Terminal Emulator - Line-at-a-time Mode/Local Editing

Line-at-a-time Mode/Local Editing

Terminal emulators may implement local editing, also known as "line-at-a-time mode". This is also mistakenly referred to as "half-duplex". In this mode, the terminal emulator only sends complete lines of input to the host system. The user enters and edits a line, but it is held locally within the terminal emulator as it is being edited and not transmitted until the user signals (usually with the ↵ Enter key on the keyboard or a "send" button of some sort on the user interface) the completion of the line. At that point the entire line is transmitted. Of course, line-at-a-time mode implies local echo, since otherwise the user will not be able to see the line as it is being edited and constructed. However, line-at-a-time mode is independent of echo mode and does not require local echo. When entering a password, for example, line-at-a-time entry with local editing is possible, but local echo is turned off (otherwise the password would be displayed).

The complexities of line-at-a-time mode are exemplified by the line-at-a-time mode option in the TELNET protocol. To implement it correctly, the Network Virtual Terminal implementation provided by the terminal emulator program must be capable of recognizing and properly dealing with "interrupt" and "abort" events that arrive in the middle of locally editing a line.

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