SUBS (extended register)

Subtract extended and scaled register, setting flags

This instruction subtracts a sign or zero-extended register value, followed by an optional left shift amount, from a register value, and writes the result to the destination register. The argument that is extended from the <Rm> register can be a byte, halfword, word, or doubleword. It updates the condition flags based on the result.

Encoding: Setting the condition flags

313029282726252423222120191817161514131211109876543210
1101011001
sfopSoptRmoptionimm3RnRd

32-bit (sf == 0)

SUBS <Wd>, <Wn|WSP>, <Wm>{, <extend> {#<amount>}}

64-bit (sf == 1)

SUBS <Xd>, <Xn|SP>, <R><m>{, <extend> {#<amount>}}

Decoding algorithm

if imm3 IN {'101', '110', '111'} then EndOfDecode(Decode_UNDEF);
constant integer d = UInt(Rd);
constant integer n = UInt(Rn);
constant integer m = UInt(Rm);
constant integer shift = UInt(imm3);
constant integer datasize = 32 << UInt(sf);
constant ExtendType extend_type = DecodeRegExtend(option);

Operation

constant bits(datasize) operand1 = if n == 31 then SP[datasize] else X[n, datasize];
constant bits(datasize) operand2 = NOT(ExtendReg(m, extend_type, shift, 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|WSP>: Is the 32-bit name of the first source general-purpose register or stack pointer, encoded in the "Rn" field.
<Wm>: Is the 32-bit name of the second general-purpose source register, encoded in the "Rm" field.
<extend>: <extend>: <amount>: Is the left shift amount to be applied after extension in the range 0 to 4, defaulting to 0, encoded in the "imm3" field. It must be absent when <extend> is absent, is required when <extend> is LSL, and is optional when <extend> is present but not LSL.
<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 first source general-purpose register or stack pointer, encoded in the "Rn" field.
<R>: <m>: Is the number [0-30] of the second general-purpose source register or the name ZR (31), encoded in the "Rm" field.

Operational Notes

If PSTATE.DIT is 1: