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the Write Bank Unlock portion of a Read-Modify-Write operation. Figure
6-13 shows the flow for this arbitration mode.
Figure 6-13
Toggle 50% High/50% Low Mode
WIN or LOSE
Fixed Low Mode
This mode causes an I/O port in node 8 to use TLSB_REQ8_LOW when re-
questing the TLSB except when arbitrating for the Write Bank Unlock por-
tion of a Read-Modify-Write operation which is always done using
TLSB_REQ8_HIGH. This mode is not recommended for normal operation
as other TLSB nodes could potentially lock out the I/O port from gaining
access to the TLSB.
Fixed High Mode
This mode causes an I/O port in node 8 to always use TLSB_REQ8_HIGH
when arbitrating for the TLSB. This mode is not recommended for normal
operation as the I/O port could potentially lock out other nodes from ac-
cessing memory.
6.5.2.2
Read-Modify-Write
The Write Bank Unlock portion of a slot 8 I/O port Read-Modify-Write op-
eration is always performed at high priority to guarantee minimum la-
tency for the node 8 I/O port. The Read Bank Lock/Write Bank Unlock
command pair is used to guarantee atomicity for Read-Modify-Write opera-
tions for I/O ports in any slot. The other ICCMSR selectable arbitration
modes permit fine-tuning of the node 8 I/O port performance relative to
other bus traffic. There is also a fixed high-priority mode. The node 8 I/O
port does not normally operate in this fixed high-priority mode unless the
system software explicitly programs the node 8 I/O port to do so through
ICCMSR<0>.
NOTE: The second I/O port, if present, always uses the single request line that is
associated with the slot it occupies (TLSB_REQ<7:4>).
In general, Read-Modify-Writes are costly operations to perform and re-
quire attention to be paid to cache coherency. This is why the Bank Lock
RESET
REQ8_HIGH
ELSE
WIN
REQ8_LOW
ELSE
Notes:
WIN = (GRANT* - NOP)
LOSE = (REQ8_LOW * PREQ<n>*-WIN)
PREQ<n>(potential request) =
(-ARB_SUPPRESS*
-BANK_AVL<n>*
-REQ8_HIGH*
-ARB_CYCLE)
BXB0810.AI
I/O Port 6-33
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