termcap - terminal capability data base
Termcap is a data base describing terminals used primarily by ex (UCB) and vi (UCB), and also by tset (UCB). Terminals are described in termcap by giving a set of capabilities which they have, and by describing how operations are performed. Padding requirements and initialization sequences are included in termcap.
Entries in termcap consist of a number of `:' separated fields. The first entry for each terminal gives the names which are known for the terminal, separated by `|' characters. The first name is always 2 characters long and is used by older version 6 systems which store the terminal type in a 16 bit word in a systemwide data base. The second name given is the most common abbreviation for the terminal, and the last name given should be a long name fully identifying the terminal. The second name should contain no blanks; the last name may well contain blanks for readability.
The following entry, which describes the Concept-100, is among the more complex entries in the termcap file as of this writing.
c1|c100|concept100:is=\EU\Ef\E7\E5\E8\El\ENH\EK\E\200\Eo&\200:\ :al=3*\E^R:am:bs:cd=16*\E^C:ce=16\E^S:cl=2*^L:cm=\Ea%+ %+ :co#80:\ :dc=16\E^A:dl=3*\E^B:ei=\E\200:eo:im=\E^P:in:ip=16*:li#24:mi:nd=\E=:\ :se=\Ed\Ee:so=\ED\EE:ta=8\t:ul:up=\E;:vb=\Ek\EK:xn:
Note that entries may continue onto multiple lines by giving a \ as the last character of a line, and that empty fields may be included for readability (here between the last field on a line and the first field on the next). Capabilities in termcap are of three types: Boolean capabilities which indicate that the terminal has some particular feature, numeric capabilities giving the size of the terminal or the size of particular delays, and string capabilities, which give a sequence which can be used to perform particular terminal operations.
All capabilities have two letter codes. For instance, the fact that the Concept has ``automatic margins'' (i.e. an automatic return and linefeed when the end of a line is reached) is indicated by the capability am. Hence the description of the Concept includes am. Numeric capabilities are followed by the character `#' and then the value. Thus co which indicates the number of columns the terminal has gives the value `80' for the Concept.
Finally, string valued capabilities, such as ce (clear to end of line sequence) are given by the two character code, an `=', and then a string ending at the next following `:'. A delay in milliseconds may appear after the `=' in such a capability, and padding characters are supplied by the editor after the remainder of the string is sent to provide this delay. The delay can be either a integer, e.g. `20', or an integer followed by an `*', i.e. `3*'. A `*' indicates that the padding required is proportional to the number of lines affected by the operation, and the amount given is the per-affected-unit padding required. When a `*' is specified, it is sometimes useful to give a delay of the form `3.5' specify a delay per unit to tenths of milliseconds.
A number of escape sequences are provided in the string valued capabilities for easy encoding of characters there. A \E maps to an ESCAPE character, ^x maps to a control-x, and the sequences \n \r \t \b \f give a newline, return, tab, backspace and formfeed. Finally, characters may be given as three octal digits after a \, and the characters ^ and \ may be given as \^ and \\. If it is necessary to place a : in a capability it must be escaped in octal as \072. If it is necessary to place a null character in a string capability it must be encoded as \200. The routines which deal with termcap use C strings, and strip the high bits of the output very late so that a \200 comes out as a \000 would.
We now outline how to prepare descriptions of terminals. The most effective way to prepare a terminal description is by imitating the description of a similar terminal in termcap and to build up a description gradually, using partial descriptions with ex to check that they are correct. Be aware that a very unusual terminal may expose deficiencies in the ability of the termcap file to describe it or bugs in ex. To easily test a new terminal description you can set the environment variable TERMCAP to a pathname of a file containing the description you are working on and the editor will look there rather than in /etc/termcap. (This only works on version 7 systems.)
The number of columns on each line for the terminal is given by the co numeric capability. If the terminal is a CRT, then the number of lines on the screen is given by the li capability. If the terminal wraps around to the beginning of the next line when it reaches the right margin, then it should have the am capability. If the terminal can clear its screen, then this is given by the cl string capability. If the terminal can backspace, then it should have the bs capability, unless a backspace is accomplished by a character other than ^H (ugh) in which case you should give this character as the bc string capability. If it overstrikes (rather than clearing a position when a character is struck over) then it should have the os capability.
