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+## READ FIRST
+https://github.com/tmk/tmk_core/blob/master/doc/keymap.md
+
+## How to get keycode
+See [Keycodes](Keycodes). Keycodes are actually defined in [common/keycode.h](https://github.com/qmk/qmk_firmware/blob/master/tmk_core/common/keycode.h).
+
+## Sysrq key
+Use keycode for Print Screen(`KC_PSCREEN` or `KC_PSCR`) instead of `KC_SYSREQ`. Key combination of 'Alt + Print Screen' is recognized as 'System request'.
+
+See [issue #168](https://github.com/tmk/tmk_keyboard/issues/168) and
+- http://en.wikipedia.org/wiki/Magic_SysRq_key
+- http://en.wikipedia.org/wiki/System_request
+
+## Power key doesn't work
+Use `KC_PWR` instead of `KC_POWER` or vice versa.
+- `KC_PWR` works with Windows and Linux, not with OSX.
+- `KC_POWER` works with OSX and Linux, not with Windows.
+
+http://geekhack.org/index.php?topic=14290.msg1327264#msg1327264
+
+## Oneshot modifier
+Solves my personal 'the' problem. I often got 'the' or 'THe' wrongly instead of 'The'.  Oneshot Shift mitgates this for me.
+https://github.com/tmk/tmk_keyboard/issues/67
+
+## Modifier/Layer stuck
+Modifier keys or layers can be stuck unless layer switching is configured properly.
+For Modifier keys and layer actions you have to place `KC_TRANS` on same position of destination layer to  unregister the modifier key or return to previous layer on release event.
+
+- https://github.com/tmk/tmk_core/blob/master/doc/keymap.md#31-momentary-switching
+- http://geekhack.org/index.php?topic=57008.msg1492604#msg1492604
+- https://github.com/tmk/tmk_keyboard/issues/248
+
+
+## Mechanical Lock Switch Support
+https://github.com/tmk/tmk_keyboard#mechanical-locking-support
+
+This feature is for *mechanical lock switch* like this Alps one.
+http://deskthority.net/wiki/Alps_SKCL_Lock
+
+Using enabling this feature and using keycodes `LCAP`, `LNUM` or `LSCR` in keymap you can use physical locking CapsLock, NumLock or ScrollLock keys as you expected.
+
+Old vintage mechanical keyboards occasionally have lock switches but modern ones don't have. ***You don't need this feature in most case and just use keycodes `CAPS`, `NLCK` and `SLCK`.***
+
+
+## Input special charactors other than ASCII like Cédille 'Ç'
+NO UNIVERSAL METHOD TO INPUT THOSE WORKS OVER ALL SYSTEMS. You have to define **MACRO** in way specific to your OS or layout.
+
+See this post for example **MACRO** code.
+
+http://deskthority.net/workshop-f7/tmk-keyboard-firmware-collection-t4478-120.html#p195620
+
+On **Windows** you can use `AltGr` key or **Alt code**.
+- http://en.wikipedia.org/wiki/AltGr_key
+- http://en.wikipedia.org/wiki/Alt_code
+
+On **Mac** OS defines `Option` key combinations.
+- http://en.wikipedia.org/wiki/Option_key#Alternative_keyboard_input
+
+On **Xorg** you can use `compose` key, instead.
+- http://en.wikipedia.org/wiki/Compose_key
+
+And see this for **Unicode** input.
+- http://en.wikipedia.org/wiki/Unicode_input
+
+
+## Apple/Mac keyboard Fn
+Not supported.
+
+Apple/Mac keyboard sends keycode for Fn unlike most of other keyboards.
+I think you can send Apple Fn key using Apple venter specific Page 0xff01 and usage 0x0003. But you have to change HID Report Descriptor for this, of course.
+
+https://opensource.apple.com/source/IOHIDFamily/IOHIDFamily-606.1.7/IOHIDFamily/AppleHIDUsageTables.h
+
+
+## Media control keys in Mac OSX
+#### KC_MNXT and KC_MPRV does not work on Mac
+Use `KC_MFFD`(`KC_MEDIA_FAST_FORWARD`) and `KC_MRWD`(`KC_MEDIA_REWIND`) instead of `KC_MNXT` and `KC_MPRV`.
+See https://github.com/tmk/tmk_keyboard/issues/195
+
+
+## Keys supported in Mac OSX?
+You can know which keycodes are supported in OSX from this source code.
+
+`usb_2_adb_keymap` array maps Keyboard/Keypad Page usages to ADB scancodes(OSX internal keycodes).
