1) assembly i am not sure… in C use
#include<reg52.h>
as a hedder file and
Pn^m=x;
n=0_3 ;for prot name
m=0_7 ; for pin
x can be 0 or 1;
eg: P0^1=0; P1^3=1;….
method two
#include<at89x51.h>
P0_0=1;
2) usb serail(RS232) converters are avialable and it works fine i tried it for windows(XP, vista, 7) it works fine… or you can made it on your own by using ic FT232(avialbale in smt pack)…
3) set your own protcols… and design you firmware(controller’s code) accordingly…..
nice project…