Test of DEC VT terminal line-wrapping semantics
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DEC VT terminal line-wrapping semantics

One aspect of DEC VT terminals, such as the VT100 and VT220, that keeps puzzling users is what happens when text goes beyond the edge of a line into the next. It turns out that the exact details are not common knowledge.

Terminal emulators, like XTerm and PuTTY, usually try to follow the behaviour of the original hardware as closely as possible. Since the rules are rarely documented and genuine hardware sometimes difficult to get hold of, emulator authors resort to guessing or inventing their own rules, something that does not necessarily serve the best interest of their users.

The sources for the following text are the actual terminals and DEC's internal reference documentation DEC STD-070 Video Systems Reference Manual (71 MB).

Basic VT line-wrapping rules

DEC VT terminals use a mechanism sometimes called "soft-wrap", "deferred wrap" or the "VT100 glitch", mainly to permit full-length newline-terminated text lines to be written without being followed by unsightly blank lines. This is what complicates matters.

The auto-wrap mode flag, DECAWM, controls whether text will wrap around the edges at all. It can be set and cleared using the DECSET and DECRST control sequences; terminal emulators usually get this bit right. In the following text, we assume that auto-wrap is enabled (DECAWM set).

When the terminal receives a printing character and the cursor is in the rightmost column, what happens depends on the hidden Last Column Flag:

  • If the LCF is 0, then the character is drawn at the cursor position as usual, but instead of advancing the cursor afterwards, the LCF is set to 1.
  • If the LCF is 1, then the cursor is moved to the first column of the next line, scrolling if necessary. Then LCF is set to 0, the character is drawn, and the cursor advanced as usual.

The Last Column flag starts out as 0 when the terminal is powered on.

Last Column Flag specifics

Only plain character output in the rightmost column is affected by the LCF; no other operation is. In particular, the Cursor Position Report (CPR) sends the same coordinates regardless of the LCF.

The following rules are all according to DEC's documentation, but not all DEC terminals obey them completely.

Save/Restore Cursor

The LCF is saved/restored by the Save/Restore Cursor (DECSC/DECRC) control sequences. The DECAWM flag is not included in the state managed by these operations.

Resetting operations

The LCF is reset by the following operations (from STD-070):

  • Plain character output, as describe above
  • Set top and bottom margins (DECSTBM)
  • Change line width/height (DECSWL, DECDWL, DECDHL)
  • Set column mode, origin mode (DECSET DECCOLM, DECOM)
  • Reset column mode, origin mode, auto-wrap mode (DECRST DECCOLM, DECOM, DECAWM)
  • Cursor up, down, forward, backward (CUU, CUD, CUF, CUB)
  • Cursor position (CUP, HVP)
  • Backspace (BS)
  • Horizontal tab, vertical tab (HT, VT)
  • Carriage return, line feed (CR, LF)
  • Form feed (FF)
  • Substitute (SUB)
  • Index, reverse index (IND, RI)
  • Next line (NEL)
  • Erase, delete, insert character (ECH, DCH, ICH)
  • Erase in line, in display (EL, ED)
  • Selective erase in line, in display (DECSEL, DECSED)

Since this part of STD-070 is from 1985, operations introduced in newer terminals are not present in this list.

Problematic operations

The fact that TAB is an LCF-resetting operation can have negative consequences. Consider a text file containing a line longer than the width of the terminal. If the file is sent unmodified to the terminal, the long line will wrap, but if it contains an ASCII TAB character at an unfortunate position, the result can be the visual disappearance of a printing character.

GNU Grep uses EL (Erase in Line) in an attempt to control the background colour. Since EL resets LCF, this can lead to characters going missing (see bug reports here, here and here).

Actual hardware and emulators

The VT100 predates the rules laid out in STD-070, so it diverges from them considerably. The VT220 appears to follow the specification to the letter, whereas the much more recent VT510 differs on a few points, perhaps on purpose.

No emulator tested so far matches either actual hardware or the specification in STD-070, but some are clearly better than others.

Test program

This little test program can be used to examine how a terminal or emulator behaves with respect to line-wrapping. Run it in the terminal, and it will spit out a report. (Stdout can be redirected for the report; real terminals usually do not have text copy-paste functions.)

Currently known results are here. If you have results for terminals or emulators not mentioned in that file, please send them to me.