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SUBS (shifted register)

Subtract (shifted register), setting flags, subtracts an optionally-shifted register value from a register value, and writes the result to the destination register. It updates the condition flags based on the result.

This instruction is used by the aliases CMP (shifted register), and NEGS.

313029282726252423222120191817161514131211109876543210
sf1101011shift0Rmimm6RnRd
opS

32-bit (sf == 0)

SUBS <Wd>, <Wn>, <Wm>{, <shift> #<amount>}

64-bit (sf == 1)

SUBS <Xd>, <Xn>, <Xm>{, <shift> #<amount>}

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

if shift == '11' then UNDEFINED;
if sf == '0' && imm6<5> == '1' then UNDEFINED;

ShiftType shift_type = DecodeShift(shift);
integer shift_amount = UInt(imm6);

Assembler Symbols

<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.

<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.

<shift> Is the optional shift type to be applied to the second source operand, defaulting to LSL and encoded in shift:
shift <shift>
00 LSL
01 LSR
10 ASR
11 RESERVED
<amount>

For the 32-bit variant: is the shift amount, in the range 0 to 31, defaulting to 0 and encoded in the "imm6" field.

For the 64-bit variant: is the shift amount, in the range 0 to 63, defaulting to 0 and encoded in the "imm6" field.

Alias Conditions

AliasIs preferred when
CMP (shifted register)Rd == '11111'
NEGSRn == '11111' && Rd != '11111'

Operation

bits(datasize) result;
bits(datasize) operand1 = X[n];
bits(datasize) operand2 = ShiftReg(m, shift_type, shift_amount);
bits(4) nzcv;

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

PSTATE.<N,Z,C,V> = nzcv;

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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