CSEL

Conditional select

This instruction writes the value of the first source register to the destination register if the condition is TRUE. If the condition is FALSE, it writes the value of the second source register to the destination register.

Encoding: Integer

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
	0	0	1	1	0	1	0	1	0	0										0	0
sf	op	S									Rm					cond					o2	Rn					Rd

32-bit (sf == 0)

CSEL <Wd>, <Wn>, <Wm>, <cond>

64-bit (sf == 1)

CSEL <Xd>, <Xn>, <Xm>, <cond>

Decoding algorithm

constant integer d = UInt(Rd);
constant integer n = UInt(Rn);
constant integer m = UInt(Rm);
constant integer datasize = 32 << UInt(sf);
constant bits(4) condition = cond;

Operation

bits(datasize) result;
if ConditionHolds(condition) then
    result = X[n, datasize];
else
    result = X[m, datasize];

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 first general-purpose source register, encoded in the "Rn" field.
<Wm>: Is the 32-bit name of the second general-purpose source register, encoded in the "Rm" field.
<cond>: <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 first general-purpose source register, encoded in the "Rn" field.
<Xm>: Is the 64-bit name of the second general-purpose source register, encoded in the "Rm" 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.