Terminal emulator

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xterm, a terminal emulator designed for the X Window System Xterm.png
xterm, a terminal emulator designed for the X Window System
Windows Terminal, an open-source terminal emulator for Windows 10 and Windows 11 Windows Terminal v1.0 1138x624.png
Windows Terminal, an open-source terminal emulator for Windows 10 and Windows 11

A terminal emulator, terminal application, or term,[ citation needed ] is a computer program that emulates a video terminal within some other display architecture. Though typically synonymous with a shell or text terminal, the term terminal covers all remote terminals, including graphical interfaces. A terminal emulator inside a graphical user interface is often called a terminal window.

Contents

A terminal window allows the user access to a text terminal and all its applications such as command-line interfaces (CLI) and text user interface (TUI) applications. These may be running either on the same machine or on a different one via telnet, ssh, dial-up, or over a direct serial connection. On Unix-like operating systems, it is common to have one or more terminal windows connected to the local machine.

Terminals usually support a set of escape sequences for controlling color, cursor position, etc. Examples include the family of terminal control sequence standards known as ECMA-48, ANSI X3.64 or ISO/IEC 6429.

History

Computer terminals

An "intelligent" terminal [1] does its own processing, usually implying a microprocessor is built in, but not all terminals with microprocessors did any real processing of input: the main computer to which it was attached would have to respond quickly to each keystroke. The term "intelligent" in this context dates from 1969. [2]

Notable examples include the IBM 2250, predecessor to the IBM 3250 and IBM 5080, and IBM 2260, [3] predecessor to the IBM 3270, introduced with System/360 in 1964.

IBM 2250 Model 4, including light pen and programmed function keyboard HypertextEditingSystemConsoleBrownUniv1969.jpg
IBM 2250 Model 4, including light pen and programmed function keyboard
Most terminals were connected to minicomputers or mainframe computers and often had a green or amber screen. Typically terminals communicate with the computer via a serial port via a null modem cable, often using an EIA RS-232 or RS-422 or RS-423 or a current loop serial interface. IBM systems typically communicated over a Bus and Tag channel, a coaxial cable using a proprietary protocol, a communications link using Binary Synchronous Communications or IBM's SNA protocol, but for many DEC, Data General and NCR (and so on) computers there were many visual display suppliers competing against the computer manufacturer for terminals to expand the systems. In fact, the instruction design for the Intel 8008 was originally conceived at Computer Terminal Corporation as the processor for the Datapoint 2200.

From the introduction of the IBM 3270, and the DEC VT100 (1978), the user and programmer could notice significant advantages in VDU technology improvements, yet not all programmers used the features of the new terminals (backward compatibility in the VT100 and later Televideo terminals, for example, with "dumb terminals" allowed programmers to continue to use older software).

Some dumb terminals had been able to respond to a few escape sequences without needing microprocessors: they used multiple printed circuit boards with many integrated circuits; the single factor that classed a terminal as "intelligent" was its ability to process user-input within the terminal—not interrupting the main computer at each keystroke—and send a block of data at a time (for example: when the user has finished a whole field or form). Most terminals in the early 1980s, such as ADM-3A, TVI912, Data General D2, DEC VT52, despite the introduction of ANSI terminals in 1978, were essentially "dumb" terminals, although some of them (such as the later ADM and TVI models) did have a primitive block-send capability. Common early uses of local processing power included features that had little to do with off-loading data processing from the host computer but added useful features such as printing to a local printer, buffered serial data transmission and serial handshaking (to accommodate higher serial transfer speeds), and more sophisticated character attributes for the display, as well as the ability to switch emulation modes to mimic competitor's models, that became increasingly important selling features during the 1980s especially, when buyers could mix and match different suppliers' equipment to a greater extent than before.

The advance in microprocessors and lower memory costs made it possible for the terminal to handle editing operations such as inserting characters within a field that may have previously required a full screen-full of characters to be re-sent from the computer, possibly over a slow modem line. Around the mid 1980s most intelligent terminals, costing less than most dumb terminals would have a few years earlier, could provide enough user-friendly local editing of data and send the completed form to the main computer. Providing even more processing possibilities, workstations like the Televideo TS-800 could run CP/M-86, blurring the distinction between terminal and Personal Computer.

