SUBS (shifted register)

Subtract optionally-shifted register, setting flags

This instruction 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.

Encoding: Setting the condition flags

313029282726252423222120191817161514131211109876543210
11010110
sfopSshiftRmimm6RnRd

32-bit (sf == 0)

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

64-bit (sf == 1)

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

Decoding algorithm

if shift == '11' then EndOfDecode(Decode_UNDEF);
if sf == '0' && imm6<5> == '1' then EndOfDecode(Decode_UNDEF);

constant integer d = UInt(Rd);
constant integer n = UInt(Rn);
constant integer m = UInt(Rm);
constant integer datasize = 32 << UInt(sf);
constant ShiftType shift_type = DecodeShift(shift);
constant integer shift_amount = UInt(imm6);

Operation

constant bits(datasize) operand1 = X[n, datasize];
constant bits(datasize) operand2 = NOT(ShiftReg(m, shift_type, shift_amount, datasize));
bits(datasize) result;
bits(4) nzcv;

(result, nzcv) = AddWithCarry(operand1, 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>: 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.
<shift>: <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.
<amount>: For the "64-bit" variant: is the shift amount, in the range 0 to 63, defaulting to 0 and encoded in the "imm6" 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.

Operational Notes

If PSTATE.DIT is 1: