| The Design and Implementation of the 4.4BSD Operating System | ||
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Terminals support the standard system I/O operations, as well as a collection of terminal-specific operations to control input-character editing and output delays. At the lowest level are the terminal device drivers that control the hardware terminal ports. Terminal input is handled according to the underlying communication characteristics, such as baud rate, and according to a set of software-controllable parameters, such as parity checking.
Layered above the terminal device drivers are line disciplines that provide various degrees of character processing. The default line discipline is selected when a port is being used for an interactive login. The line discipline is run in canonical mode; input is processed to provide standard line-oriented editing functions, and input is presented to a process on a line-by-line basis.
Screen editors and programs that communicate with other computers generally run in noncanonical mode (also commonly referred to as raw mode or character-at-a-time mode). In this mode, input is passed through to the reading process immediately and without interpretation. All special-character input processing is disabled, no erase or other line editing processing is done, and all characters are passed to the program that is reading from the terminal.
It is possible to configure the terminal in thousands of combinations between these two extremes. For example, a screen editor that wanted to receive user interrupts asynchronously might enable the special characters that generate signals and enable output flow control, but otherwise run in noncanonical mode; all other characters would be passed through to the process uninterpreted.
On output, the terminal handler provides simple formatting services, including
Converting the line-feed character to the two-character carriage-return-line-feed sequence
Inserting delays after certain standard control characters
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Displaying echoed nongraphic ASCII characters as a two-character sequence of the form ``^C'' (i.e., the ASCII caret character followed by the ASCII character that is the character's value offset from the ASCII ``@'' character).
Each of these formatting services can be disabled individually by a process through control requests.