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IBM21P100BGB
IBM 133 PCI-X Bridge R1.1
ppb11_pcix_regs.fm.03
July 9, 2001
Configuration Registers
Page 79 of 131
5.2.5.21 Secondary Bus Upstream Split Transaction Register
The Secondary Bus Upstream Split Transaction register controls the behavior of the bridge buffers for
forwarding Split Transactions from a secondary bus requester to a primary bus completer. When the compl-
eter bus is in PCI-X mode, the Split Transaction Commitment Limit field only affects the byte count used when
issuing read requests.
Address Offset
x‘88’
Access
See bit descriptions
Reset Value
x‘0020 0020’
Split Transaction Commitment Limit
Split Transaction Capacity
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10
98
76543210
Bit(s)
Access
Field Name and Description
31:16
RW
Split Transaction Commitment Limit
This field indicates the cumulative Sequence size for all memory read transactions forwarded by the bridge
from requesters on the secondary bus addressing completers on the primary bus.
This field indicates the size of the commitment limit in units of ADQs.
Software is permitted to program this field to any value greater than or equal to the contents of the Split Trans-
action Capacity field. A value less than the contents of the Split Transaction Capacity field causes unspecified
results. If this field is set to x‘FFFF’, the bridge is permitted to forward all Split Requests of any size regardless
of the amount of buffer space available.
A value of x‘0100’ or greater will cause the bridge to forward accepted Split Requests of any size regardless of
the amount of buffer space available.
Software is permitted to change this field at any time. The most recent value of the field is used each time the
bridge forwards a Split Transaction.
State after RST# is the same as the Split Transaction Capacity field.
15:0
RO
Split Transaction Capacity
This read-only field indicates the size of the buffer (in number of ADQs) for storing Split Completions for mem-
ory reads for requesters on the secondary bus addressing completers on the primary bus.
The bridge returns x‘0020’ to indicate that there are 32 ADQs (4K bytes) available buffer space.