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Can alphanumeric terminals and desktop PCs from the 1980s and early 1990s handle the features of today's full-screen applications? Not of Mosaic and the like, of course; just modern full-screen alphanumeric interfaces that come with programs like the Lynx Web browser and various front ends to Unix. Experience seems to say ``No; we've tried using them and they can't keep up''. But most often the limitations are not in the terminal or the terminal section of the desktop system; they're in poorly- done configuration files and databases, especially Termcap and Terminfo entries.
Termcap and the later Terminfo started out as a great concept, even a necessity. When display terminals moved beyond the ``glass TTY'' stage, more control of the screen was needed than could be provided by the 32 ASCII control characters, and every new terminal seemed to have a different set of escape control sequences for the additional functions. To let a full-screen program work with more than one or a few models of terminals, it was necessary to create a database of different terminals' idiosyncratic control sequences, so a program could look up what it needed to know about whatever terminal it was connected to. But these databases have been shakey from the start.
Back then it was often the case that, while starry-eyed hardware engineers were building terminals with capabilities galore, the software department was more pessimistic. Someone in software would say, "Our customers don't need half those features --don't bother to write Termcap items for the stuff they won't use because it would only get them in trouble". That's if we were lucky--otherwise the terminal manufacturer wasn't concerned with Unix and released his product with no Termcap entry at all. Then one of the first Unix-using buyers would cobble up a bare-bones Termcap entry to meet personal needs, start passing it around among friends, eventually post it on the net--and so a miserably inadequate but quasi-official standard was born.
When a terminal's performance is inadequate or erratic on full-screen programs, a little detective work starting with Termcap or Terminfo will very often set things right, and having no documentation for the terminal is usually not a fatal obstacle.
A good example of all this was my recent effort to link some vintage desktop systems to the Internet.
The desktop systems were AT&T PC7300s, with small monochrome screens and little more than alphanumeric capabilities. They had been purchased at a closeout sale, without manuals, and AT&T said it had no PC7300 manuals to sell. Nonetheless, with intuition and knowledge of Berkeley Unix we'd been able to do everything we wanted to with them, up to the present.
But now I wanted to connect them to the Net, via shell accounts at an access provider running Sun servers. At first we couldn't even log into the system. As soon as the menu front end came up, it printed the start of an error message and disconnected the line. A system administrator there finally gave me direct access to command-line Unix, after which I could log in, at least. Both PC7300s and Sun servers are based on Berkeley Unix, which uses Termcap, but a look around showed that these Suns had no entry for the PC7300 in their Termcap files. This was a surprise--most vendors use the Berkeley-standard Termcap file for all but their own terminals, and the PC7300 has been in the standard file for years. I uploaded the Termcap entry the PC7300s use internally, the system administrator installed it, and then we could get into full-screen programs, including that menu front end.
But running those full-screen programs generated problems. There was some strange behavior, mostly involving character attributes such as reverse video and underlining. Ominously, this strangeness was not consistent: various surprises cropped up while running different programs.
The worst problem was with Lynx, a Web browser designed for alphanumeric terminals. A Web document may have several phrases the user can click on within a single page. Because Lynx is intended for systems without mouse pointers, it is supposed to use one attribute mode to display the phrase that would be activated by pressing the return key now, and another attribute to show the phrases the user could move to with the up and down arrow keys. With our PC7300s, unfortunately, Lynx usually displayed all the phrases in the same mode: reverse video.
Discouraging? Actually, the problem was halfway to being diagnosed when I thought over that symptom list. When character attributes malfunction differently in different programs, the problem is almost certain to be missing information in the Termcap or Terminfo entry. Programs that fail to find an entry telling how to produce one character attribute mode must default to another mode or skip the attribute altogether. Each programmer makes his own decision about these substitutions, so each program defaults in its own way. The Lynx problem just as certainly indicated that character attribute information was present but not correct. No sane program replaces a missing attribute with one it already uses for another purpose on the same screen.
