As I said before:

"Look up the operation of the UART Interrupt, and the RI and TI flags.
See "the bible" in the 'Links' section."

Have you done that?

These are the only means by which the processor knows that it has sent or received a character; there is no other way to tell.
The fact that you are using a "null-modem"^[1] connection is totally irrelevant to this problem!
Adding modem control lines (RTS, DTS, etc) will make no difference whatsoever.

For an example of interrupt-driven serial IO, see:
http://www.keil.com/download/docs/intsio.zip.asp


[1] Strictly, a "Null Modem" cable connects all of the modem control signals - what you're using is a simple 3-wire "crossover" lead, that just connects the 8051's transmitter to the PC's receiver, and vice-versa