hi,
     Here I will try to explain how to program com ports using interrupts.
you can communicate with the com port using the int 14h. The value in the DX register determines the port(ie, 0=com1, 1=com2, 2=com3, 3=com4).
we can initialize the port using function 00h (ie, 0h in al register). We can set the parameters in AL register. The format of the parameter is:
bit 0-1 determines the length of the word.(00=7bit,01 = 8bit).
bit 2   determines stop bit.
bit 3-4 determines the parity bit.(00=none,01=odd,10=none,11=even).
bit 7-5 determines the transfer rate(100=1200bit/sec, 101=2400, 110=4800, 111 = 9600).
eg. mov ah, 00h
    mov al, 11101011
    mov dx, 0h
    int 14h
the above sample initialize the com0 port with transfer rate 9600bits/sec and has odd parity and the size of word is 8bits.

now, similarly you can transmit a character with function 01h.the character to be transmitted should be in al register.
eg. mov ah, 01h
    mov al, 61h ;'a'
    mov dx, 0h
    int 14h

similarly you can receive a character by using function 2h.it returns a received character in al and non zero value in ah if error.

regards manish.