Encoding:

Format:

BC2FL cc, offset

Purpose:

Branch on COP2 False Likely

To test a COP2 condition code and make a PC-relative conditional branch; execute the instruction in the delay slot only if the branch is taken.

Description:

if COP2Condition(cc) = 0 then branch_likely

An 18-bit signed offset (the 16-bit offset field shifted left 2 bits) is added to the address of the instruction following the branch (not the branch itself) in the branch delay slot to form a PC-relative effective target address. If the COP2 condition specified by cc is false (0), the program branches to the effective target address after the instruction in the delay slot is executed. If the branch is not taken, the instruction in the delay slot is not executed.

Restrictions:

Processor operation is UNPREDICTABLE if a branch, jump, ERET, DERET, or WAIT instruction is placed in the delay slot of a branch or jump.

Availability and Compatibility:

This instruction has been removed in Release 6.

Operation:

This operation specification is for the general Branch On Condition operation with the tf (true/false) and nd (nullify delay slot) fields as variables. The individual instructions BC2F, BC2FL, BC2T, and BC2TL have specific values for

tf and nd.

I:    condition = COP2Condition(cc) = 0
      target_offset = (offset15)GPRLEN-(16+2) || offset || 02
I+1:  if condition then
         PC = PC + target_offset
      else
         NullifyCurrentInstruction()
      endif

Exceptions:

Coprocessor Unusable, Reserved Instruction

Implementation Note:

Some implementations always predict that the branch will be taken, and do not use nor do they update the branch internal processor branch prediction tables for this instruction. To maintain performance compatibility, future implementations are encouraged to do the same.

Programming Notes:

With the 18-bit signed instruction offset, the conditional branch range is ± 128 KBytes. Use jump (J) or jump register

(JR) to branch to addresses outside this range.

In Pre-Release 6 implementations, software is strongly encouraged to avoid the use of the Branch Likely instructions, as they will be removed from a future revision of the MIPS Architecture.

Some implementations always predict the branch will be taken, so there is a significant penalty if the branch is not taken. Software should only use this instruction when there is a very high probability (98% or more) that the branch will be taken. If the branch is not likely to be taken or if the probability of a taken branch is unknown, software is encouraged to use the BC2F instruction instead.