Multiple command is issued, even though the conditions for Read Line are met.

If the Read Multiple mode is enabled and the Read Line mode is disabled, Read Multiple commands are issued if the Read Multiple conditions are met.

PCI Functional Description

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LSI 53C875A technical manual PCI Functional Description