2. Interface Operation
42
Tsi308 User Manual
80D4000_MA001_02
Tundra Semiconductor Corporation
www.tundra.com
I/O writes are not allowed to stream and always disconnect after a single data beat on the PCI-X
(32-bits). Each I/O write is issued to HyperTransport as an independent request.
2.4.4
Prefetching (PCI mode only)
While operating in standard PCI mode, the Tsi308 supports a variety of prefetching options
configured under CSR control using two Read Control CSRs, Read Control 1 at 62h:60h and
Read Control 2 at 5Eh:5Ch. Read Control 2 is applied for requests passing through DMA
window and is reserved in Tsi301 compatible mode. However:
•
I/O reads are never prefetchable.
•
MemRdLines and MemRdMult may have prefetching individually configured.
•
For systems in which MemRds are known to be side-effect free, MemReadPrefEn can be
set to enable prefetching behavior for MemReads using the same parameters as
MemRdLines.
•
PrefEn can be used to globally enable or disable all prefetching.
•
Nonprefetchable reads always request only the bytes required to satisfy the initial data beat
of four or eight bytes on the PCI bus, which may result in either one or two HyperTransport
requests.
Transactions for which prefetching is enabled issue a HyperTransport read for the remainder of
the 64-byte aligned block containing the original request. These transactions also issue
HyperTransport reads for the zero to seven complete 64-byte blocks following, as determined
by the Read Control CSRs. The total number of reads that may be outstanding to
HyperTransport at one time is limited by the Outbound Data Buffers.
When multiple reads to HyperTransport are issued for a single PCI read request due to
prefetching or due to clear byte enables in a 64-bit nonprefetchable read on a 64-bit bus, each
HyperTransport request is referred to as a subrequest of the PCI request. Each Delayed Request
Buffer can track up to 8 subrequests at once. The total number of configured subrequests
(number of enabled delayed request buffers * (the maximum number of subrequests each,
rounded up to the next power of 2)) must not exceed the number of entries in the Outbound Data
Buffers.
2.4.5
Memory Read Block (PCI-X mode only)
MemRdBlk command of PCI-X is analogous to MemRdLine or MemRdMultiple of PCI.
Since PCI-X request provides the byte count to be satisfied during the attribute phase of a PCI-X
read transaction, read data for PCI-X requests are never prefetched, instead data is read just
enough to satisfy the byte count of the original request.
Содержание TSI308
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