18. SBB Instruction, Integer Subtraction with Borrow

• After subtraction, the carry flag CF = 1 indicates a need for a borrow.

• The SBB (subtract with borrow) instruction subtracts both a source operand and the value of the Carry flag CF from a destination operand:

      SBB op1, op2   ; op1 -= op2, op1 -= CF
  

• The possible operands are the same as for the ADC instruction.

• The following fragment of code performs 64-bit subtraction:

      mov edx, 1 ; upper half
      mov eax, 0 ; lower half
      sub eax, 1 ; subtract 1 from the lower half, set CF.
      sbb edx, 0 ; subtract carry CF from the upper half.
  

• The example logic:

  1. Sets EDX:EAX to 00000001h:00000000h.

  2. Subtracts 1 from the value in EDX:EAX.

     • The lower 32 bits are subtracted first, setting the Carry flag CF.

     • The upper 32 bits are subtracted next, including the Carry flag.