CNT

Count bits

This instruction counts the number of binary one bits in the value of the source register, and writes the result to 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	1
sf		S									opcode2					opcode						Rn					Rd

32-bit (sf == 0)

CNT <Wd>, <Wn>

64-bit (sf == 1)

CNT <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 = BitCount(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.