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Computer Group Literature Center Web Site
Raven PCI Host Bridge & Multi-Processor Interrupt Controller
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If the write post FIFO is full, any other accesses to the Raven are delayed
(AACK* will not be asserted) until there is room in the FIFO to store the
complete transaction.
All write posted transfers will be completed before a non-write posted read
or write is begun to assure that all transfers are completed in the order
issued. All write posted transfers will also be completed before any access
to the Raven’s registers is begun.
PPC Master
Wherever possible, the PPC master will attempt to consolidate data
movement into a pair of burst transfers called couplets. If there is not
enough data movement to perform a couplet, the PPC master will attempt
singular burst transfers. The PPC master will perform single beat transfers
as required during all non-cache aligned writes and some non-cache
aligned reads. A 64-bit by 16 entry FIFO is used to hold data between the
PCI slave and the PPC master to ensure that optimum data throughput is
maintained. While the PCI slave is filling the FIFO with one cache line
worth of data, the PPC master can be moving another cache line worth onto
the PPC bus. This will allow the PCI slave to receive long block transfers
without stalling.
The PPC master has an optional read ahead mode controlled by the RAEN
bit in the PCISATTx registers that allows the PPC master to prefetch data
in bursts and store it in the FIFO. The contents of the FIFO will then be
used to satisfy the data requirements for the remainder of the PCI read
transaction. A second control bit within the PCISATTx registers called
TDIS (Threshold Disable) determines how the PPC master will continue
to fill the FIFO during prefetched reads. The following table shows the
read ahead options.