To what extent are the xterm, xterm-color, and linux terminal emulators based on VT100?

Question

My understanding is that all of these terminal emulators derive the ASCII control code behaviors and ANSI escape sequences from the VT100 standard. I also understand that there is an ANSI standard for terminal behavior that is based on either VT100 or VT102. Am I correct in this understanding? Also, what other features or behaviors are derived from VT100?

Answer 1

While there were several popular terminals during the 1970s and 1980s, for whatever reason the original developers of xterm starting in the late 1980s chose to use vt100 as a model. From the outset there were differences (such as the alternate screen mode, which may have been influenced by Hewlett Package terminals).

ANSI x3.64 was not based on vt100; DEC was one of several manufacturers who participated in developing the standard. Like most standards, it is compromise. Much of vt100 corresponds to the ANSI x3.64 standard, replaced long ago by ISO-6429 (ECMA-48). It implements perhaps a quarter of ECMA-48, and provides some features not in the standard.

Some features of vt100 not in the standard include:

• scrolling (and scrolling margins)
• double-size characters
• save/restore cursor position

You've used two of those three, knowing or not. ECMA-48 describes different controls for scrolling which vt100 did not implement, but are supported in xterm (and some of the xterm imitators).

Other ANSI-compliant terminals such as Sun's console emulator did not implement these features (and sending the scrolling region escape there has interesting results).

Strictly speaking, the vt100 did not support the controls for inserting and deleting lines. That was done in vt102 (which is what most people think of as "vt100"). They're part of a series: a complete vt100 emulator can emulate vt52, vt220 can emulate vt100, vt420 can emulate vt220 / vt100 / vt52.

By the mid-1990s, xterm had some of the character-set switching features associated with vt220. Further development added all but soft fonts from the vt220 repertoire (like double-sized characters, this is not used from many programs other than vttest). This is summarized in the manual page section on Emulations.

While ISO-6429 defined color escape sequences, those were not supported by the vt100/vt220 models. Those were supported in the vt525 model, which I've been told was designed and manufactured by Wyse. Neither DEC nor Wyse has made terminals for quite a while, and the behavior of colors in that terminal had no impact on the development of xterm. That had some influence from Linux console — but Linux console's color palette escapes fall completely outside any standard. Its color selection escapes are based on ANSI, but likely in imitation of AT&T (and SCO) consoles rather than by reading the standard itself.

Answer 2

http://vt100.net/ is a great resource. You are broadly right; however, the ANSI standard in question, ANSI X3.64, and the VT100 were developed simultaneously. The VT100 was the first terminal to support the standard, and the standard was developed with input from DEC. See https://en.wikipedia.org/wiki/ANSI_escape_code for a longer discussion. xterm and its relatives emulate just enough of VT220 to support vi, vim, mc and generally the applications using curses. They don't attempt a full emulation. And xterm includes a Tektronix 4010 emulation, for use by graphical applications.

The most important thing that xterm and its siblings have and VT220 did not is color. Most VTs were monochrome devices (black and purchase-time choice of amber, green or white); the first DEC terminal to support color was VT340. Color selection commands are defined in the international standard ISO 6429, the successor of ANSI X3.64. xterm can also load and display user-defined fonts.

VT200-series terminals introduced the numerical keypad (with a gold key where the NumLock key is on a regular PC keyboard, it was actually colored yellow), which was used by EDT, EVE, TPU and other wonderful text editors made by DEC (for RSX and VMS) for text editing commands instead of using regular keys with Alt. This introduced the idea of an "application keypad mode", where the keys on numerical keypad do not send the digits, but special codes for use by full-screen applications.

Trying an answer a request in the comments, the Linux console (that is, the character-cell interface available when Linux boots in text mode, or accessible from many GUIs with Ctrl-Alt-F1 to Ctrl-Alt-F6, which identifies itself as linux) emulates a VT102, but is can load and display user-defined fonts and is capable of color; the color selection commands are the same as for xterm.