code: def instruction_SBC(self, opcode, m, register):
          
          a = register.value
          r = a - m - self.C
          register.set(r)
  #        log.debug("$%x %02x SBC %s: %i - %i - %i = %i (=$%x)" % (
  #            self.program_counter, opcode, register.name,
  #            a, m, self.C, r, r
  #        ))
          self.clear_NZVC()
          self.update_NZVC_8(a, m, r)
docs: Subtracts the contents of memory location M and the borrow (in the C
          (carry) bit) from the contents of the designated 8-bit register, and
          places the result in that register. The C bit represents a borrow and is
          set to the inverse of the resulting binary carry.
  
          source code forms: SBCA P; SBCB P
  
          CC bits "HNZVC": uaaaa