A software port (usually just called a 'port') is a virtual data connection that can be used by programs to exchange data directly, instead of going through a file or other temporary storage location. The most common of these are TCP and UDP ports which are used to exchange data between computers on the Internet. In Flow-based programming, a 'port' is a (named) point of contact between a process and a connection.
I/O or machine port mechanism - port-mapped I/O
For Input or Output (I/O) operations nearly all processor families use similar assembly instructions for both memory access and hardware I/O (see memory-mapped I/O for details). However, Intel microprocessors have assembly instructions (IN and OUT) that are used specifically for hardware I/O. These instructions figure out which hardware device to communicate with using the concept of an I/O port or machine port. These ports are numbered based on which hardware device they refer to. Intel microprocessors generally allow one octet (8-bit byte or word) to be sent or received during each instruction. The hardware device decides how to interpret data sent to it and what data to send to the processor. For example, a common use is to ask a hardware device which byte (in a data transfer) it will be sending next.
