COM
Encyclopedia
COM is the original, yet still common, name of the serial port
interface on IBM PC-compatible computer
s. It might not only refer to physical ports, but also to virtual ports, such as ports created by bluetooth
or USB-to-Serial adapters.
Most PC-compatible systems in the 1980s and 1990s had one or two ports, with communication interfaces defined like this:
As of 2007, most systems ship with one or no physical COM ports.
. This IC has seven internal 8-bit registers
which hold information and configuration data about which data is to be sent or was received, the baud rate, interrupt configuration and more. In the case of COM1, these registers can be accessed by writing to or reading from the I/O address
es 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 holds the information that is to be sent out.
Serial port
In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...
interface on IBM PC-compatible computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
s. It might not only refer to physical ports, but also to virtual ports, such as ports created by bluetooth
Bluetooth
Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances from fixed and mobile devices, creating personal area networks with high levels of security...
or USB-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, IRQInterrupt requestThe computing phrase "interrupt request" is used to refer to either the act of interrupting the bus lines used to signal an interrupt, or the interrupt input lines on a Programmable Interrupt Controller...
4 - COM2: I/O port 0x2F8, IRQ 3
- COM3: I/O port 0x3E8, IRQ 4
- COM4: I/O port 0x2E8, IRQ 3
As 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 UART16550 UART
The 16550 UART is an integrated circuit designed for implementing the interface for serial communications...
. This IC has seven internal 8-bit registers
Hardware register
In digital electronics, especially computing, a hardware register stores bits of information, in a way that all the bits can be written to or read out simultaneously.The hardware registers inside a central processing unit are called processor registers....
which hold information and configuration data about which data is to be sent or was received, the baud rate, interrupt configuration and more. In the case of COM1, these registers can be accessed by writing to or reading from the I/O address
Memory-mapped I/O
Memory-mapped I/O and port I/O are two complementary methods of performing input/output between the CPU and peripheral devices in a computer...
es 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 holds the information that is to be sent out.