How Does a Keystroke Travel From Keyboard to Your Screen?

Hardware detection

A keyboard controller continuously scans the key matrix. When a key is pressed it generates a scan code that identifies the physical key.

Interrupt generation

The controller asserts an interrupt request (IRQ) line to the CPU, signalling that a new scan code is available.

CPU interrupt handling

The CPU saves the current execution context and jumps to the interrupt vector defined by the operating system, typically a routine such as do_irq().

Keyboard driver processing

The interrupt handler recognises the source as the keyboard controller and invokes the keyboard driver. The driver translates the raw scan code into a key symbol (e.g., the letter “a”) using a keymap table.

Device file interface (Unix‑like systems)

The driver writes the resulting key event to an input device node such as /dev/input/eventX. This node behaves like a regular file: user‑space programs can read from it to obtain key events.

From kernel to X Window System

The X server monitors the appropriate /dev/input node, extracts key events, and forwards them to the X client (the graphical application) that currently has input focus. The forwarding uses inter‑process communication (IPC) mechanisms such as X protocol over a Unix domain socket.

Application receives the key event

The X client receives the IPC message, interprets the key symbol, and renders the corresponding character on the screen.