The TTY Demystified

Summary

Beware, though: What you are about to see is not particularly elegant. In fact, the TTY subsystem — while quite functional from a user's point of view — is a twisty little mess of special cases. To understand how this came to be, we have to go back in time. Meanwhile, however, the computers — still quite large and primitive, but able to multitask — were becoming powerful enough to be able to interact with users in realtime. When the command line eventually replaced the old batch processing model, teletypes were used as input and output devices, because they were readily available on the market. There was a plethora of teletype models around, all slightly different, so some kind of software compatibility layer was called for. In the UNIX world, the approach was to let the operating system kernel handle all the low-level details, such as word length, baud rate, flow control, parity, control codes for rudimentary line editing and so on. Fancy cursor movements, colour output and other advanced features made possible in the late 1970s by solid state video terminals such as the VT-100, were left to the applications. In present time, we find ourselves in a world where physical teletypes and video terminals are practically extinct. Unless you visit a museum or a hardware enthusiast, all the TTYs you're likely to see will be emulated video terminals — software simulations of the real thing. But as we shall see, the legacy from the old cast-iron beasts is still lurking beneath the surface. A user types at a terminal (a physical teletype). This terminal is connected through a pair of wires to a UART (Universal Asynchronous Receiver and Transmitter) on the computer. The operating system contains a UART driver which manages the physical transmission of bytes, including parity checks and flow control. In a naïve system, the UART driver would then deliver the incoming bytes directly to some application process. But such an approach would lack the following essential features: Inci...