INT 0bH: IRQ 3 COM2/4 / Segment Not Present

  INT 0bH is a hardware-generated interrupt (IRQ 3) forced by COM2 or COM4
  when it is done sending or receiving a byte of data.

Ř›CPU Exception InterruptřŘ
  286+ computer execute INT 0bH when the CPU is in protected mode and it
  detects a Segment Not Present exception.

See Also: IRQs: Hardware Interrupts
          Serial Ports
          BIOS Data Area
          ROM-BIOS Functions
          DOS Functions