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The earliest interactive computers used teletypewriters. Setups such as the Dartmouth time-sharing system (dating from the early 1960s) that I described in the preceding chapter supported multiple teletypewriters that could be used at the same time. In such a system, a user types a line at the teletypewriter, and the computer replies with one or more lines in response. The exchange of information between teletypewriter and computer consists entirely of streams of ASCII (or another character set), which are almost entirely character codes with some simple control codes, such as the carriage return and linefeed. The transaction proceeds only in one direction down the roll of paper.

The cathode-ray tube (which became more common during the 1970s) shouldn't have such restrictions, however. Software can instead treat the entire screen in a more flexible manner—as a two-dimensional platform for information. Yet, possibly in an attempt to keep the display output logic of an operating system generalized, much early software written for small computers continued to treat the CRT as a "glass teletypewriter"—displaying output line by line going down the screen and scrolling the contents of the screen up when the text reached the bottom. All the utilities in CP/M and most utilities in MS-DOS used the video display in a teletypewriter mode. Perhaps the archetypal teletypewriter operating system is UNIX, which still proudly upholds that tradition.

Interestingly enough, the ASCII character set isn't entirely inadequate in dealing with the cathode-ray tube. When ASCII was originally designed, the code 1Bh was labeled Escape and was specifically intended for handling extensions of the character set. In 1979, the American National Standards Institute (ANSI) published a standard entitled "Additional Controls for Use with American National Standard Code for Information Interchange." The purpose of this standard was "to accommodate the foreseeable needs for input/output control of two-dimensional character-imaging devices, including interactive terminals of both the cathode ray tube and printer types…"

Of course, the Escape code 1Bh is just 1 byte and can mean only one thing. The Escape code works by prefacing variable-length sequences that perform a variety of functions. For example, the sequence

1Bh 5Bh 32h 4Ah

which is the Escape code followed by the characters [2J, is defined to erase the entire screen and move the cursor to the upper left corner. This isn't something that can be done on a teletypewriter. The sequence

1Bh 5Bh 35h 3Bh 32h 39h 48h

which is the Escape code followed by the characters [5;29H, moves the cursor to row 5 and column 29.

A combined keyboard and CRT that responds to ASCII codes (and possibly to a collection of Escape sequences) coming from a remote computer is sometimes called a dumb terminal. Such terminals are faster than teletypewriters and somewhat more flexible, but they're not quite fast enough for real innovations in the user interface. Such innovations came with small computers in the 1970s that—like the hypothetical computer we built in Chapter 21—included the video display memory as part of the microprocessor's address space.

The first indication that home computers were going to be much different from their larger and more expensive cousins was probably the application VisiCalc. Designed and programmed by Dan Bricklin (born 1951) and Bob Frankston (born 1949) and introduced in 1979 for the Apple II, VisiCalc used the screen to give the user a two-dimensional view of a spreadsheet. Prior to VisiCalc, a spreadsheet (or worksheet) was a piece of paper with rows and columns generally used for doing series of calculations. VisiCalc replaced the paper with the video display, allowing the user to move around the spreadsheet, enter numbers and formulas, and recalculate everything after a change.

What was amazing about VisiCalc is that it was an application that could not be duplicated on larger computers. A program such as VisiCalc needs to update the screen very