Keyboard controller (computing)

Its main function is to inform the computer when a key is pressed or released.

If a keyboard is a separate peripheral system unit (such as in most modern desktop computers), the keyboard controller is not directly attached to the keys, but receives scancodes from a microcontroller embedded in the keyboard via some kind of serial interface.

The IBM PC AT used an Intel 8042 chip to interface to the keyboard.

[1] The latter was a problem because the BIOS and services provided by real mode operating systems such as MS-DOS and similar operating systems could only be called by programs in real mode.

These behaviors have been used by plenty of software that expects this behavior, and therefore keyboard controllers have continued controlling the A20 line and performing software CPU resets even when the need for a reset via the keyboard controller was obviated by the Intel 80386's ability to switch to real mode from protected mode without a CPU reset.