layouts

If "USE_LAYOUT s" pre-configuration command is present in config.txt,
the default US keyboard layout is replaced with the new one. You need to
have the necessary layout file in /kblayout/ directory on the device and
specify it's filename as argument to the command, e.g. "USE_LAYOUT fra_FR".
Device then will get new layout data from that file. Keep in mind that
the filename must be in 8.3 format, and only lowercase letters a-z,
upepercase letters A-Z, digits 0-9, underscore and dot symbols are allowed.

Keyboard layout files are used as a mapping of ASCII-printable
characters (used in payload scripts) into HID keycodes along with
appropriate modifier keys. The mapping is done by taking an ascii symbol
value and subtracting 32 from it (to avoid non-printable characters),
then using resulting number as an index to a byte array of 95 elements.
This byte array is located at the beginning of the file, so byte 0 of
the file corresponds to ASCII code 32 (spacebar), byte 1 to ASCII code 33
(exclamation mark), byte 2 to ASCII 34 (quotation mark) and so on,
right up until (and including) byte index 94.

These bytes contain corresponding HID keycodes in 7 least significant
bits, and a SHIFT indicator in the most significant bit. If MSb is
set, the shift modifier will be applied. As an example, in a US layout
file, byte 33 (for ASCII "A") would contain 0x84 value, while byte 65
(for ASCII "a") would contain 0x04 value. Both have the same
HID keycode 0x04, but for "A" the SHIFT indicator bit is set.

Sometimes, however, you also need to apply an AltGr modifier key.
This is what the last 12 bytes are for. In this portion of the file
the bit array of AltGr indicators is located. That is, in byte 95
the least significant bit corresponds to ASCII code 32, next more
significant bit corresponds to ASCII code 33 and so on. For ASCII
code 40 the least significant bit of the next byte (byte 96) is used,
and this keeps on going until the end of file.
Example: in french layout you need AltGr to type "#" symbol. ASCII
code for "#" is 35, and after subtracting 32 you get that bit with
index 3 in AltGr array must be set. This bit is located in byte 95
of layout file. Other ASCII symbols (codes 32-39) do not require
AltGr to be applied, so this bit is the only one that is set,
giving a final value of 0x08 to be stored in byte 95 of layout file.

There are some existing keyboard layout files in this repository
( in /extra/payloads/FeatureTesting/LayoutTest/kblayout/ ).
You can, however, make your own if necessary. I suggest copying
an existing layout file (preferably with a layout similar to yours)
and then inserting all the right values with some hex editor.

For more info on which HID keycodes correspond to which US keys read
USB HID usage tables document located here, pages 89-96.
The HID keycode is bound to key position, not to the symbol on the key;
e.g. french keyboards have AZERTY layout, that means french "a" is
on the same position as US "q", hence they will have the same keycode;
So, whenever PocketAdmin sees ASCII "a" in the script, it needs to
send HID keycode for US "q". To do that, go to the layout file, find
byte 65 (for ASCII "a") and put HID keycode for US "q" in it.