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# Understanding QMK's Code

This document attempts to explain how the QMK firmware works from a very high level. It assumes you understand basic programming concepts but does not (except where needed to demonstrate) assume familiarity with C. It assumes that you have a basic understanding of the following documents:

* [Introduction](getting_started_introduction.md)
* [How Keyboards Work](how_keyboards_work.md)
* [FAQ](faq.md)

## Startup

You can think of QMK as no different from any other computer program. It is started, performs its tasks, and then ends. The entry point for the program is the `main()` function, just like it is on any other C program. However, for a newcomer to QMK it can be confusing because the `main()` function appears in multiple places, and it can be hard to tell which one to look at.

The reason for this is the different platforms that QMK supports. The most common platform is `lufa`, which runs on AVR processors such at the atmega32u4. We also support `chibios` and `vusb`.

We'll focus on AVR processors for the moment, which use the `lufa` platform. You can find the `main()` function in [tmk_core/protocol/lufa/lufa.c](https://github.com/qmk/qmk_firmware/blob/661ca4440cc42f3b60697e98985c44b0571ccfc1/tmk_core/protocol/lufa/lufa.c#L1019). If you browse through that function you'll find that it initializes any hardware that has been configured (including USB to the host) and then it starts the core part of the program with a [`while(1)`](https://github.com/qmk/qmk_firmware/blob/661ca4440cc42f3b60697e98985c44b0571ccfc1/tmk_core/protocol/lufa/lufa.c#L1060). This is [The Main Loop](#the-main-loop).

## The Main Loop

This section of code is called "The Main Loop" because it's responsible for looping over the same set of instructions forever. This is where QMK dispatches out to the functions responsible for making the keyboard do everything it is supposed to do. At first glance it can look like a lot of functionality but most of the time the code will be disabled by `#define`'s.

```
    keyboard_task();
```

This is where all the keyboard specific functionality is dispatched. The source code for `keyboard_task()` can be found in [tmk_core/common/keyboard.c](https://github.com/qmk/qmk_firmware/blob/661ca4440cc42f3b60697e98985c44b0571ccfc1/tmk_core/common/keyboard.c#L206), and it is responsible for detecting changes in the matrix and turning status LED's on and off.