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 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 <-'
### 0.2 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.
Keymap in this QMK is **`static const uint16_t PROGMEM keymaps[]`** C array in fact and you can define layers in it with **`KEYMAP()`** C macro and keycodes. To use complex actions you need to define `Fn` keycode in **`fn_actions[]`** array. It holds the 16 bit quantum keycode (action code).
> Please note that keymap in the TMK, which QMK was forked from, is **`static const uint8_t PROGMEM keymaps[]`** C array which holds the 8 bit keycode (~USB HID usage code).
See [`tmk_core/common/keycode.h`](../tmk_core/common/keycode.h) or keycode table below for the detail. Keycode is internal **8bit code** to indicate action performed on key in keymap. Keycodes are based on [HID Usage Keyboard/Keypad Page(0x07)](http://www.usb.org/developers/hidpage/Hut1_12v2.pdf) plus special codes in the `0xA5-DF` range.
Keycode has `KC_` prefixed symbol respectively. Most of keycodes like `KC_A` have simple action registers key to host on press and unregister on release, while some of other keycodes has some special actions like `Fn` keys, Media control keys, System control keys and Mousekeys.
In `KEYMAP()` macro, TMK recommends you to keep prefix part `KC_` of keycode to keep keymap compact. For example, just use `A` instead you place `KC_A` in `KEYMAP()`. But this doesn't apply for QMK.
You don't need to use this functionality under QMK since this is a backward compatibility functionality. Unlike TMK, you can write action code itself directly in **`static const uint16_t PROGMEM keymaps[]`** C array using `MO(layer)`, etc.
`KC_FNnn` are keycodes for `Fn` key which not given any actions at the beginning unlike most of keycodes has its own inborn action. To use these keycodes in `KEYMAP()` you need to assign action you want at first. Action of `Fn` key is defined in `fn_actions[]` and its index of the array is identical with number part of `KC_FNnn`. Thus `KC_FN0` keycode indicates the action defined in first element of the array. ***Only 32 `Fn` keys can be defined at most.***
See keycode table in [`doc/keycode.txt`](./keycode.txt) for description of keycodes.
In regard to implementation side most of keycodes are identical with [HID usage][HID_usage](pdf) sent to host for real and some virtual keycodes are defined to support special actions.
See [`common/action_code.h`](../common/action_code.h). Action is a **16bit code** and defines function to perform on events of a key like press, release, holding and tapping.
Most of keys just register 8bit scancode to host, but to support other complex features needs 16bit extended action codes internally. However, using 16bit action codes in keymap results in double size in memory compared to using just keycodes. To avoid this waste 8bit keycodes are used in `KEYMAP()` instead of action codes.
***You can just use keycodes of `Normal key`, `Modifier`, `Mousekey` and `System & Media key` in keymap*** to indicate corresponding actions instead of using action codes. While ***to use other special actions you should use keycode of `Fn` key defined in `fn_actions[]`.***
### 2.1 Key Action
This is a simple action that registers scancodes(HID usage in fact) to host on press event of key and unregister on release.
***This action usually won't be used expressly in keymap*** because you can just use keycodes in `KEYMAP()` instead.
You can define these actions on *'A'* key and *'left shift'* modifier with:
ACTION_KEY(KC_A)
ACTION_KEY(KC_LSFT)
#### 2.1.2 Modified key
This action is comprised of strokes of modifiers and a key. `Macro` action is needed if you want more complex key strokes.
Say you want to assign a key to `Shift + 1` to get character *'!'* or `Alt + Tab` to switch application windows.
ACTION_MODS_KEY(MOD_LSFT, KC_1)
ACTION_MODS_KEY(MOD_LALT, KC_TAB)
Or `Alt,Shift + Tab` can be defined. `ACTION_MODS_KEY(mods, key)` requires **4-bit modifier state** and a **keycode** as arguments. See `keycode.h` for `MOD_BIT()` macro.
ACTION_MODS_KEY(MOD_LALT | MOD_LSFT, KC_TAB)
#### 2.1.3 Multiple Modifiers
Registers multiple modifiers with pressing a key. To specify multiple modifiers use `|`.
ACTION_MODS(MOD_ALT | MOD_LSFT)
#### 2.1.3 Modifier with Tap key([Dual role][dual_role])
Works as a modifier key while holding, but registers a key on tap(press and release quickly).
ACTION_MODS_TAP_KEY(MOD_RCTL, KC_ENT)
### 2.2 Layer Action
These actions operate layers of keymap.
#### Parameters
You can specify a **target layer** of action and **when the action is executed**. Some actions take a **bit value** for bitwise operation.
This C function is called every time key is operated, argument `id` selects action to be performed and `opt` can be used for option. Function `id` can be 0-255 and `opt` can be 0-15.
`keyrecord_t` is comprised of key event and tap count. `keyevent_t` indicates which and when key is pressed or released. From `tap_count` you can know tap state, 0 means no tap. These information will be used in user function to decide how action of key is performed.
