kernel: add an add-in restart mechanism using gint_osmenu()

This new mechanism allows an add-in to be restarted after exiting by
just never exiting in the first place, calling gint_osmenu() instead.
This makes sure that we can relaunch the add-in immediately, which is
normally possible through an option in the OS though no OS-independent
method of setting it is currently known.

Because this is gint_osmenu(), known pitfalls apply. On all platforms,
it is necessary to prepare the first frame before leaving. On fx-CG 50,
the inevitable display border is also there.

TODO

@@ -13,7 +13,6 @@ Extensions on existing code:
 * core:      review forgotten globals and MPU addresses not in <gint/mpu/*.h>
 * build:     make the build system simpler (two targets are enough by default)
 * core:      run destructors when a task-switch results in leaving the app
-* core:      invoke main menu instead of returning after main() ends
 * core rtc:  use qdiv10 to massively improve division performance
 * topti:     let the font specify letter and word spacing
 

include/gint/gint.h

@@ -43,6 +43,18 @@ void gint_switch(void (*function)(void));
    call to getkey(), but can also be called manually. */
 void gint_osmenu(void);
 
+/* gint_setrestart(): Set whether to restart the add-in after exiting
+
+   An add-in that reaches the end of its code exits. On the calculator, except
+   using OS-dependent settings, it cannot be started again unless another
+   application is launched first.
+
+   This setting allows the add-in to restart by calling gint_osmenu() instead
+   of exiting. This can give a proper illusion of restarting if used correctly.
+
+   @restart  0 to exit, 1 to restart by using gint_osmenu() */
+void gint_setrestart(int restart);
+
 /* gint_inthandler(): Install interrupt handlers
 
    This function installs (copies) interrupt handlers in the VBR space of the

src/core/start.c

@@ -36,6 +36,15 @@ extern void (*bdtors)(void), (*edtors)(void);
 /* User-provided main() function */
 int main(int isappli, int optnum);
 
+/* Whether to restart main through the OS menu rather than returning */
+int gint_restart = 0;
+
+/* gint_setrestart(): Set whether to restart the add-in after exiting */
+void gint_setrestart(int restart)
+{
+	gint_restart = restart;
+}
+
 /* regcpy(): Copy a memory region using symbol information
    @l  Source pointer (load address)
    @s  Size of area (should be a multiple of 16)
@@ -77,8 +86,9 @@ static void callarray(void (**f)(void), void (**l)(void))
 	while(f < l) (*(*f++))();
 }
 
-/* start() - this is where it all starts
-   Returns a status code. Invoking main menu is better than returning! */
+/* start(): Where it all starts
+   Returns a status code. Invoking main menu is better than returning, see
+   gint_setrestart() for that. */
 GSECTION(".text.entry")
 int start(int isappli, int optnum)
 {
@@ -126,7 +136,15 @@ int start(int isappli, int optnum)
 	   destructors to work with. */
 
 	callarray(&bctors, &ectors);
-	int ret = main(isappli, optnum);
+
+	int ret;
+	while(1)
+	{
+		ret = main(isappli, optnum);
+		if(!gint_restart) break;
+		gint_osmenu();
+	}
+
 	callarray(&bdtors, &edtors);
 
 	/* Before leaving the application, we need to clean everything we
@@ -137,7 +155,5 @@ int start(int isappli, int optnum)
 	/* Unload gint and give back control to the system. Driver settings
 	   will be restored while interrupts are disabled */
 	kquit();
-
-	/* TODO: Invoke main menu so that add-in can restart? */
 	return ret;
 }