Encoding:

Format:

BNEL rs, rt, offset

Purpose:

Branch on Not Equal Likely

To compare GPRs then do a PC-relative conditional branch; execute the delay slot only if the branch is taken.

Description:

 if GPR[rs] != GPR[rt] 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 contents of GPR rs and GPR rt are not equal, branch 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:

I:    target_offset = sign_extend(offset || 02)
      condition = (GPR[rs] != GPR[rt])
I+1:  if condition then
         PC = PC + target_offset
      else
         NullifyCurrentInstruction()
      endif

Exceptions:

None

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 BNE instruction instead.

Historical Information:

In the MIPS I architecture, this instruction signaled a Reserved Instruction exception.