The swap-hands action allows support for one-handed keyboards without requiring a separate layer. Set `ONEHAND_ENABLE` in the Makefile and define a `hand_swap_config` entry in your keymap. Now whenever the `ACTION_SWAP_HANDS` command key is pressed the keyboard is mirrored. For instance, to type "Hello, World" on QWERTY you would type `^Ge^s^s^w^c W^wr^sd`
Note that the array indices are reversed same as the matrix and the values are of type `keypos_t` which is `{col, row}` and all values are zero-based. In the example above, `hand_swap_config[2][4]` (third row, fifth column) would return {7, 2} (third row, eighth column).
There are some ways to switch layer with 'Layer' actions.
### 3.1 Momentary switching
Momentary switching changes layer only while holding Fn key.
This action makes 'Layer 1' active(valid) on key press event and inactive on release event. Namely you can overlay a layer on lower layers or default layer temporarily with this action.
ACTION_LAYER_MOMENTARY(1)
Note that after switching on press the actions on destination layer(Layer 1) are performed.
***Thus you shall need to place an action to go back on destination layer***, or you will be stuck in destination layer without way to get back. Usually you need to place same action or 'KC_TRNS` on destination layer to get back.
### 3.2 Toggle switching
Toggle switching performed after releasing a key. With this action you can keep staying on the destination layer until you type the key again to return.
This performs toggle switching action of 'Layer 2'.
ACTION_LAYER_TOGGLE(2)
### 3.3 Momentary switching with Tap key
These actions switch a layer only while holding a key but register the key on tap. **Tap** means to press and release a key quickly.
ACTION_LAYER_TAP_KEY(2, KC_SCLN)
With this you can place a layer switching action on normal key like ';' without losing its original key register function. This action allows you to have layer switching action without necessity of a dedicated key. It means you can have it even on home row of keyboard.
### 3.4 Momentary switching with Tap Toggle
This switches layer only while holding a key but toggle layer with several taps. **Tap** means to press and release key quickly.
ACTION_LAYER_TAP_TOGGLE(1)
Number of taps can be configured with `TAPPING_TOGGLE` in `config.h`, `5` by default.
### 3.5 Momentary switching with Modifiers
This registers modifier key(s) simultaneously with layer switching.
ACTION_LAYER_MODS(2, MOD_LSFT | MOD_LALT)
## 4. Tapping
Tapping is to press and release a key quickly. Tapping speed is determined with setting of `TAPPING_TERM`, which can be defined in `config.h`, 200ms by default.
### 4.1 Tap Key
This is a feature to assign normal key action and modifier including layer switching to just same one physical key. This is a kind of [Dual role key][dual_role]. It works as modifier when holding the key but registers normal key when tapping.
This is a feature to assign both toggle layer and momentary switch layer action to just same one physical key. It works as momentary layer switch when holding a key but toggle switch with several taps.
This runs onetime effects which modify only on just one following key. It works as normal modifier key when holding down while oneshot modifier when tapping. The behavior of oneshot modifiers is similar to the [sticky keys](https://en.wikipedia.org/wiki/StickyKeys) functionality found in most operating systems.
Say you want to type 'The', you have to push and hold Shift key before type 't' then release it before type 'h' and 'e', otherwise you'll get 'THe' or 'the' unintentionally. With Oneshot Modifier you can tap Shift then type 't', 'h' and 'e' normally, you don't need to holding Shift key properly here. This mean you can release Shift before 't' is pressed down.
Oneshot effect is cancel unless following key is pressed down within `ONESHOT_TIMEOUT` of `config.h`. No timeout when it is `0` or not defined.
Most implementations of sticky keys allow you to lock a modifier by double tapping the modifier. The layer then remains locked untill the modifier is tapped again. To enable this behaviour for oneshot modifiers set `ONESHOT_TAP_TOGGLE` to the number taps required. The feature is disabled if `ONESHOT_TAP_TOGGLE<2` or not defined.
Similar to layer tap toggle, this works as a momentary modifier when holding, but toggles on with several taps. A single tap will 'unstick' the modifier again.
Legacy Keymap uses two arrays `fn_layer[]` and `fn_keycode[]` to define Fn key. The index of arrays corresponds with postfix number of `Fn` key. Array `fn_layer[]` indicates destination layer to switch and `fn_keycode[]` has keycodes to send when tapping `Fn` key.
In the following legacy keymap setting example, `Fn0`, `Fn1` and `Fn2` switch layer to 1, 2 and 2 respectively. `Fn2` registers `Space` key when tapping while `Fn0` and `Fn1` doesn't send any key.
Under QMK, these can be realized using action code ACTION_LAYER_TAP_KEY(1, KC_NO), ACTION_LAYER_TAP_KEY(2, KC_NO), and ACTION_LAYER_TAP_KEY(2, KC_SPC) in the `keymaps` directly.
