Decrement accumulator

def instruction_DEC_register(self, opcode, register):
        """ Decrement accumulator """
        a = register.value
        r = self.DEC(a)
#        log.debug("$%x DEC %s value $%x -1 = $%x" % (
#            self.program_counter,
#            register.name, a, r
#        ))
        register.set(r)