A very important point here is that the local cursor motions encoded in termcap are undefined at the left and top edges of a CRT terminal. The editor will never attempt to backspace around the left edge, nor will it attempt to go up locally off the top. The editor assumes that feeding off the bottom of the screen will cause the screen to scroll up, and the am capability tells whether the cursor sticks at the right edge of the screen. If the terminal has switch selectable automatic margins, the termcap file always assumes that this is on, i.e. am.
These capabilities suffice to describe hardcopy and ``glass-tty'' terminals. Thus the model 33 teletype is described as
while the Lear Siegler ADM-3 is described as
Cursor addressing in the terminal is described by a cm string capability, with printf (3s) like escapes %x in it. These substitute to encodings of the current line or column position, while other characters are passed through unchanged. If the cm string is thought of as being a function, then its arguments are the line and then the column to which motion is desired, and the % encodings have the following meanings:
%d as in printf, 0 origin %2 like %2d %3 like %3d %. like %c %+x adds x to value, then %. %<xy if value < x adds y; then in any case %. %r reverses order of line and column, no output %i increments line/column (for 1 origin) %% gives a single % %n exclusive or row and column with 0140 (DM2500)
Consider the HP2645, which, to get to row 3 and column 12, needs to be sent \E&a12c03Y padded for 6 milliseconds. Note that the order of the rows and columns is inverted here, and that the row and column are printed as two digits. Thus its cm capability is ``cm=6\E&%r%2c%2Y''. The Microterm ACT-IV needs the current row and column sent preceded by a ^T, with the row and column simply encoded in binary, ``cm=^T%.%.''. Terminals which use ``%.'' need to be able to backspace the cursor (bs or bc), and to move the cursor up one line on the screen (up introduced below). This is necessary because it is not always safe to transmit \t, \n ^D and \r, as the system may change or discard them.
A final example is the LSI ADM-3a, which uses row and column offset by a blank character, thus ``cm=\E=%+ %+ ''.
If the terminal can move the cursor one position to the right, leaving the character at the current position unchanged, then this sequence should be given as nd (non-destructive space). If it can move the cursor up a line on the screen in the same column, this should be given as up. If the terminal has no cursor addressing capability, but can home the cursor cursor (to very upper left corner of screen) then this can be given as ho; similarly a fast way of getting to the lower left hand corner can be given as ll; this may involve going up with up from the home position, but the editor will never do this itself (unless ll does) because it makes no assumption about the effect of moving up from the home position.
If the terminal can clear from the current position to the end of the line, leaving the cursor where it is, this should be given as ce. If the terminal can clear from the current position to the end of the display, then this should be given as cd. The editor only uses cd from the first column of a line.
If the terminal can open a new blank line before the line where the cursor is, this should be given as al; this is done only from the first position of a line. The cursor must then appear on the newly blank line. If the terminal can delete the line which the cursor is on, then this should be given as dl; this is done only from the first position on the line to be deleted. If the terminal can scroll the screen backwards, then this can be given as sb, but just al suffices. If the terminal can retain display memory above then the da capability should be given; if display memory can be retained below then db should be given. These let the editor understand that deleting a line on the screen may bring non-blank lines up from below or that scrolling back with sb may bring down non-blank lines.
There are two basic kinds of intelligent terminals with respect to insert/delete character which can be described using termcap. The most common insert/delete character operations affect only the characters on the current line and shift characters off the end of the line rigidly. Other terminals, such as the Concept 100 and the Perkin Elmer Owl, make a distinction between typed and untyped blanks on the screen, shifting upon an insert or delete only to an untyped blank on the screen which is either eliminated, or expanded to two untyped blanks. You can find out which kind of terminal you have by clearing the screen and then typing text separated by cursor motions. Type ``abc\ \ \ \ def'' using local cursor motions (not spaces) between the ``abc'' and the ``def''. Then position the cursor before the ``abc'' and put the terminal in insert mode. If typing characters causes the rest of the line to shift rigidly and characters to fall off the end, then your terminal does not distinguish between blanks and untyped positions. If the ``abc'' shifts over to the ``def'' which then move together around the end of the current line and onto the next as you insert, you have the second type of terminal, and should give the capability in, which stands for ``insert null''. If your terminal does something different and unusual then you may have to modify the editor to get it to use the insert mode your terminal defines. We have seen no terminals which have an insert mode not not falling into one of these two classes.
