bclr

Branch Conditional to Link Register

bclr BO,BI,BH

Branches to the address in the Link Register (LR) if the condition is met. Used for function returns.

Encoding

Binary Layout

0:5

6:10

11:15

000

16:20

21:30

Format XL-form

Opcode 0x4C000020

Extension Base

Operands

BO
Branch Options
BI
CR Bit Index
BH
Hint bits
LK
Link Register Update field

Example

bclr 20, 0

// Unconditional return (blr).

Registers Altered

CTR, LR

Reference

View in PowerISA v3.1C Specification ↗ OPF Power ISA v3.1C

Description

Branches to the address in the Link Register (LR) if the condition specified by BO and BI is satisfied. The CTR is decremented if BO[2]=0. If LK=1, the return address (CIA + 4) is saved in LR, enabling exception-safe subroutine returns. The BH field provides a branch prediction hint to the processor.

Operation

ctr_ok ← (BO[2] = 1) | (CTR ≠ 0 ⊕ BO[3])
if BO[2] = 0 then CTR ← CTR - 1
cr_ok ← (BO[0] = 1) | (CR[BI] = BO[1])
if ctr_ok & cr_ok then NIA ← LR[0:61] || 0b00
if LK = 1 then LR ← CIA + 4

Extended Mnemonics

Extended Mnemonic	Equivalent Instruction
bcctr
bltctr
bnectr
bclr
bltlr
bnelr
bdnzlr
bcctr
bcctrl
bclr 4,6
bnelr cr2

Programming Note

bclr, bclrl, bcctr, and bcctrl each serve as both a basic and an extended mnemonic. The Assembler will recognize a bclr, bclrl, bcctr, or bcctrl mnemonic with three operands as the basic form, and a bclr, bclrl, bcctr, or bcctrl mnemonic with two operands as the extended form. In the extended form the BH operand is omitted and assumed to be 0b00.