Another of the motivations for development of the microprocessor was to simplify and reduce the electronics required in a terminal. That also made it practicable to load several "personalities" into a single terminal, so a Qume QVT-102 could emulate many popular terminals of the day, and so be sold into organizations that did not wish to make any software changes. Frequently emulated terminal types included:

The ANSI X3.64 escape code standard produced uniformity to some extent, but significant differences remained. For example, the VT100, Heathkit H19 in ANSI mode, Televideo 970, Data General D460, and Qume QVT-108 terminals all followed the ANSI standard, yet differences might exist in codes from function keys, what character attributes were available, block-sending of fields within forms, "foreign" character facilities, and handling of printers connected to the back of the screen.

Emulators

Local echo

Terminal emulators may implement a local echo function, which may erroneously be named "half-duplex", or still slightly incorrectly "echoplex" (which is formally an error detection mechanism rather than an input display option). [4] [5] [6] [7]

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".[ citation needed ] 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. It is not transmitted until the user signals its completion, usually with the ↵ Enter key on the keyboard or a "send" button of some sort in the user interface. At that point, the entire line is transmitted. 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. [4] [8] 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). [9]

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. [10]

Synchronous terminals

In asynchronous terminals data can flow in any direction at any time. In synchronous terminals a protocol controls who may send data when. IBM 3270-based terminals used with IBM mainframe computers are an example of synchronous terminals. They operate in an essentially "screen-at-a-time" mode (also known as block mode). Users can make numerous changes to a page, before submitting the updated screen to the remote machine as a single action.

Terminal emulators that simulate the 3270 protocol are available for most operating systems, for use both by those administering systems such as the z9, as well as those using the corresponding applications such as CICS.

Other examples of synchronous terminals include the IBM 5250, ICL 7561, Honeywell Bull VIP7800 and Hewlett-Packard 700/92.

Virtual consoles

Virtual consoles, also called virtual terminals, are emulated text terminals, using the keyboard and monitor of a personal computer or workstation. The word "text" is key since virtual consoles are not GUI terminals and they do not run inside a graphical interface. Virtual consoles are found on most Unix-like systems. They are primarily used to access and interact with servers, without using a graphical desktop environment.

Examples of terminals emulated

Many terminal emulators have been developed for terminals such as VT52, VT100, VT220, VT320, IBM 3270/8/9/E, IBM 5250, IBM 3179G, Data General D211, Hewlett Packard HP700/92, Sperry/Unisys 2000-series UTS60, Burroughs/Unisys A-series T27/TD830/ET1100, ADDS ViewPoint, Sun console, QNX, AT386, SCO-ANSI, SNI 97801, Televideo, and Wyse 50/60. Additionally, programs have been developed to emulate other terminal emulators such as xterm and assorted console terminals (e.g., for Linux). Finally, some emulators simply refer to a standard, such as ANSI. Such programs are available on many platforms ranging from DOS and Unix to Windows and macOS to embedded operating systems found in cellphones and industrial hardware.

Implementation details

Unix-like systems

In the past, Unix and Unix-like systems used serial port devices such as RS-232 ports, and provided /dev/* device files for them. [11]

With terminal emulators those device files are emulated by using a pair of pseudoterminal devices. This pair is used to emulate a physical ports/connection to the host computing endpoint - computer's hardware provided by operating system APIs, some other software like rlogin, telnet or SSH or else. [12] For example, in Linux systems these would be /dev/ptyp0 (for the master side) and /dev/ttyp0 (for the slave side) pseudoterminal devices respectively.

There are also special virtual console files like /dev/console. In text mode, writing to the file displays text on the virtual console and reading from the file returns text the user writes to the virtual console. As with other text terminals, there are also special escape sequences, control characters and functions that a program can use, most easily via a library such as ncurses . For more complex operations, the programs can use console and terminal special ioctl system calls. One can compare devices using the patterns vcs ("virtual console screen") and vcsa ("virtual console screen with attributes") such as /dev/vcs1 and /dev/vcsa1. [13]

The virtual consoles can be configured in the file /etc/inittab read by init -- typically it starts the text mode login process getty for several virtual consoles. X Window System can be configured in /etc/inittab or by an X display manager. A number of Linux distributions use systemd instead of init, which also allows virtual console configuration.