The editor can handle both terminals which have an insert mode, and terminals which send a simple sequence to open a blank position on the current line. Give as im the sequence to get into insert mode, or give it an empty value if your terminal uses a sequence to insert a blank position. Give as ei the sequence to leave insert mode (give this, with an empty value also if you gave im so). Now give as ic any sequence needed to be sent just before sending the character to be inserted. Most terminals with a true insert mode will not give ic, terminals which send a sequence to open a screen position should give it here. (Insert mode is preferable to the sequence to open a position on the screen if your terminal has both.) If post insert padding is needed, give this as a number of milliseconds in ip (a string option). Any other sequence which may need to be sent after an insert of a single character may also be given in ip.
It is occasionally necessary to move around while in insert mode to delete characters on the same line (e.g. if there is a tab after the insertion position). If your terminal allows motion while in insert mode you can give the capability mi to speed up inserting in this case. Omitting mi will affect only speed. Some terminals (notably Datamedia's) must not have mi because of the way their insert mode works.
Finally, you can specify delete mode by giving dm and ed to enter and exit delete mode, and dc to delete a single character while in delete mode.
If your terminal has sequences to enter and exit standout mode these can be given as so and se respectively. If the terminal has a way of flashing the screen to indicate an error quietly (a bell replacement) then this can be given as vb; it must not move the cursor. Finally, if the terminal should be placed in a different mode during open and visual modes of ex, this can be given as vs and ve, sent at the start and end of these modes respectively. These can be used to change, e.g., from a underline to a block cursor and back.
If your terminal correctly generates underlined characters (with no special codes needed) even though the it does not overstrike, then you should give the capability ul; If overstrikes are erasable with a blank, then this should be indicated by giving eo.
If the terminal requires other than a null (zero) character as a pad, then this can be given as pc.
If it has cursor motion keys which generate single control characters, then the ma option can be set up to map these to the normal editor functions; the map ``ma=^K^P'' maps a control-K, which moves the cursor up one line on an ADM-3A, to a control-P, which is the editor function to move the cursor up one line. Many pairs of characters may be given to ma.
If tabs on the terminal require padding, or if the terminal uses a character other than ^I to tab, then this can be given as ta.
Hazeltine terminals, which don't allow `~' characters to be printed should indicate hz. Datamedia terminals, which echo carriage-return newline for carriage return and then ignore a following newline should indicate nc. Early Concept terminals, which ignore a newline immediately after an am wrap, should indicate xn. Other specific terminal problems may be corrected by adding more capabilities of the form xx.
Other capabilities utilized only by tset (UCB) include is, an initialization string for the terminal, and if, the name of a file containing long initialization strings.
/etc/termcap file containing terminal descriptions
ex (UCB), termlib (UCB), tset (UCB), vi (UCB)
Ex allows only 128 characters for string capabilities, and the routines in termlib do not check for overflow of this buffer.
(P) indicates padding may be specified (P*) indicates that padding may be based on no. lines affected
|al||str||(P*)||Add new blank line|
|am||bool||Terminal has automatic margins|
|bc||str||Back cursor if not ^H|
|bs||bool||Terminal can backspace with ^H|
|cd||str||(P*)||Clear to end of display|
|ce||str||(P)||Clear to end of line|
|co||num||Number of columns in a line|
|da||bool||Display may be retained above|
|db||bool||Display may be retained below|
|dm||str||Delete mode (enter)|
|ed||str||End delete mode|
|ei||str||End insert mode; give ``:ei=:'' if ic|
|eo||str||Can erase overstrikes with a blank|
|ho||str||Home cursor (if no cm)|
|hz||str||Hazeltine; can't print ~'s|
|if||-||Name of file containing is|
|im||bool||Insert mode (enter); give ``:im=:'' if ic|
|in||bool||Insert mode distinguishes nulls on display|
|ip||str||(P*)||Insert pad after character inserted|
|is||str||Terminal initialization string|
|li||num||Number of lines on CRT screen|
|ll||str||Last line, first column (if no cm)|
|ma||str||Control character map for arrow keys|
|mi||bool||Safe to move while in insert mode|
|nc||bool||No correctly working carriage return (DM2500)|
|nd||str||Non-destructive space (cursor right)|
|pc||str||Pad character (rather than null)|
|se||str||End stand out mode|
|so||str||Begin stand out mode|
|sr||str||(P)||Scroll reverse (backwards)|
|ta||str||(P)||Tab (other than ^I or with padding)|
|ul||bool||Terminal underlines even though it doesn't overstrike|
|up||str||Upline (cursor up)|
|vb||str||Visible bell (may not move cursor)|
|ve||str||Sequence to end open/visual mode|
|vs||str||Sequence to start open/visual mode|
|xn||str||A newline is ignored after a wrap (Concept)|