sbc

Subtract with Carry

SBC <Wd>, <Wn>, <Wm>

Subtracts with borrow (Carry - 1).

Details

Subtracts one register from another with a borrow (carry-in), computing Wd = Wn - (Wm + NOT(C)), where the carry flag is inverted before use as a borrow. All condition flags (N, Z, C, V) are updated: N and Z reflect the result, C is set if no borrow occurred (result ≥ 0 in unsigned arithmetic), and V is set if signed overflow occurred. This is a 32-bit operation; a 64-bit variant (SBC for X-registers) exists with identical semantics.

Pseudocode Operation

borrow ← NOT(C)
result ← Wn - (Wm + borrow)
Wd ← result
N ← result[31]
Z ← (result == 0)
C ← NOT(BorrowFrom(Wn - (Wm + borrow)))
V ← OverflowFrom(Wn - (Wm + borrow))

Example

SBC w0, w1, w2

Encoding

Binary Layout
0
1
0
11010000
Rm
000000
Rn
Rd
 
Format Data Processing
Opcode 0x5A000000
Extension Base

Operands

  • Wd
    Destination 32-bit integer register
  • Wn
    First source / base 32-bit integer register
  • Wm
    Second source / offset 32-bit integer register

Reference (Arm A64 ISA)

Instruction Forms

Encoding Instruction ISA Bit pattern
0x5A000000 SBC <Wd>, <Wn>, <Wm> A64 0 | 1 | 0 | 11010000 | Rm | 000000 | Rn | Rd
0xDA000000 SBC <Xd>, <Xn>, <Xm> A64 1 | 1 | 0 | 11010000 | Rm | 000000 | Rn | Rd

Description

Subtract with Carry subtracts a register value and the value of NOT (Carry flag) from a register value, and writes the result to the destination register.

Operation

bits(datasize) result;
bits(datasize) operand1 = X[n, datasize];
bits(datasize) operand2 = X[m, datasize];

operand2 = NOT(operand2);

(result, -) = AddWithCarry(operand1, operand2, PSTATE.C);

X[d, datasize] = result;