IRQs are known as interrupt requests. Basically, an IRQ is assigned to any device in your computer that needs to communicate with the system. When information needs to be sent, the device uses an IRQ to get the system’s attention. In simplest terms, it’s like raising your hand in class in order for the teacher to call on you for your answer. 

Once the device is recognized, it needs to transmit its data. To do this, it requires a channel to transmit on—which is where the COM (or communication) ports come in. These ports are used for serial (one bit at a time) communication and each device requires its own private port to communicate across.

To keep the communication chatter down, the system keeps tight control over the IRQ and COM settings. If more than one device attempts to use a common resource (either IRQ or COM) out-of-turn, then errors occur—to include the possibility of entire system freezing up. Think of this as the confusion that might occur if every student in the class tried to share their answer with the teacher at the same time.

To help solve the problem, we need to set the modem to use resources that are commonly available for serial communication. There are standards to follow where these settings are concerned. 

Perhaps you’ve noticed the one or two nine-pin male connectors on the back of your machine? These are the standard COM (or serial) ports, and are commonly used for serial mouses, external modems, and some older digital devices (such as cameras, etc.) The standard resources assigned to these ports are as follows: COM 1 = IRQ 4 and COM 2 = IRQ 3. When adding an additional COM port (which is what adding an internal modem does), an unused COM port number must be assigned. Following the example above, if you have two serial ports already, the next available is COM 3, then COM 4, and so forth. If you have a device already connected to one of the external ports, then COM 1 or COM 2 is considered in use.

Even-numbered COM ports can share an IRQ with other even-numbered COM ports; odd-numbered ports can share with other odd-numbered COM ports as well. This means that both COM 1 and COM 3 can be assigned IRQ 4 simultaneously, while COM 2 and COM 4 can both hold IRQ 3. 

However, there is a caution when sharing IRQs. If both COM ports are frequently needed at the same time, it’s not wise to assign them to the same IRQ. For instance, if you have a serial mouse connected to COM 1, do not assign your internal modem to COM 3—you might find it difficult to surf the Internet and control your cursor at the same time!