Terminal Emulation - What Is It?
Years ago, before the PC revolution, people used terminals to interact
directly with computers (they also submitted batch jobs on decks of punched
cards or rolls of paper tape, but that's another story). A terminal has a
keyboard, a display, and a communication port. The earliest terminals were
Teletypes (in wide use since the late 1920s), a kind of communicating
typewriter. (What's a typewriter?) By the mid-1970s, most terminals
had video displays (like a PC monitor) instead of printing on paper.
Typically many terminals were connected to one large computer, which all the
users shared; this was called timesharing. Well-known timesharing systems
included MULTICS, UNIX, RSTS/E, TOPS-10, TENEX, TOPS-20, AOS/VS, VMS, VOS,
VM/CMS, and TSO; some of these are long gone, others are still popular today.
Video terminals did more than simply display incoming letters, digits,
and punctuation, as a typewriter would. They also included formatting
capabilities allowing for host-directed cursor positioning, erasing,
highlighting, printing, coloration, forms-filling, and so forth. The
formatting was accomplished by embedding escape
sequences in the data
stream. Of course, every kind of terminal used different different escape
sequences, and over the years escape sequences became more powerful and
Meanwhile, special keys began to appear on terminal keyboards: arrow
keys, function keys, editing keys, and so on. Unlike the regular typewriter
keys (letters, digits, punctuation), these keys did not have a one-to-one
correspondence to the ASCII character set,
which does not include arrows, functions, and the like, so each terminal
vendor assigned differing and incompatible escape sequences to them.
The phrase terminal emulation applies to software on a personal
computer that does what a specific real terminal would do:
Point 1 is important because very few terminals have the same keyboard as
a PC. The number of keys might be different, the keys might be labeled
differently, and keys that are labeled the same way might have different
placement. For this reason, no single mapping (of say, VT220 to PC) will
please everybody. Thus most terminal emulators give you a way to
change the mapping to suit your preferences.
- It maps terminal keys to PC keys.
- When you press a key, it sends what the corresponding terminal key would
- It displays incoming characters on the screen.
- It responds to incoming escape sequences as the real terminal would.
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