SUBS (immediate)

Subtract immediate value, setting flags

This instruction subtracts an optionally-shifted immediate value from a register value, and writes the result to the destination register. It updates the condition flags based on the result.

Encoding: Setting the condition flags

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	1	1	0	0	0	1	0
sf	op	S							sh	imm12												Rn					Rd

32-bit (sf == 0)

SUBS <Wd>, <Wn|WSP>, #<imm>{, <shift>}

64-bit (sf == 1)

SUBS <Xd>, <Xn|SP>, #<imm>{, <shift>}

Decoding algorithm

constant integer d = UInt(Rd);
constant integer n = UInt(Rn);
constant integer datasize = 32 << UInt(sf);

constant bits(24) imm = if sh == '0' then Zeros(12):imm12 else imm12:Zeros(12);

Operation

constant bits(datasize) operand1 = if n == 31 then SP[datasize] else X[n, datasize];
constant bits(datasize) operand2 = ZeroExtend(imm, datasize);
bits(datasize) result;
bits(4) nzcv;

(result, nzcv) = AddWithCarry(operand1, NOT(operand2), '1');

X[d, datasize] = result;
PSTATE. = nzcv;

Explanations

<Wd>: Is the 32-bit name of the general-purpose destination register, encoded in the "Rd" field.
<Wn|WSP>: Is the 32-bit name of the source general-purpose register or stack pointer, encoded in the "Rn" field.
<imm>: Is an unsigned immediate, in the range 0 to 4095, encoded in the "imm12" field.
<shift>: <Xd>: Is the 64-bit name of the general-purpose destination register, encoded in the "Rd" field.
<Xn|SP>: Is the 64-bit name of the source general-purpose register or stack pointer, 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.