code: def SBB(cpu, dest, src):
          
          cpu._SUB(dest, src, carry=True)
docs: Integer subtraction with borrow.
  
          Adds the source operand (second operand) and the carry (CF) flag, and
          subtracts the result from the destination operand (first operand). The
          result of the subtraction is stored in the destination operand. The
          destination operand can be a register or a memory location; the source
          operand can be an immediate, a register, or a memory location.
          (However, two memory operands cannot be used in one instruction.) The
          state of the CF flag represents a borrow from a previous subtraction.
          When an immediate value is used as an operand, it is sign-extended to
          the length of the destination operand format.
          The SBB instruction does not distinguish between signed or unsigned
          operands. Instead, the processor evaluates the result for both data
          types and sets the OF and CF flags to indicate a borrow in the signed
          or unsigned result, respectively. The SF flag indicates the sign of the
          signed result.  The SBB instruction is usually executed as part of a
          multibyte or multiword subtraction in which a SUB instruction is
          followed by a SBB instruction::
  
                  DEST  =  DEST - (SRC + CF);
  
          :param cpu: current CPU.
          :param dest: destination operand.
          :param src: source operand.