- COM (hardware interface)
COM is the original, yet still common, name of the
serial port interface on IBM PC-compatiblecomputer s. It might not only refer to physical ports, but also to virtual ports, such as ports created bybluetooth orUSB -to-Serial adapters.Most PC-compatible systems in the 1980s and 1990s had one or two ports, with communication interfaces defined like this:
*COM1:
I/O port 0x3F8, IRQ 4
*COM2: I/O port 0x2F8, IRQ 3
*COM3: I/O port 0x3E8, IRQ 4
*COM4: I/O port 0x2E8, IRQ 3As of 2007, most systems ship with one or no physical COM ports.
I/O addresses
The COM ports are interfaced by an integrated circuit such as
16550 UART . This IC has seven internal 8-bit registers which hold information and configuration data about which data is to be sent or was received, thebaud rate , interrupt configuration and more. In the case of COM1, these registers can be accessed by writing to or reading from the I/O addresses 0x3F8 to 0x3FF.If the CPU, for example, wants to send information out on COM1, it writes to I/O port 0x3F8, as this I/O port is "connected" to the UART IC register which holds the information that is to be sent out.
ee also
*
Device file
*LPT
*Input/Output Base Address
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