Answer:
To help interrupt serial driven communications, the PC utilizes two interrupts, IRQ 3 and IRQ 4.
INT 0Bh and INT 0Ch are the BIOS interrupt
It patches the COM1,COM2 when you call the Com Init Intr routine: interrupt vectors (int 0Ch int 0Bh), allowing IRQ 4 and IRQ3 in the 8259A PIC.
IRQ 3 0Bh Serial port 2
IRQ 4 0Ch Serial port 1
9 pins of DB-9 connector -
1 RTS, Request to Send
2 CTS, Clear to send
3 GND,Ground
4 RXD,Receive Data
5 TXD, Transmit Data
6 DTR, Data Terminal Ready
7 DSR, Data Set Ready
8 RI, Ring Indicator
9 DCD,Data carrier detect