QMK keymaps are defined inside a C source file. The data structure is an array of arrays. The outer array is a list of layer arrays while the inner layer array is a list of keys. Most keyboards define a `KEYMAP()` macro to help you create this array of arrays.
## Keymap and layers
In QMK, **`const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS]`** holds multiple **layers** of keymap information in **16 bit** data holding the **action code**. You can define **32 layers** at most.
For trivial key definitions, the higher 8 bits of the **action code** are all 0 and the lower 8 bits holds the USB HID usage code generated by the key as **keycode**.
Respective layers can be validated simultaneously. Layers are indexed with 0 to 31 and higher layer has precedence.
Keymap: 32 Layers Layer: action code matrix
----------------- ---------------------
stack of layers array_of_action_code[row][column]
____________ precedence _______________________
/ / | high / ESC / F1 / F2 / F3 ....
31 /___________// | /-----/-----/-----/-----
30 /___________// | / TAB / Q / W / E ....
29 /___________/ | /-----/-----/-----/-----
: _:_:_:_:_:__ | : /LCtrl/ A / S / D ....
: / : : : : : / | : / : : : :
2 /___________// | 2 `--------------------------
1 /___________// | 1 `--------------------------
0 /___________/ V low 0 `--------------------------
Sometimes, the action code stored in keymap may be referred as keycode in some documents due to the TMK history.
### Keymap layer status
Keymap layer has its state in two 32 bit parameters:
* **`default_layer_state`** indicates a base keymap layer(0-31) which is always valid and to be referred.
* **`layer_state`** () has current on/off status of the layer on its each bit.
Keymap has its state in two parameter **`default_layer`** indicates a base keymap layer(0-31) which is always valid and to be referred, **`keymap_stat`** is 16bit variable which has current on/off status of layers on its each bit.
Keymap layer '0' is usually `default_layer` and which is the only valid layer and other layers is initially off after boot up firmware, though, you can configured them in `config.h`.
To change `default_layer` will be useful when you switch key layout completely, say you want Colmak instead of Qwerty.
Initial state of Keymap Change base layout
----------------------- ------------------
31 31
30 30
29 29
: :
: : ____________
2 ____________ 2 / /
1 / / ,->1 /___________/
,->0 /___________/ | 0
| |
`--- default_layer = 0 `--- default_layer = 1
layer_state = 0x00000001 layer_state = 0x00000002
On the other hand, you shall change `layer_state` to overlay base layer with some layers for feature such as navigation keys, function key(F1-F12), media keys or special actions.
Overlay feature layer
--------------------- bit|status
____________ ---+------
31 / / 31 | 0
30 /___________// -----> 30 | 1
29 /___________/ -----> 29 | 1
: : | :
: ____________ : | :
2 / / 2 | 0
,->1 /___________/ -----> 1 | 1
| 0 0 | 0
| +
`--- default_layer = 1 |
layer_state = 0x60000002 <-'
### Layer Precedence and Transparency
Note that ***higher layer has higher priority on stack of layers***, namely firmware falls down from top layer to bottom to look up keycode. Once it spots keycode other than **`KC_TRNS`**(transparent) on a layer it stops searching and lower layers aren't referred.
You can place `KC_TRANS` on overlay layer changes just part of layout to fall back on lower or base layer.
Key with `KC_TRANS` (`KC_TRNS` and `_______` are the alias) doesn't has its own keycode and refers to lower valid layers for keycode, instead.
For this example we will walk through the [default Clueboard 66% keymap](https://github.com/qmk/qmk_firmware/blob/master/keyboards/clueboard_66/keymaps/default/keymap.c). You'll find it helpful to open that file in another browser window so you can look at everything in context.
These are some handy definitions we can use when building our keymap and our custom function. The `GRAVE_MODS` definition will be used later in our custom function. The `_______` define makes it easier to see what keys a layer is overriding, while the `_BL`, `_FL`, and `_CL` defines make it easier to refer to each of our layers.
### Layers and Keymaps
The main part of this file is the `keymaps[]` definition. This is where you list your layers and the contents of those layers. This part of the file begins with this definition:
After this you'll find a list of KEYMAP() macros. A KEYMAP() is simply a list of keys to define a single layer. Typically you'll have one or more "base layers" (such as QWERTY, Dvorak, or Colemak) and then you'll layer on top of that one or more "function" layers. Due to the way layers are processed you can't overlay a "lower" layer on top of a "higher" layer.
`keymaps[][MATRIX_ROWS][MATRIX_COLS]` in QMK holds the 16 bit action code (sometimes referred as the quantum keycode) in it. For the keycode representing typical keys, its high byte is 0 and its low byte is the USB HID usage ID for keyboard.
> TMK from which QMK was forked uses `const uint8_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS]` instead and holds the 8 bit keycode. Some keycode values are reserved to induce execution of certain action codes via the `fn_actions[]` array.
* From a C source point of view it's only a single array, but we have embedded whitespace to more easily visualize where each key is on the physical device.
* Plain keyboard scancodes are prefixed with KC_, while "special" keys are not.
* The upper left key activates custom function 0 (`F(0)`)
* The "Fn" key is defined with `MO(_FL)`, which moves to the `_FL` layer while that key is being held down.
#### Function Overlay Layer
Our function layer is, from a code point of view, no different from the base layer. Conceptually, however, you will build that layer as an overlay, not a replacement. For many people this distinction does not matter, but as you build more complicated layering setups it matters more and more.
* We have used our `_______` definition to turn `KC_TRNS` into `_______`. This makes it easier to spot the keys that have changed on this layer.
* While in this layer if you press one of the `_______` keys it will activate the key in the next lowest active layer.
### Custom Functions
At the bottom of the file we've defined a single custom function. This function defines a key that sends `KC_ESC` when pressed without modifiers and `KC_GRAVE` when modifiers are held. There are a couple pieces that need to be in place for this to work, and we will go over both of them.
#### `fn_actions[]`
We define the `fn_actions[]` array to point to custom functions. `F(N)` in a keymap will call element N of that array. For the Clueboard's that looks like this:
In this case we've instructed QMK to call the `ACTION_FUNCTION` callback, which we will define in the next section.
> This `fn_actions[]` interface is mostly for backward compatibility. In QMK, you don't need to use `fn_actions[]`. You can directly use `ACTION_FUNCTION(N)` or any other action code value itself normally generated by the macro in `keymaps[][MATRIX_ROWS][MATRIX_COLS]`. N in `F(N)` can only be 0 to 31. Use of the action code directly in `keymaps` unlocks this limitation.
#### `action_function()`
To actually handle the keypress event we define an `action_function()`. This function will be called when the key is pressed, and then again when the key is released. We have to handle both situations within our code, as well as determining whether to send/release `KC_ESC` or `KC_GRAVE`.
We are actively working to improve these docs. If you have suggestions for how they could be made better please [file an issue](https://github.com/qmk/qmk_firmware/issues/new)!
