Encoding:

Format:

BC1TL cc, offset

Purpose:

Branch on FP True Likely

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

Description:

if FPConditionCode(cc) = 1 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 FP Condition Code bit cc is true (1), 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.

An FP condition code is set by the FP compare instruction, C.cond fmt.

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 BC1F, BC1FL, BC1T, and BC1TL have specific values for

tf and nd.

I:    condition = FPConditionCode(cc) = 1
      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

Floating Point Exceptions:

Unimplemented Operation

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 BC1T instruction instead.

Historical Information:

The MIPS I architecture defines a single floating point condition code, implemented as the coprocessor 1 condition signal (Cp1Cond) and the C bit in the FP Control/Status register. MIPS I, II, and III architectures must have the CC field set to 0, which is implied by the first format in the "Format" section.

The MIPS IV and MIPS32 architectures add seven more Condition Code bits to the original condition code 0. FP compare and conditional branch instructions specify the Condition Code bit to set or test. Both assembler formats are valid for MIPS IV and MIPS32.