The first thing to do was make a working copy of the PC7300
Termcap entry, reproduced below:
s4|PC7300|unixpc|pc7300|3b1|Safari 4:\ :md=\E[1m:me=\E[0m:\ :al=\E[1L:am:bs:\ :cd=\E[0J:ce=\E[0K:cl=\E[2J\E[H:cm=\E[%i%2;%2H:co#80:\ :dc=\E[1P:dl=\E[1M:do=\E[B:ei=:ho=\E[H:\ :ic=\E[1\@:im=:kb=\10:kd=\E[B:kl=\E[D:kr=\E[C:ku=\E[A:li#24:\ :k1=\EOc:k2=\EOd:k3=\EOe:k4=\EOf:k5=\EOg:k6=\EOh:k7=\EOi:k8=\EOj:\ :nd=\E[C:se=\E[m:so=\E[7m:ue=\E[m:up=\E[A:us=\E[4m:\ :EE=\E[m:BO=\E[1m:DS=\E[2m:XS=\E[9m:CV=\E[=C:CI=\E[=1C:\ :KM=/usr/lib/ua/kmap.s4:
That's not quite as cryptic as it looks. The items between
the vertical bar symbols (
|) are alternative names
for the PC7300 (and for related machines that use exactly the
same codes). Between the colons you'll find terse descriptions of
what the PC7300 can do and the character strings needed to drive
A pound sign (
#) in the middle of an item
indicates a numeric value, as in
co#80, which tells
the calling program that the screen holds 80 columns.
An equal sign shows the character string needed to produce an
effect. For instance, in
cd= directive indicates that what follows will, when
sent to the terminal, clear everything the screen from the cursor
\E represents the ASCII escape character
(a control-[), and a backslash (
\) followed by two
or three digits represents in octal the value of the ASCII
character to be placed there.
An item with neither a
# or an
within it states that the terminal has a certain capability. For
am says that when a line of characters is
about to overflow the right edge of the screen the terminal will
wrap it around to the next line.
Comparing lines 2 through 8 of the Termcap entry above to my
showed that there were items giving the strings needed
to turn on underlining (
md=\E[1m), and whatever character attribute the
configurer regarded as particularly distinctive, called standout mode
so=\E[7m); but no other character attributes. Whatever
this Termcap entry's standout mode might be, this collection of
attributes is not enough for all full-screen programs, which could
explain why some of them were not painting screens correctly.
The last two lines contained items from a Termcap extension called Tam, used almost exclusively on AT&T systems for drawing windows and forms. One item gave again the string that starts extra-bright characters; two others enabled dim and overstruck characters, respectively. But most programs cannot read Tam capabilities.
The next question was whether the additional character attributes were missing from the PC7300's capabilities, or only from the Termcap entry. I had no PC7300 manual to consult, but I did notice that the attribute-on strings I'd found in the Termcap entry were identical except for one numeric digit in each string. Might it be that substituting other digits would bring out other attributes?
To test this idea I wrote a single-line file that read
abc^[[9mdefg^[[0mhij in full. By sending contents
of this file to the screen (with
cat), I would get
the first ten letters of the alphabet displayed, with the letters
"defg" in whatever attribute style the numeral 9 produces, if
any. (Note that since
cat does not use Termcap, I
\E with the actual ASCII escape
character, control-[; and that I followed "defg" with the string
said to turn attributes off again, so I wouldn't leave my
terminal locked in an attribute mode.) I tested all ten numerals
this way, with mixed results. No new attributes turned up, but
all the strings Termcap and/or Tam gave as attribute switches
worked as promised (standout mode turned out to be just reverse
One of the items in the Tam section,
gives the string that turns all attributes off at once, also
implies that any or all of the character attributes may be
combined to yield still other attribute modes. I tested this
implication by putting multiple attribute-on strings in my file
to be displayed and the combinations all functioned. Even the
unlikely cases proved workable: all five attributes at once
produced a distinct though unattractive effect; and combining
extra-bright with dim, which too many programmers assume is a
no-op, created one of the nicest typefaces of all.
