Updated Keymap (markdown)

Keymap.md

@@ -2,6 +2,81 @@
 
 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.
+
 ## Anatomy Of A `keymap.c`
 
 For this example we will walk through the [default Clueboard keymap](https://github.com/qmk/qmk_firmware/blob/master/keyboards/clueboard/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.
@@ -140,7 +215,6 @@ To actually handle the keypress event we define an `action_function()`. This fun
 
 This should have given you a basic overview for creating your own keymap. For more details see the following resources:
 
-* https://github.com/qmk/qmk_firmware/blob/master/doc/keymap.md (If you see `static const uint8_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS]`, it is still the TMK code example code.)
 * https://github.com/qmk/qmk_firmware/wiki/Keycodes
 * https://github.com/qmk/qmk_firmware/wiki/FAQ-Keymap
 * https://github.com/qmk/qmk_firmware/wiki/Keymap-examples