clz

Count Leading Zeros (Thumb)

CLZ <Rd>, <Rm>

Counts consecutive zeros.

Details

Count the number of leading zero bits in the 32-bit value in Rm and place the result in Rd. The result ranges from 0 (all bits set) to 32 (all bits clear). No condition flags are affected. This instruction is available in T32 (Thumb) and executes in all privilege levels.

Pseudocode Operation

Rd ← CountLeadingZeros(Rm[31:0])

Example

CLZ r0, r2

Encoding

Binary Layout
111110101
011
Rn
1111
Rd
10
00
Rm
 
Format Thumb Misc
Opcode 0xFAB0F080
Extension A32 (Base)

Operands

  • Rd
    Destination general-purpose register
  • Rm
    Second source / offset general-purpose register

Reference (Arm AArch32 ISA)

Instruction Forms

Encoding Instruction ISA Bit pattern
0x016F0F10 CLZ{<c>}{<q>} <Rd>, <Rm> A32 cond | 00010110 | 1 | 1 | 1 | 1 | Rd | 1 | 1 | 1 | 1 | 0001 | Rm
0xFAB0F080 CLZ{<c>}{<q>} <Rd>, <Rm> T32 111110101 | 011 | Rn | 1111 | Rd | 10 | 00 | Rm

Description

Count Leading Zeros returns the number of binary zero bits before the first binary one bit in a value.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    result = CountLeadingZeroBits(R[m]);
    R[d] = result<31:0>;