ProcUI Library Overview


Process switching is a cooperative procedure between every process and the Cafe operating system. The ProcUI library was developed to simplify this process.

To initialize the ProcUI library call ProcUIInit, and to shut it down, call ProcUIShutdown.

When ProcUIProcessMessages is called at every frame, all system messages are handled automatically. The ProcUIProcessMessages function performs the following.

The DEMO library includes an implementation that offers one possible solution for process switching by using the ProcUI library. See $CAFE_ROOT/system/src/lib/demo/demoSystem.c

Before Process Switching To Background

Finalize all rendering operations

While a process transitions to the background (i.e., it is in the "RELEASING" state), it may continue to use GX2 and MEM1 as normal. However, it should complete all drawing activity, display its final frame, and release all MEM1 resources before doing its final transition to the background state. Prior to releasing the foreground, the main graphics core (the one which called GX2Init) must call GX2DrawDone when all rendering is complete. This guarantees that all command buffer data has been dispatched to the GPU and completed. There are two ways to do this using the ProcUI system:

  1. If ProcUIProcessMessages returns PROCUI_STATUS_RELEASING:
    1. Render any last transition frames.
    2. Call GX2SetTVEnable(TRUE) if a frame was rendered to the TV.
    3. Call GX2SetDRCEnable(TRUE) if a frame was rendered to the DRC.
    4. Call GX2DrawDone just prior to calling ProcUIDrawDoneRelease
    5. Call ProcUIProcessMessages
  2. Register a PROCUI_MESSAGE_RELEASE callback using ProcUIRegisterCallback that performs the following.
    1. Finalize and swap the current frame.
    2. Call GX2SetTVEnable(TRUE) if a frame was rendered to the TV.
    3. Call GX2SetDRCEnable(TRUE) if a frame was rendered to the DRC.
    4. Call GX2DrawDone
    5. Call ProcUIDrawDoneRelease

In addition to guaranteeing that the main core's command buffer has been flushed, all display list generation on the other threads/cores must also stop. The application must call GX2EndDisplayList in all other rendering threads/cores if a display list is being generated prior to switching to the background. While in the background state, no further GX2 operations should be performed until the main core has reentered the foreground. After the main core switches to the background, other threads/cores may partially overrun their time slice. To avoid overrunning and writing new GX2 commands on these cores, stop all threads that issue GX2 commands prior to calling GX2DrawDone in the above sequence.

Stop the threads on which a GX2 command is issued to avoid an overrun.

After performing a process switch using the ProcUIProcessMessages function, threads overrun slightly, except the thread that called ProcUIProcessMessages. To avoid the overrun when calling ProcUIProcessMessages, stop the thread on which a GX2 command is issued.

For example, if you have an asynchronous thread for loading that also creates a display list, you must use a mutex or other exclusive locking construct to lock the place where the display list is created and the place where ProcUIProcessMessages is called.

Call GX2SetTVEnable and GX2SetDRCEnable before switching processes.

The GX2SetTVEnable and GX2SetDRCEnable functions sets a flag after the last frame is cleared that tells the system that the application allowed rendering to occur. When a process releases the foreground, GX2 can store the last frame that the application displayed. The flag that these functions set determines whether the frame is stored.

The last frame is not stored unless the GX2SetTVEnable or the GX2SetDRCEnable function is called with TRUE as the argument, which sets the flag.

In addition, under the current implementation, at each process switch, this flag is cleared after the last frame is stored. Although the black screen is cleared when the application acquires the foreground, the application must pass TRUE to these functions every time.

To simplify your implementation:

Mutex Locking When Moving to Background

When a user presses the HOME Button during gameplay, the game moves to the background. Some games continue to run in the background. For example, an online game might communicate in the background to maintain its connection. Only Core 2 is available in the background. Core 0 and Core 1 are not available.

If the game moves to the background with a thread on Core 0 or Core 1, and the thread leaves a mutex locked:

Recovering from a Process Switch

Call GX2SetContextState after returning from a process switch.

Upon reacquiring the foreground state, the application should make few assumptions about what GPU state remains from its previous foreground instance. MEM2 resources remained where they were. Any MEM1 resources will need to be reloaded. Display settings (see GX2DisplayPage) should not require resetting, though it does not hurt to do so.

The application must start the rendering with a context state reset. This means it should begin the frame by calling either GX2ClearColor (or similar APIs, see GX2 Functions Which Disable State Shadowing) or GX2SetContextState. This is mandatory. It is not acceptable to attempt to reset every GX2 state manually, since this may not reset all of the GPU pipeline state.


ProcUIProcessMessages handles releasing and acquiring the foreground silently. Callbacks may be used to release and reset resources as needed. For more information, see ProcUIRegisterCallback.

When initialized, ProcUI issues a callback thread on each of the cores. These threads are responsible for calling the callback functions set by the user. They have a priority of 16, and a default stack size of 8×1024.

Exit callbacks are called when ProcUIShutdown is called or the exit message is received.

If they are not called from main, they will be called shortly after.

Revision History

2013/05/08 Automated cleanup pass.
2012/05/04 Initial version.