CLI tools

Typical Linux system programs used to access the virtual consoles include:

  • chvt to switch the current virtual console
  • openvt to run a program on a new virtual console
  • deallocvt to close a currently unused virtual console

System loading

The program startx starts the X Window System on a new virtual console. There are also other graphical programs that can start from the console (e.g. LinuxTV and MPlayer etc.)

See also

Notes

  1. "intelligent terminal Definition from PC Magazine Encyclopedia".
  2. Twentieth Century Words; by John Ayto; Oxford Unity Press; page 413
  3. "What is 3270 (Information Display System)". 3270 .. over its predecessor, the 2260
  4. 1 2 Daintith 2004, p. 171.
  5. Weik 2000, "echo" p. 478.
  6. Weik 2000, "echoplex" p. 479.
  7. Weik 2000, "echoplex mode" p. 479.
  8. Bangia 2010, p. 324.
  9. Stevens & Wright 1994, p. 413.
  10. Miller 2009, p. 590, 591.
  11. "A Brief History of Terminal Emulation | Turbosoft". www.ttwin.com. Retrieved 2021-10-04.
  12. "Ubuntu Manpage: tty — general terminal interface". manpages.ubuntu.com. Retrieved 2021-10-04.
  13. "Screen dumps". The Linux keyboard and console HOWTO. The Linux Documentation Project . Retrieved 2008-07-31. The current contents of the screen of /dev/ttyN can be accessed using the device /dev/vcsN (where `vcs' stands for `virtual console screen'). [...] From a program it is usually better to use /dev/vcsaN (`virtual console screen with attributes') instead - it starts with a header giving the number of rows and columns and the location of the cursor. See vcs(4).

Related Research Articles

In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and other computer programs to access hardware functions without needing to know precise details about the hardware being used.

IBM 3270 Family of block-oriented display terminals and printers made by IBM

The IBM 3270 is a family of block oriented display and printer computer terminals introduced by IBM in 1971 and normally used to communicate with IBM mainframes. The 3270 was the successor to the IBM 2260 display terminal. Due to the text color on the original models, these terminals are informally known as green screen terminals. Unlike a character-oriented terminal, the 3270 minimizes the number of I/O interrupts required by transferring large blocks of data known as data streams, and uses a high speed proprietary communications interface, using coaxial cable.

Telnet Network protocol for bidirectional communication using a virtual terminal connection

Telnet is an application protocol used on the Internet or local area network to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection. User data is interspersed in-band with Telnet control information in an 8-bit byte oriented data connection over the Transmission Control Protocol (TCP).

VT100 Computer terminal from Digital Equipment Corporation

The VT100 is a video terminal, introduced in August 1978 by Digital Equipment Corporation (DEC). It was one of the first terminals to support ANSI escape codes for cursor control and other tasks, and added a number of extended codes for special features like controlling the status lights on the keyboard. This led to rapid uptake of the ANSI standard, becoming the de facto standard for terminal emulators.

ANSI escape code Method used for display options on video text terminals

ANSI escape sequences are a standard for in-band signaling to control cursor location, color, font styling, and other options on video text terminals and terminal emulators. Certain sequences of bytes, most starting with an ASCII escape character and a bracket character, are embedded into text. The terminal interprets these sequences as commands, rather than text to display verbatim.

Conversational Monitor System

The Conversational Monitor System is a simple interactive single-user operating system. CMS was originally developed as part of IBM's CP/CMS operating system, which went into production use in 1967. CMS is part of IBM's VM family, which runs on IBM mainframe computers. VM was first announced in 1972, and is still in use today as z/VM.

Computer terminal Computer input/output device for users

A computer terminal is an electronic or electromechanical hardware device that can be used for entering data into, and transcribing data from, a computer or a computing system. The teletype was an example of an early day hardcopy terminal, and predated the use of a computer screen by decades.

System console

One meaning of system console, computer console, root console, operator's console, or simply console is the text entry and display device for system administration messages, particularly those from the BIOS or boot loader, the kernel, from the init system and from the system logger. It is a physical device consisting of a keyboard and a screen, and traditionally is a text terminal, but may also be a graphical terminal. System consoles are generalized to computer terminals, which are abstracted respectively by virtual consoles and terminal emulators. Today communication with system consoles is generally done abstractly, via the standard streams, but there may be system-specific interfaces, for example those used by the system kernel.

