Encoding:

Format:

BLTZALL rs, offset

Purpose:

Branch on Less Than Zero and Link Likely

To test a GPR then do a PC-relative conditional procedure call; execute the delay slot only if the branch is taken.

Description:

 if GPR[rs] < 0 then procedure_call_likely

Place the return address link in GPR 31. The return link is the address of the second instruction following the branch, where execution continues after a procedure call.

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 are less than zero (sign bit is 1), 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 control transfer instruction (CTI) is placed in the delay slot of a branch or jump.

Branch-and-link Restartability: GPR 31 must not be used for the source register rs, because such an instruction does

not have the same effect when reexecuted. The result of executing such an instruction is UNPREDICTABLE. This restriction permits an exception handler to resume execution by reexecuting the branch when an exception occurs in the branch delay slot.

Availability and Compatibility:

This instruction has been removed in Release 6.

Operation:

I:    target_offset = sign_extend(offset || 02)
      condition = GPR[rs] < 0GPRLEN
      GPR[31] = PC + 8
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 and link (JAL) or jump and link register (JALR) instructions for procedure calls 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 BLTZAL instruction instead.

Historical Information:

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