subs

Subtract and Set Flags (Immediate)

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

Subtracts immediate and updates flags.

Details

Subtracts a 12-bit immediate value (optionally shifted left by 0 or 12 bits) from the source register and updates all condition flags. The N, Z, C, and V flags are set according to the result. This instruction executes in AArch64 state and requires no special privileges.

Pseudocode Operation

imm_val ← (imm12 << (sh * 12)); result ← Wn - imm_val; Wd ← result; N ← result[31]; Z ← (result == 0); C ← BorrowFrom(Wn, imm_val); V ← OverflowFrom(Wn, imm_val, result)

Example

SUBS w0, Wn, #16

Encoding

Binary Layout
0
1
1
100010
sh
imm12
Rn
Rd
 
Format Data Processing
Opcode 0x71000000
Extension Base

Operands

  • Wd
    Destination 32-bit integer register
  • Wn
    First source / base 32-bit integer register
  • imm
    Imm

Reference (Arm A64 ISA)

Instruction Forms

Encoding Instruction ISA Bit pattern
0x6B200000 SUBS <Wd>, <Wn|WSP>, <Wm>{, <extend> {#<amount>}} A64 0 | 1 | 1 | 01011 | 00 | 1 | Rm | option | imm3 | Rn | Rd
0xEB200000 SUBS <Xd>, <Xn|SP>, <R><m>{, <extend> {#<amount>}} A64 1 | 1 | 1 | 01011 | 00 | 1 | Rm | option | imm3 | Rn | Rd
0x71000000 SUBS <Wd>, <Wn|WSP>, #<imm>{, <shift>} A64 0 | 1 | 1 | 100010 | sh | imm12 | Rn | Rd
0xF1000000 SUBS <Xd>, <Xn|SP>, #<imm>{, <shift>} A64 1 | 1 | 1 | 100010 | sh | imm12 | Rn | Rd
0x6B000000 SUBS <Wd>, <Wn>, <Wm>{, <shift> #<amount>} A64 0 | 1 | 1 | 01011 | shift | 0 | Rm | imm6 | Rn | Rd
0xEB000000 SUBS <Xd>, <Xn>, <Xm>{, <shift> #<amount>} A64 1 | 1 | 1 | 01011 | shift | 0 | Rm | imm6 | Rn | Rd

Description

Subtract (immediate), setting flags, 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.

Operation

bits(datasize) result;
bits(datasize) operand1 = if n == 31 then SP[]<datasize-1:0> else X[n, datasize];
bits(datasize) operand2;
bits(4) nzcv;

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

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

X[d, datasize] = result;