code: def SXTB(self, params):
          
          Ra, Rb = self.get_two_parameters(r'\s*([^\s,]*),\s*([^\s,]*)(,\s*[^\s,]*)*\s*', params)
  
          self.check_arguments(low_registers=(Ra, Rb))
  
          def SXTB_func():
              if self.register[Rb] & (1 << 7):
                  self.register[Ra] = 0xFFFFFF00 + (self.register[Rb] & 0xFF)
              else:
                  self.register[Ra] = (self.register[Rb] & 0xFF)
  
          return SXTB_func
docs: STXB Ra, Rb
  
          Sign extend the byte in Rb and store the result in Ra