cbz

Compare and Branch Zero (64-bit)

CBZ <Xt>, <label>

Branches if 64-bit register is zero.

Pseudocode Operation

if Xt == 0 then
  PC ← PC + SignExtend(imm19 << 2, 64)

Example

CBZ x3, label

Encoding

Binary Layout

011010

30:25

imm19

23:5

4:0

Format Branch

Opcode 0xB4000000

Extension Base

Operands

Xt
Reg
label
Label

Other forms of cbz

cbz Compare and Branch Zero
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