Filter
Top Products
PC Serial Ports
A-5
The table below lists the default I/O port addresses and IRQs used by the
serial ports of IBM-PC/AT-compatible computers:
Serial Port
Device Name
I/O Port Address IRQ
COM1 03F8 4
COM2 02F8 3
COM3 03E8 4
COM4 02E8 3
The values in this table play an important part in understanding the "wrinkles"
associated with serial ports: although there is provision for up to four serial
ports, with four unique I/O addresses, there are only two unique IRQs
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 is 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.