Things Every Hacker Once Knew
ESR forgot to give the reason for XON/XOFF: physical terminals often couldn't keep up with an output stream even at a low 9600 baud, so they'd have to back-pressure the sender (usually a dial-up or direct-connect host) to let them know when to stop and when to start.Plus, people used them manually (control-S/control-Q) on systems to stop output scrolling by, and restart it when they've read what's on the screen, before built-in pagination filters (e.g., more(1) or less(1)) became common. (Specially back in the DECsystem-10/-20 days.)