+
+https://opensource.apple.com/source/IOHIDFamily/IOHIDFamily-606.1.7/IOHIDFamily/Cosmo_USB2ADB.c
+
+And `IOHIDConsumer::dispatchConsumerEvent` handles Consumer page usages.
+
+https://opensource.apple.com/source/IOHIDFamily/IOHIDFamily-606.1.7/IOHIDFamily/IOHIDConsumer.cpp
+
+
+## JIS keys in Mac OSX
+Japanese JIS keyboard specific keys like `無変換(Muhenkan)`, `変換(Henkan)`, `ひらがな(hiragana)` are not recognized on OSX. You can use **Seil** to enable those keys, try following options.
+
+* Enable NFER Key on PC keyboard
+* Enable XFER Key on PC keyboard
+* Enable KATAKANA Key on PC keyboard
+
+https://pqrs.org/osx/karabiner/seil.html
+
+
+## RN-42 Bluetooth doesn't work with Karabiner
+Karabiner - Keymapping tool on Mac OSX - ignores inputs from RN-42 module by default. You have to enable this option to make Karabiner working with your keyboard.
+https://github.com/tekezo/Karabiner/issues/403#issuecomment-102559237
+
+See these for the deail of this problem.
+https://github.com/tmk/tmk_keyboard/issues/213
+https://github.com/tekezo/Karabiner/issues/403
+
+
+## Esc and `~ on a key
+
+
+You can define FC660 and Poker style ESC with `ACTION_LAYER_MODS`.
+https://github.com/tmk/tmk_core/blob/master/doc/keymap.md#35-momentary-switching-with-modifiers
+
+```
+#include "keymap_common.h"
+
+
+/* Leopold FC660
+ * https://elitekeyboards.com/products.php?sub=leopold,compact&pid=fc660c
+ * Shift + Esc = ~
+ * Fn    + Esc = `
+ *
+ * Votex Poker II
+ * https://adprice.fedorapeople.org/poker2_manual.pdf
+ * Fn         + Esc = `
+ * Fn + Shift + Esc = ~
+ */
+const uint8_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
+    /* 0: qwerty */
+    [0] = KEYMAP( \
+        ESC, 1,   2,   3,   4,   5,   6,   7,   8,   9,   0,   MINS,EQL, NUHS,BSPC, \
+        TAB, Q,   W,   E,   R,   T,   Y,   U,   I,   O,   P,   LBRC,RBRC,BSLS, \
+        LCTL,A,   S,   D,   F,   G,   H,   J,   K,   L,   SCLN,QUOT,ENT,  \
+        FN0, NUBS,Z,   X,   C,   V,   B,   N,   M,   COMM,DOT, SLSH,RSFT,ESC, \
+        LCTL,LGUI,LALT,          SPC,                     RALT,FN1, RGUI,RCTL),
+    [1] = KEYMAP( \
+        GRV, TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS, \
+        TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,\
+        TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS, \
+        TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS, \
+        TRNS,TRNS,TRNS,          TRNS,                    TRNS,TRNS,TRNS,TRNS),
+    [2] = KEYMAP( \
+        GRV, F1,  F2,  F3,  F4,  F5,  F6,  F7,  F8,  F9,  F10, F11, F12, TRNS,TRNS, \
+        TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,\
+        TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS, \
+        TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS, \
+        TRNS,TRNS,TRNS,          TRNS,                    TRNS,TRNS,TRNS,TRNS),
+};
+
+const uint16_t PROGMEM fn_actions[] = {
+    // https://github.com/tmk/tmk_core/blob/master/doc/keymap.md#35-momentary-switching-with-modifiers
+    [0] = ACTION_LAYER_MODS(1, MOD_LSFT),
+    [1] = ACTION_LAYER_MOMENTARY(2),
+};
+```
+
+Otherwise, you can write code, see this.
+https://github.com/p3lim/keyboard_firmware/commit/fd799c12b69a5ab5addd1d4c03380a1b8ef8e9dc
+
+
+## 32 Fn keys are not enough?
+### actionmap
+It uses 16 bit codes and has no limitation of 32 Fn at the expense of memory space. TMK keymap is actually is 8 bit codes as subset of the actionmap.
+https://github.com/tmk/tmk_keyboard/issues?utf8=%E2%9C%93&q=is%3Aissue+actionmap
+
+### extension for modified keys
+https://geekhack.org/index.php?topic=41989.msg1885526#msg1885526
+
+
+## Arrow on Right Modifier keys with Dual-Role
+This turns right modifer keys into arrow keys when the keys are tapped while still modifiers when the keys are hold. In TMK the dual-role function is dubbed **TAP**.
