cbz

Compare and Branch Zero

CBZ <Wt>, <label>

Branches if register is zero.

Pseudocode Operation

if Wt == 0 then PC ← PC + (imm19 << 2)

Example

CBZ w3, label

Encoding

Binary Layout

011010

30:25

imm19

23:5

4:0

Format Branch

Opcode 0x34000000

Extension Base

Operands

Wt
Reg
label
Label

Other forms of cbz

cbz Compare and Branch Zero (64-bit)
cbz Compare and Branch Zero (Thumb)

Across architectures

Compare and Branch : how x86, ARM, RISC-V, and PowerISA each do this.

Reference

View in Arm A64 ISA Reference ↗ Arm A64 ISA

Instruction Forms

Encoding	Instruction		ISA		Bit pattern
0x34000000	CBZ <Wt>, <label>		A64		0 \| 011010 \| 0 \| imm19 \| Rt
0xB4000000	CBZ <Xt>, <label>		A64		1 \| 011010 \| 0 \| imm19 \| Rt

Description

Compare and Branch on Zero compares the value in a register with zero, and conditionally branches to a label at a PC-relative offset if the comparison is equal. It provides a hint that this is not a subroutine call or return. This instruction does not affect condition flags.

Operation

bits(datasize) operand1 = X[t, datasize];
if IsZero(operand1) == TRUE then
    BranchTo(PC64 + offset, BranchType_DIR, TRUE);
else
    BranchNotTaken(BranchType_DIR, TRUE);

cbz