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SUB (immediate)

Subtract (immediate) subtracts an optionally-shifted immediate value from a register value, and writes the result to the destination register.

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sf10100010shimm12RnRd
opS

32-bit (sf == 0)

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

64-bit (sf == 1)

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

integer d = UInt(Rd);
integer n = UInt(Rn);
integer datasize = if sf == '1' then 64 else 32;
bits(datasize) imm;

case sh of
    when '0' imm = ZeroExtend(imm12, datasize);
    when '1' imm = ZeroExtend(imm12:Zeros(12), datasize);

Assembler Symbols

<Wd|WSP>

Is the 32-bit name of the destination general-purpose register or stack pointer, 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.

<Xd|SP>

Is the 64-bit name of the destination general-purpose register or stack pointer, 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.

<imm>

Is an unsigned immediate, in the range 0 to 4095, encoded in the "imm12" field.

<shift> Is the optional left shift to apply to the immediate, defaulting to LSL #0 and encoded in sh:
sh <shift>
0 LSL #0
1 LSL #12

Operation

bits(datasize) result;
bits(datasize) operand1 = if n == 31 then SP[] else X[n];
bits(datasize) operand2;

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

if d == 31 then
    SP[] = result;
else
    X[d] = result;

Operational information

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.
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