CTZ

Count Trailing Zeros

CTZ rd, rs1

Counts the number of 0 bits at the LSB end of the register.

Encoding

Binary Layout

011000000001

31:20

rs1

19:15

001

14:12

rd

11:7

0010011

6:0

 

Format I-Type

Opcode 0x60101013

Extension Zbb

Operands

• rd
  Destination register (integer)
• rs1
  Source

Example

CTZ x10, x11

// Find lowest set bit index.

Related

Across architectures

Count Trailing Zeros : how x86, ARM, RISC-V, and PowerISA each do this.

More in Zbb

Reference

View in RISC-V Unprivileged ISA Specification ↗ RISC-V ISA Manual

Description

CTZ counts the number of trailing (least-significant) zero bits in rs1, writing the count to rd. Returns XLEN if rs1 is zero.

Operation

int count = 0; while(rs1[count] == 0) count++; R[rd] = count;