cnttzw

Count Trailing Zeros Word

cnttzw RA, RS

Counts the number of trailing zeros in the low 32-bits.

Encoding

Binary Layout

31

0:5

RS

6:10

RA

11:15

/

16:20

538

21:30

/

31

 

Format X-form

Opcode 0x7C000434

Extension Base

Operands

• RA
  Target
• RS
  Source

Example

cnttzw r4, r3

Registers Altered

CR0

Related

Across architectures

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

More in Base

Reference

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

Description

Counts the number of trailing zero bits in the low 32 bits of general-purpose register RS and stores the count in RA. The count ranges from 0 to 32; if all 32 bits are zero, the result is 32. No condition register or status flags are affected.

Operation

count ← 0
for i in 0..31:
  if RS[32+i] == 0 then count ← count + 1
  else break
RA ← count

Programming Note

The cnttzw instruction is useful for quickly determining the number of trailing zeros in a word, which can be helpful in bit manipulation tasks. Be cautious with Rc=1 as it modifies CR0 based on the result, affecting subsequent conditional operations. Ensure that the input register (RS) is correctly aligned and contains valid data to avoid unexpected results.