You are connecting an external modem to a legacy (non-plug and play) PC which has 4 existing com ports. A serial mouse is already using com1. Which of the following ports could you use?

You are connecting an external modem to a legacy (non-plug and play) PC which has 4 existing com ports. A serial mouse is already using com1. Which of the following ports could you use?

• 2 or 4
• 3 or 4
• 1 or 3
• 2 or 3 

EXPLANATION

Com ports 1 and 3 share the same IRQ (4), whereas Com ports 2 and 4 share IRQ 3. Under normal circumstances,
serial devices can share the IRQ, however, in the case of a mouse, it would have required a driver which would constantly be active, not giving another serial device the opportunity to use the IRQ. So any answer with COM3 would be invalid.

SOURCE

https://en.wikibooks.org/wiki/A%2B_Certification/Exam_Objectives/Hardware/Basics/CPU/IRQ