MIPS R10000 Microprocessor User's Manual - SGI TechPubs Library

Coprocessor 0 263
Errata
There are two ways to read the branch prediction state from the Branch Prediction
Table (BPT):
• Place an mfc0 rx, C0_Diag (a Move From Diagnostic register to GPR rx)
in the delay slot of the conditional branch. This read of the Diagnostic
register returns the next predicted state from the branch stacks before
the BPT is updated.
• Move the Index and the BPT read operation into the Idx and BPOp field
of the Diagnostic register. This mtc0 into CP0_Diag graduates as soon as
the write is completed; however, there could be a significant delay in
transferring the data from BPT to CP0_Diag. This delay occurs because
C0_Diag has a lower priority to access the BPT as compared to the
accesses by IFETCH and other processes. Thus, the prediction state
read from the C0_Diag may not reflect the content of the BPT. Use the
code sequence shown below to get the correct prediction state from
the BPT:
li rx # rx has index and BPT read for
# Idx and BPOp, respectively.
mtc0 rx, C0_Diag # Set the Diagnostic register for reading the BPT
la ry, label # ry !=r31; la could be replaced by a dla for 64-bits
jr ry # This gives priority for C0_Diag to access BPT
label: mfc0 rz, C0_Diag # rz holds the state from BPT entry pointed by Idx
MIPS R10000 Microprocessor User's Manual Version 2.0 of January 29, 1997