This document describes the requirements of the QMK XAP ("extensible application protocol") API.
## Types
**All integral types are little-endian.**
| Name | Definition |
| -- | -- |
| _type[n]_ | An array of `type`, with array extent of `N` -- e.g. `u8[2]` signifies two consecutive octets. |
| _u16_ | An unsigned 16-bit integral, commonly seen as `uint16_t` from _stdint.h_. |
| _u32_ | An unsigned 32-bit integral, commonly seen as `uint32_t` from _stdint.h_. |
| _u8_ | An unsigned 8-bit integral (octet, or byte), commonly seen as `uint8_t` from _stdint.h_. |
## Definitions
This list defines the terms used across the entire set of XAP protocol documentation.
| Name | Definition |
| -- | -- |
| _Handler_ | A piece of code that is executed when a specific _route_ is received. |
| _ID_ | A single octet / 8-bit byte. |
| _Payload_ | Any received data appended to the _route_, which gets delivered to the _handler_ when received. |
| _Response_ | The data sent back to the host during execution of a _handler_. |
| _Response Flags_ | An `u8` containing the status of the request. |
| _Route_ | A sequence of _IDs_ describing the route to invoke a _handler_. |
| _Subsystem_ | A high-level area of functionality within XAP. |
| _Token_ | A `u16` associated with a specific request as well as its corresponding response. |
## Requests and Responses
Communication generally follows a request/response pattern.
Each request needs to include a _token_ -- this `u16` value prefixes each outbound request from the host application and its corresponding response, allowing repsonse messages to be correlated with their request, even if multiple host applications are communicating with the firmware simultaneously. Host applications should randomly generate a token ID for **every** outbound request, unless using a reserved token defined below.
This token is followed by a `u8` signifying the length of data in the request.
Response messages will always be prefixed by the originating request _token_, directly followed by that request's _response flags_, then the response payload length:
| Bit 7 | Bit 6 | Bit 5 | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0 |
* Bit 0 (`SUCCESS`): When this bit is set, the request was successfully handled. If not set, all payload data should be disregarded, and the request retried if appropriate (with a new token).
