From XON/XOFF to Forward Incremental Search
In the olden days of computing, software flow control with control codes XON and XOFF was a necessary feature that dumb terminals needed to support. When a terminal received more data than it could display, there needed to be a way for the terminal to tell the remote host to pause sending more data. The control code 19 was chosen for this. The control code 17 was chosen to tell the remote host to resume transmission of data.
The control code 19 is called Device Control 3 (DC3) in the ASCII chart. It is also known as "transmit off" (XOFF). The control code 17 is called Device Control 1 (DC1) as well as "transmit on" (XON). Now how does a user of the terminal really send these control codes? Well, how do they send any control code? Using the ctrl key of course.
Let us take a step back and see how a user can send familiar control codes on modern terminal emulators, like say, the terminal software we find on a Unix or Linux desktop environment. While any sane computer user would just type the tab key to insert a tab character, one could also type ctrl+i to insert a tab character. The character I has code 73 (binary 1001001) and holding the ctrl key while typing it results in a control code made by taking 73 (binary 1001001), keeping its five least significant bits, and discarding the rest to get the control code 9 (binary 1001) which is the code of the tab character. In other words, we get the control code by performing a bitwise AND operation on the modified character with binary code of 31 (binary 0011111).
