Avaya Partner 18 Connection Manual page 298

Appendix B: PC Serial Ports
associated with them (recall that most PCs require the IRQs used by each
active device to be unique to avoid conflicts). To understand why, it's useful
to recall what the PC world was like before the widespread availability of
products like Windows. When the architecture of the current generation of
PCs was first being designed (for the IBM PC/AT), the concept of multitasking
was not nearly as important in the PC marketplace as it is today.
Consequently, since DOS (before Windows) did not permit multiple
applications to run simultaneously (with the notable exception of certain
specialized programs such as mouse drivers), there was little need to provide
a mechanism by which several serial ports could be operated simultaneously.
Consequently, the strategy used was to conserve IRQs by assigning the same
IRQ to more than one COM port (that is, the COM1 and COM3 ports were
both assigned to IRQ4, and the COM2 and COM4 ports were both assigned to
IRQ3). Then, under the assumption that at most two serial ports would be
active simultaneously (for example, COM1 and COM2, which have unique
IRQs), conflicts would not occur.
B-5