The Linux keyboard driver
When you press a key on the console keyboard, the corresponding character is not simply added to the tty (generic terminal handling) input buffers as if it had come in over a serial port. A lot of processing is required before the kernel knows what the correct characters are. Only after processing can the generic tty code, which handles all interactive terminal devices, take over.
Roughly speaking, the picture is this: the keyboard produces scancodes, the scancodes are assembled into keycodes (one unique code for each key), and keycodes are converted to tty input characters using the kernel keymaps. After that, the normal `stty' processing takes place, just as for any other terminal.
The usual PC keyboards are capable of producing three sets of scancodes. Writing 0xf0 followed by 1, 2 or 3 to port 0x60 will put the keyboard in scancode mode 1, 2 or 3. Writing 0xf0 followed by 0 queries the mode, resulting in a scancode byte 0x43, 0x41 or 0x3f from the keyboard. (Don't try this at home, kids. If you are not very careful, you will end up in a situation where rebooting is the only way out—and control-alt-delete will not be available to shut the computer down correctly. See the accompanying listing of kbd_cmd.c for details.)
Scancode mode 2 is the default. In this mode, a key press usually produces a value s in the range 0x01-0x5f and the corresponding key release produces s+0x80. In scancode mode 3, the only key releases that produce a scan code are of both Shift keys, and the left Ctrl and Alt keys; for all other keys only the key presses are noted. The produced scancodes are mostly equal to those for scancode mode 2.
In scancode mode 1 most key releases produce the same values as in scancode mode 2, but for key presses there are entirely different, unrelated values. The details are somewhat messy.
A program can request the raw scancodes by
ioctl(0, KDSKBMODE, K_RAW);
For example, X, dosemu, svgadoom, and showkey -s do this. The default keycode translation mode is restored by
ioctl(0, KDSKBMODE, K_XLATE);
See the keyboard FAQ (in kbd-0.90.tar.gz) for some advice about how to get out of raw scancode mode from the shell prompt. (At a shell prompt the commands kbd_mode [-s|-k|-a|-u] will set the keyboard mode to scancode mode, keycode mode, translation (`ASCII') mode and Unicode mode, respectively. But it is difficult to type this command when the keyboard is in raw scancode mode.)
Life would have been easy had there been a 1-1 correspondence between keys and scancodes. (And in fact there is, in scancode mode 3, but that does not suffice for Linux, since X requires both the key press and the key release events.)
But as it is, a single key press can produce a sequence of up to six scancodes, and the kernel has to parse the stream of scancodes and convert it into a series of key press and key release events. To this end, each key is provided with a unique keycode k in the range 1-127, and pressing key k produces keycode k, while releasing it produces keycode k+128. The assignment of key codes is in principle arbitrary (and has no relation to the key codes used by X), but at present the key code equals the scan code for those keys that produce a single scancode in the range 0x01-0x58.
The parsing works by
recognizing the special sequence 0xe1 0x1d 0x45 0xe1 0x9d 0xc5 produced by the Pause key
throwing out any fake Shift-down and Shift-up codes, inserted by the keyboard to make the kernel believe that you pressed Shift to undo the effect of NumLock
recognizing scancode pairs 0xe0 s
recognizing single scancodes s.
Since s can take 127 values (0 is a keyboard error condition, and the high order bit indicates press/release) this means that parsing could result in 1+127+126=254 distinct keycodes. However, at present keycodes are restricted to the range 1-127 and we have to work a little to make things fit. (No doubt sooner or later keycodes will be integers instead of 7-bit quantities, and the keymaps will be sparse, but for the time being we can avoid that—since to my knowledge no actual PC keyboard has more than 127 keys.) So, there are small tables that assign a keycode to a scancode pair 0xe0 s or to a single scancode in the range 0x59-0x7f. In the default setting everything works for most current keyboards, but in case you have some strange keyboard, you can get the kernel to recognize an otherwise unrecognized key by filling an entry in these tables using the KDSETKEYCODE ioctl; see setkeycodes(8).
Two keys are unusual in the sense that their keycode is not constant, but depends on modifiers. The PrintScrn key will yield keycode 84 when combined with either Alt key, but keycode 99 otherwise. The Pause key will yield keycode 101 when combined with either Ctrl key, but keycode 119 otherwise. (This has historic reasons, but might change, to free keycodes 99 and 119 for other purposes.)
At present there is no way to tell X about strange key(board)s. The easiest solution would be to make X use keycodes instead of scancodes, so that the information about strange keys and the scancodes they produce is located a single place.
A program can request to get keycodes by doing
ioctl(0, KDSKBMODE, K_MEDIUMRAW);
For example, showkey does this. Warning: the details of the function of both the KDSETKEYCODE ioctl and the K_MEDIUMRAW keyboard mode are likely to change in the future.