CTZ

Count trailing zeros

This instruction counts the number of consecutive binary zero bits, starting from the least significant bit in the source register, and places the count in the destination register.

Encoding: Integer

Variants: FEAT_CSSC (ARMv8.9)

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
	1	0	1	1	0	1	0	1	1	0	0	0	0	0	0	0	0	0	1	1	0
sf		S									opcode2					opcode						Rn					Rd

32-bit (sf == 0)

CTZ <Wd>, <Wn>

64-bit (sf == 1)

CTZ <Xd>, <Xn>

Decoding algorithm

if !IsFeatureImplemented(FEAT_CSSC) then EndOfDecode(Decode_UNDEF);
constant integer datasize = 32 << UInt(sf);
constant integer d = UInt(Rd);
constant integer n = UInt(Rn);

Operation

constant bits(datasize) operand1 = X[n, datasize];
constant integer result = CountLeadingZeroBits(BitReverse(operand1));
X[d, datasize] = result;

Explanations

<Wd>: Is the 32-bit name of the general-purpose destination register, encoded in the "Rd" field.
<Wn>: Is the 32-bit name of the general-purpose source register, encoded in the "Rn" field.
<Xd>: Is the 64-bit name of the general-purpose destination register, encoded in the "Rd" field.
<Xn>: Is the 64-bit name of the general-purpose source register, encoded in the "Rn" field.

Operational Notes

If PSTATE.DIT is 1:

• The execution time of this instruction is independent of:
  • The values of the data supplied in any of its registers.
  • The values of the NZCV flags.
• The response of this instruction to asynchronous exceptions does not vary based on:
  • The values of the data supplied in any of its registers.
  • The values of the NZCV flags.