When a computer's interrupt handler receives a scancode from the keyboard, a series of processes are executed to transform this low-level input into actions that can be recognized by the operating system and applications. This is a crucial part of the input handling process. In this detailed outline, we will explore each step and the layers that make this transformation possible.

Introduction to Interrupt Handlers and Scan Codes

When a key on the keyboard is pressed or released, a low-level interrupt is triggered that sends a scancode to the interrupt handler. A scancode is a unique identifier assigned to each physical key on the keyboard. This scancode is essentially a raw, hardware-level input that requires further processing before it can be used by the software.

Logical Key Code Mapping

After the interrupt handler receives a scancode, the next step is to map this scancode to a logical key code. Logical key codes are higher-level representations of the key that have meaning in the context of the keyboard layout and language settings. For example, a scancode for the letter "A" might be mapped to KEY_A_UP if the key is released, or KEY_A_DOWN if the key is pressed. This mapping deals with the key's physical characteristics and visual representation on the keyboard.

Processing of Logical Key Codes

Once the logical key code is determined, the next step is to process it into a sequence of key events. This involves recognizing key combinations, such as a sequence like:

These key events might be further processed into a compound event, such as:

This layer also deals with special keys like SHIFT, ALT, and CTRL. It also manages the function of CAPS LOCK by keeping track of the state of this key and adjusting the input accordingly.

Event Collapsing for National and Multimedia Keys

Some national and multimedia keys require special handling. For instance, certain combinations of keys on a Polish keyboard might be represented as a single key event. For example, the combination of ALTGR (AltGr) and A might be mapped to P8 on a Polish keyboard, which is represented as {AOGONEK}. This layer also deals with multimedia keys, such as the Print Screen key, ensuring that these special functions are properly recognized and handled.

Event Dispatching

Finally, the processed key events are dispatched to the relevant programs and services. The operating system, as well as different applications running on the system, receive these events and interpret them to perform the appropriate actions, such as changing text in a document, navigating a web page, or launching a specific application.

Understanding the detailed process of how a scancode is transformed into meaningful key events is crucial for anyone interested in the intricacies of computer input handling. This knowledge can be particularly valuable for software developers, system administrators, and anyone involved in creating or optimizing input handling for keyboard-based applications.