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The Termcap Library

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1.5.1 Why Pad, and How

Most types of terminal have commands that take longer to execute than they do to send over a high-speed line. For example, clearing the screen may take 20msec once the entire command is received. During that time, on a 9600 bps line, the terminal could receive about 20 additional output characters while still busy clearing the screen. Every terminal has a certain amount of buffering capacity to remember output characters that cannot be processed yet, but too many slow commands in a row can cause the buffer to fill up. Then any additional output that cannot be processed immediately will be lost.

To avoid this problem, we normally follow each display command with enough useless charaters (usually null characters) to fill up the time that the display command needs to execute. This does the job if the terminal throws away null characters without using up space in the buffer (which most terminals do). If enough padding is used, no output can ever be lost. The right amount of padding avoids loss of output without slowing down operation, since the time used to transmit padding is time that nothing else could be done.

The number of padding characters needed for an operation depends on the line speed. In fact, it is proportional to the line speed. A 9600 baud line transmits about one character per msec, so the clear screen command in the example above would need about 20 characters of padding. At 1200 baud, however, only about 3 characters of padding are needed to fill up 20msec.

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