Next I had to make use of all this new information. I started by writing a revised Termcap entry in which every attribute I'd found got an item in both the standard and Tam vocabularies. (As far as this was possible; there is no standard Termcap item for the overstruck attribute, for instance.) Not that we could expect to encounter Tam-using programs on a Sun server, but this would let us have identical Termcap entries on the Sun servers and our own PC7300s, which keeps things simple.
Putting new items into the existing entry only required inserting each of them in one of the lines, with a colon before and after each new item. The order of the items in a Termcap entry is not significant in operation, and human readability was already hopeless in this entry. To keep the new Tam items with the existing ones I created a new line, which required putting 8 spaces at the start of the line and escaping the ending newline with a backslash.
That left the problem of blinking mode. Some programs use
blinking text, and the PC7300 has no way to blink anything but
the cursor. In the end I resolved this by setting the Termcap
blink item to a string which turns on a combination of reverse
video and dim--this produces reverse video with a screened
background. It doesn't blink, of course, but it's visibly
different from any of the other character attributes. To remind
users that this mode is what they should look for when a program
displays messages about ``the blinking text'', I wrote a text
file that held just the single line ``Text in this typeface
should be regarded as blinking''--with the words ``this
typeface'' surrounded by the codes that would display them in my
pseudo blink mode--and I put a line in everyone's C shell
initialization file (
.login) that would display this
new file at login.
With reverse video declared as a mode of its own within my revised Termcap entry, having standout mode also be reverse video was redundant. Someday we might come across a program that used both those attributes, and there would be no way for users to tell the standout-highlighted characters from the ones using reverse video under its own identity. So I revised the entry again, to make reverse video combined with extra-bright the new standout mode.
Although I had less to go on in tracing the other minor
problems--most often a few stray characters here and there--I
decided to make a cursory check of the Termcap entry for possible
causes. The entry's assertions about what the PC7300 does--
automatically wraps overlength lines, backspaces, has 80 columns
and 24 lines--I knew to be correct. I knew nothing about the
strings to move the cursor, erase all or part of the screen,
etcetera, but those are easy to test. I only needed to put the
string alleged to perform a task into a file (using actual escape
characters instead of Termcap representations), then send the
contents of the file to the screen (with
cat) to see
that the action was as expected. (In a few cases I would have to
use more than one such string in a file to run a test accurately.
For instance, the string to erase from the cursor to the end of
the screen. When working from the command line, the part of the
screen following the cursor is normally empty, so it's necessary
to precede the erase-to-end string with another string that moves
the cursor up into the already-written-on screen area in order to
see something get erased.) I tested a few of the likely-suspect
strings this way, but found nothing incorrect.
The last question was how to put my revised Termcap entry into
effect on the Sun servers. The
generally can't be modified without the superuser password. System
administrators usually don't want to put an experimental entry
like this into the main Termcap file, nor did I want to have to
hunt up a system administrator every time I needed to make
another little Termcap change. But there are other places where
a Termcap entry can live. It's possible to set a
TERMCAP variable within a user's environment to
override the default selection of
the place where programs look for a Termcap entry. This variable
can be set from the command line, or set automatically at each
login by putting the appropriate definition line in the user's
shell initialization file (
.login for C shell and
.profile for Bourne and Korn shells).
TERMCAP variable is set to a string that
does not start with a
/ mark, then the string itself
is taken as the Termcap entry. A possibility, but rather
cumbersome when the entry is subject to continuing experiments
and changes. But if the variable string does begin with a
/ mark, it's taken as the full path name of a file to
be used in place of the
/etc/termcap file. That's
the convenient place for a Termcap entry that's likely to be
tested and modified from time to time, which is why I chose
What I'd done so far should have cleared up most of the
character attribute problems. Testing showed that it did--Hallelujah!
No new problems had cropped up along with the improvements, so I
set all our users'
.login files to use my revised
Termcap entry, and downloaded it to our PC7300s so we could try
it there, too.
But I'd found nothing that could be causing the Lynx attribute confusion or the other miscellaneous problems not involving attributes, so I wasn't disappointed when I didn't see any improvement there. Next time I'll explain how I cracked those problems.
