code: def SUB(cpu, dest, src):
          
          cpu._SUB(dest, src, carry=False)
docs: Subtract.
  
          Subtracts the second operand (source operand) from the first operand
          (destination operand) and stores the result in the destination operand.
          The destination operand can be a register or a memory location; the
          source operand can be an immediate, register, or memory location.
          (However, two memory operands cannot be used in one instruction.) When
          an immediate value is used as an operand, it is sign-extended to the
          length of the destination operand format.
          The SUB 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::
  
              DEST  =  DEST - SRC;
  
          :param cpu: current CPU.
          :param dest: destination operand.
          :param src: source operand.