+```
+#include "keymap_common.h"
+
+
+/* Arrow keys on right modifier keys with TMK dual role feature
+ *
+ *  https://github.com/tmk/tmk_core/blob/master/doc/keymap.md#213-modifier-with-tap-keydual-role
+ *  https://en.wikipedia.org/wiki/Modifier_key#Dual-role_keys
+ */
+const uint8_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
+    /* 0: qwerty */
+    [0] = KEYMAP( \
+        ESC, 1,   2,   3,   4,   5,   6,   7,   8,   9,   0,   MINS,EQL, NUHS,BSPC, \
+        TAB, Q,   W,   E,   R,   T,   Y,   U,   I,   O,   P,   LBRC,RBRC,BSLS, \
+        LCTL,A,   S,   D,   F,   G,   H,   J,   K,   L,   SCLN,QUOT,ENT,  \
+        LSFT,NUBS,Z,   X,   C,   V,   B,   N,   M,   COMM,DOT, SLSH,FN0, ESC, \
+        FN4, LGUI,LALT,          SPC,                     APP, FN2, FN1, FN3),
+    [1] = KEYMAP( \
+        GRV, F1,  F2,  F3,  F4,  F5,  F6,  F7,  F8,  F9,  F10, F11, F12, TRNS,TRNS, \
+        TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,\
+        TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS, \
+        TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,TRNS,FN5, TRNS, \
+        TRNS,TRNS,TRNS,          TRNS,                    TRNS,FN7, FN6, FN8),
+};
+
+const uint16_t PROGMEM fn_actions[] = {
+    [0] = ACTION_MODS_TAP_KEY(MOD_RSFT, KC_UP),
+    [1] = ACTION_MODS_TAP_KEY(MOD_RGUI, KC_DOWN),
+    [2] = ACTION_MODS_TAP_KEY(MOD_RALT, KC_LEFT),
+    [3] = ACTION_MODS_TAP_KEY(MOD_RCTL, KC_RIGHT),
+    [4] = ACTION_LAYER_MOMENTARY(1),
+    [5] = ACTION_MODS_TAP_KEY(MOD_RSFT, KC_PGUP),
+    [6] = ACTION_MODS_TAP_KEY(MOD_RGUI, KC_PGDN),
+    [7] = ACTION_MODS_TAP_KEY(MOD_RALT, KC_HOME),
+    [8] = ACTION_MODS_TAP_KEY(MOD_RCTL, KC_END),
+};
+
+```
+
+
+Dual-role key: https://en.wikipedia.org/wiki/Modifier_key#Dual-role_keys
+
+
+## Eject on Mac OSX
+`EJCT` keycode works on OSX. https://github.com/tmk/tmk_keyboard/issues/250
+It seems Windows 10 ignores the code and Linux/Xorg recognizes but has no mapping by default.
+
+Not sure what keycode Eject is on genuine Apple keyboard actually. HHKB uses `F20` for Eject key(`Fn+f`) on Mac mode but this is not same as Apple Eject keycode probably.
+
+
+
+## What's weak_mods and real_mods in action_util.c
+___TO BE IMPROVED___
+
+real_mods is intended to retains state of real/physical modifier key state, while
+weak_mods retains state of virtual or temprary modifiers which should not affect state real modifier key.
+
+Let's say you hold down physical left shift key and type ACTION_MODS_KEY(LSHIFT, KC_A), 
+
+with weak_mods,
+* (1) hold down left shift: real_mods |= MOD_BIT(LSHIFT)
+* (2) press ACTION_MODS_KEY(LSHIFT, KC_A): weak_mods |= MOD_BIT(LSHIFT)
+* (3) release ACTION_MODS_KEY(LSHIFT, KC_A): waek_mods &= ~MOD_BIT(LSHIFT)
+real_mods still keeps modifier state.
+
+without weak mods,
+* (1) hold down left shift: real_mods |= MOD_BIT(LSHIFT)
+* (2) press ACTION_MODS_KEY(LSHIFT, KC_A): real_mods |= MOD_BIT(LSHIFT)
+* (3) release ACTION_MODS_KEY(LSHIFT, KC_A): real_mods &= ~MOD_BIT(LSHIFT)
+here real_mods lost state for 'physical left shift'.
+
+weak_mods is ORed with real_mods when keyboard report is sent.
+https://github.com/tmk/tmk_core/blob/master/common/action_util.c#L57
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