Text-based user interface Type of interface based on outputting to or controlling a text display

In computing, text-based user interfaces (TUI), is a retronym describing a type of user interface (UI) common as an early form of human–computer interaction, before the advent of graphical user interfaces (GUIs). Like GUIs, they may use the entire screen area and accept mouse and other inputs. They may also use color and often structure the display using special graphical characters such as ┌ and ╣, referred to in Unicode as the "box drawing" set. The modern context of use is usually a terminal emulator.

Text mode is a computer display mode in which content is internally represented on a computer screen in terms of characters rather than individual pixels. Typically, the screen consists of a uniform rectangular grid of character cells, each of which contains one of the characters of a character set; at the same time, contrasted to all points addressable (APA) mode or other kinds of computer graphics modes.

Pseudoterminal

In some operating systems, including Unix and Linux, a pseudoterminal, pseudotty, or PTY is a pair of pseudo-device endpoints (files) which establish asynchronous, bidirectional communication (IPC) channel between two or more processes. The master provides means by which a terminal emulator process controls the slave. The slave, emulates a hardware text terminal device. PTY are similar to bidirectional pipes.

IBM 5250

IBM 5250 is a family of block-oriented terminals originally introduced with the IBM System/34 midrange computer systems in 1977. It also connects to the later System/36, System/38, and IBM AS/400 systems, and to IBM Power Systems systems running IBM i.

3270 emulator

A 3270 Emulator is a terminal emulator that duplicates the functions of an IBM 3270 mainframe computer terminal on a computer, usually a PC or similar microcomputer.

ZOC (software)

ZOC is a popular computer-based terminal emulator and Telnet software client for the Microsoft Windows and Apple Macintosh macOS operating systems that supports telnet, modem, SSH 1 and 2, ISDN, serial, TAPI, Rlogin and other means of communication. Its terminal emulator supports Xterm emulation with full colors, meta-keys and local printing, VT102, VT220 and several types of ANSI as well as Wyse, TVI, TN3270, and Sun's CDE. It supports full keyboard remapping, scripting in REXX and other languages, and support for named pipes.

A headless computer is a computer system or device that has been configured to operate without a monitor, keyboard, and mouse. A headless system is typically controlled over a network connection, although some headless system devices require a serial connection to be made over RS-232 for administration of the device. Headless operation of a server is typically employed to reduce operating costs.

Linux console Console of the Linux kernel

The Linux console is a system console internal to the Linux kernel. A system console is the device which receives all kernel messages and warnings and which allows logins in single user mode). The Linux console provides a way for the kernel and other processes to send text output to the user, and to receive text input from the user. The user typically enters text with a computer keyboard and reads the output text on a computer monitor. The Linux kernel supports virtual consoles – consoles that are logically separate, but which access the same physical keyboard and display. The Linux console are implemented by the VT subsystem of the Linux kernel, and do not rely on any user space software. This is in contrast to a terminal emulator, which is a user space process that emulates a terminal, and is typically used in a graphical display environment.

A terminal multiplexer is a software application that can be used to multiplex several separate pseudoterminal-based login sessions inside a single terminal display, terminal emulator window, PC/workstation system console, or remote login session, or to detach and reattach sessions from a terminal. It is useful for dealing with multiple programs from a command line interface, and for separating programs from the session of the Unix shell that started the program, particularly so a remote process continues running even when the user is disconnected.

The IBM WebSphere Host On-Demand Server, or HOD as it is commonly known is a Java application that runs on a Server that is deliverable via modern web servers such as the Apache web server. The application allows the end user to access IBM 3270, IBM 5250 and other Virtual terminals using the Telnet protocol whether through a secure or unsecured mode of communication. The product in its present form runs on AIX, UNIX, HP-UX, IBM i, z/OS, Linux, Solaris and Windows Server.

devpts

devpts is a virtual filesystem directory available in the Linux kernel since version 2.1.93. It is normally mounted at /dev/pts and contains solely devices files which represent slaves to the multiplexing master located at /dev/ptmx which is in turn is used to implement terminal emulators.

References