through vblank_cb_battle

include/contest.h

@@ -401,6 +401,7 @@ struct ContestResources
     void * field_3c;
 };
 
+#define shared15800 (gHeap + 0x18000)
 #define shared18000 (*(struct Shared18000 *)(gHeap + 0x1a000))
 
 extern struct ContestResources *gContestResources;

include/gba/types.h

@@ -28,6 +28,17 @@ typedef u8  bool8;
 typedef u16 bool16;
 typedef u32 bool32;
 
+struct BgCnt
+{
+    u16 priority:2;
+    u16 charBaseBlock:2;
+    u16 dummy:4;
+    u16 screenBaseBlock:5;
+    u16 areaOverflowMode:1;
+    u16 screenSize:2;
+};
+typedef volatile struct BgCnt vBgCnt;
+
 struct PlttData
 {
     u16 r:5; // red