Typically, protected-mode operating systems such as Windows® have required each serial port to have its own interrupt request line (IRQ). This is true for Windows 3.x and Windows 95. The only way to share interrupts under Windows 3.x or Windows 95 without risking data loss is to install a replacement communications driver.

With Windows NT®, interrupt sharing is possible "right out of the box." But in order to share interrupts, the serial-port card must have a built-in interrupt status port (ISP). Without one, the software that services the interrupts and requests processor time (or interrupt service routine) has to poll each port separately to determine which port generated the interrupt. Polling each port separately is efficient and risks data loss.

However, if your serial-port card has an ISP and your operating system supports it, the interrupt service routine reads the ISP and is immediately directed to the correct port.