Get exit code of subprogram (INT 21h, 4Dh - WAIT)

        Entry:
          AH = 4Dh
        Return:
          AL = exit code of subprogram (functions 31h or 4Ch)
          AH = circumstance which caused termination
                  0 = Terminate/abort
                  1 = Control-C
                  2 = Hard error
                  3 = Terminate and stay resident