If you think you just need the
man pages to fathom
Termcap, but they're not online on the system you're using right now,
the place to find them is the 4.4BSD Berkeley Software
Distribution book series, especially the Programmer's
Reverence Manual volume. These are published jointly by the
Usenix Association and O'Reilly & Associates, and should be
around any site that uses Berkeley Unix.
But if you are about to buy Termcap documentation, consider instead
termcap & terminfo, a book by John Strang, Linda Mui
and Tim O'Reilly that is published by O'Reilly & Associates. This
book explains both the systems named in its title without the terseness
man pages. A third of the pages overview the two systems
and offer strategies for using them. Most of the rest is discussion of
individual capabilities, grouped into chapters with related items.
The finale is an alphabetical guide to capability descriptors,
cross-referenced between Termcap and Terminfo. (I only wish the
references to text pages were more direct.) The entire book is
pretty lucid and pretty accurate. It's paperbound, has 253 pages,
and is priced at $21.95.
My other caveat regarding these books is that they don't even
mention ad hoc Termcap extensions such as Tam. You may
not need to know about Tam, but if you do you'll have to find the
man pages for it, or pay close attention to
the summary of Tam capability descriptors attached to this
You can order O'Reilly books directly from the publisher at:
(Return to main tutorial)
The Tam capability items live inside the standard Termcap entry,
usually on a few lines by themselves at the end of the entry.
But they're used only by programs that understand
tam(3T) system, which is primarily for
drawing windows and forms on alphanumeric terminal screens.
There are just a dozen Tam items for termcap entries, and all of
them are string variables. They're expressed in standard Termcap
style: first a two-letter code to indicate what capability is being
given, then an equal
= sign, finally the string that
the program should send to the terminal to perform the action.
Most of the items deal with character attributes. Tam uses
BO= to turn on extra-bright characters,
DS= for dim characters and
overstruck characters. The corresponding items to turn these
attributes off are
XE=, respectively. But you may not need those
last three items. If there is one string that will turn off any
and all attributes, and if (very important) any or all character
attributes can be combined on this terminal without restriction,
EE=to give the string that turns off
everything and skip the individual attribute-off items.
Tam's use of these attribute items interacts in complex ways
with the standard Termcap items
ug#. Be sure that any of these
latter which have values are set correctly.
CI= gives the string that turns the cursor
invisible on the screen;
CV= tells how to make it
visible again. If the terminal you're describing has hardware
SLK keys, the string following
FL= should put a label on
a key and
FE= should turn off the SLK labels. The
string with the
FL= item must be a
format string with provision for two runtime arguments: the number
of the key to be labeled and the text which will be the label.
KM= item gives the full path name of a
file which tells what strings the various function keys send.
Customarily, such files are in the directory
and have names that start with
kmap. and end with the usual
short name for the terminal being described.
(Return to main tutorial)
June 10, 1995
Dear Mr. Zintz:
I saw and enjoyed your article ``Teaching Old Terminals New Tricks'' in UnixWorld Online, but as a former AT&T UNIXpc enthusiast (I still have several of the poor dead things lying around my house), I feel I must object to the characterization:
``The desktop systems were AT&T PC7300s, with small monochrome screens and little more than alphanumeric capabilities.''
Well, to be honest, I can't argue with the ``small monochrome screens'', but I will take issue with the latter part. The UNIXpc had full bitmap graphics, a mouse, and a desktop windowing environment with the stock OS. Also, MGR, the Bellcore window manager--now enjoying a small resurgence in popularity with some Linux users--was available and widely used. Actually, I prefer MGR to the X11 desktop I'm using right now; unfortunately, MGR never really took off, and so lacks clients, and other support that have made X popular.
Anyway, while the UNIXpc is now well past its prime, it was at one time fairly advanced, quite useful, and certainly had more than ``little more than alphanumeric capabilities.''John R. MacMillan / Toronto, Ontario, CANADA
Last Modified: Wednesday, 30-Aug-95 08:44:52 PDT