Display Management

The BOLOS SDKs contain a toolkit for building GUIs called the BOLOS Application Graphics Library (BAGL). BAGL defines a few useful types, most notably bagl_element_t. This type defines a single display element, such as a rectangle, a line of text, a touchable button (in the case of the Ledger Blue), et cetera. Therefore, an entire GUI consists of an entire array of such elements. As the device hardware, including the screen, is managed by the MCU, the BAGL elements need to be transported to the MCU over SEPROXYHAL in order to display them. This is done using a Display Status.

A Display Status may be used to send a single BAGL element to the MCU. However, due to the design of the E/Cs/S protocol, in order to send a sequence of BAGL elements the application must asynchronously send Display Statuses and wait for Display Processed Events before sending the next one, as well as handling the other Events that are received from the MCU in the mean time. As this is something that must be done by every BOLOS application, a set of utilities have been defined in the SDK to facilitate this process. These utilities also simplify the process of handling user input events, such as button presses.

Asynchronous Display and Interaction Helpers

To facilitate the process of implementing an asynchronous display manager loop, a set of macros have been defined in the BOLOS SDKs. All these macros have the prefix UX_, and use a global variable of type ux_state_t called ux to maintain the user interface state.

This macro is to be called when initializing the application, prior to sending the first Display Status.
This macro takes an array of BAGL elements to be displayed and renders them, asynchronously. It is to be called when a new screen is to be displayed over the current one. This macro only sends the first element of the given array using a Display Status. Therefore, further Commands and Statuses are prohibited until the Display Processed Event is sent by the MCU.
This macro restarts the process of drawing the current screen. It behaves like UX_DISPLAY(...), but takes no arguments.
This macro returns 0 when the array requested to be displayed by UX_DISPLAY(...) or UX_REDISPLAY() has not been displayed entirely, or a non-zero value when it has.
This macro is to be called to handle Display Processed Events (generally in the io_event(...) function). It displays the next element in the array given as a parameter to UX_DISPLAY(...). This macro sends a Display Status if an element remains to be displayed in the given array. Therefore, further Commands and Statuses are prohibited until the Display Processed Event is sent by the MCU.
This macro facilitates handling of Button Push Events, by setting the button released flag and calling the button handler implicitly passed to UX_DISPLAY(...).

See the sample apps for examples of how to use these macros. The main concept to remember is that after a Display Status has been sent, the application must wait, asynchronously, for the Display Processed Event before being able to continue to display more elements of the GUI.


The BOLOS UX is the implementation of the device-wide user interface; it is a component of the dashboard. Applications delegate certain jobs to the BOLOS UX in order to retain consistency across all apps for certain UI components (like the status bar on the Ledger Blue), as well as to allow the operating system to override the application’s UI when necessary (for example, when locking the screen). The application interfaces with the BOLOS UX using os_ux(...), which is a syscall. However, applications don’t need to call this function directly as it is automatically called by the display interaction helpers (the UX_ macros).

Applications should delegate Events like Button Push Events to the BOLOS UX (in this case, using UX_BUTTON_PUSH_EVENT(...)) instead of handling them directly in case the BOLOS UX needs to override the application’s UI. If the event is consumed by the BOLOS UX (for example, a button press while the user is unlocking the screen) then the event is not passed on to the application.