CertiGuide to A+ (Core Hardware)  9  Chapter 0001:  Power Supplies - System Board       9  XV  Demanding Attention Digitally

IRQ 1 XVI  DMA and Bus Mastering

Until the PCI slot, sharing an interrupt was a bad idea. The CPU wouldn't know which device was really talking. This is known as an IRQ conflict. This is not as an uncommon experience as we would like. As folks added more toys to the expansion bus, the more IRQs were used. When the IBM PC came out, it had one serial (communications) port. In theory, it could handle up to four communication ports.

However, the designers allowed both communications port 1 and 3 (COM1 & COM3) to have the same IRQ! In addition, COM2 and COM4 also share IRQ 3. Now this wasn't an issue when there was no mouse, and a 300-baud modem was over a thousand bucks.

Today the answer is assign additional needs for the communication ports to non-standard IRQs or install a serial board that uses one IRQ and has a dedicated processor.

IRQ 1 XVI  DMA and Bus Mastering