code: def DEC(self, a):
          
          r = a - 1
          self.clear_NZV()
          self.update_NZ_8(r)
          if r == 0x7f:
              self.V = 1
          return r
docs: Subtract one from the register. The carry bit is not affected, thus
          allowing this instruction to be used as a loop counter in multiple-
          precision computations. When operating on unsigned values, only BEQ and
          BNE branches can be expected to behave consistently. When operating on
          twos complement values, all signed branches are available.
  
          source code forms: DEC Q; DECA; DECB
  
          CC bits "HNZVC": -aaa-