Update palette only when screen buffers flip

Previously, any palette updates would be immediately visible, but any
screen updates would draw to the back buffer and would only become
visible when the back buffer was copied to the front buffer. This could
result in a flash of a new palette before the new screen had a chance
to draw and become visible.

To prevent this, a second palette was created to receive palette
updates, with the updates only being copied to the visible palette when
the screen buffers flip.

3 files changed, 36 insertions, 22 deletions

diff --git a/arm9/source/devices/screen.c b/arm9/source/devices/screen.c
index 0d2fc36..5c16d8f 100644
--- a/arm9/source/devices/screen.c
+++ b/arm9/source/devices/screen.c
@@ -19,6 +19,31 @@ a 16-bit tile index (low 10 bits are tile index, then h-flip, then v-flip, then
 #include "screen.h"
 #include "../bang.h"
 
+Screen scr_main = {
+    .bgv  = BG_TILE_RAM(BG_SLOT_VIS),
+    .fgv  = BG_TILE_RAM(FG_SLOT_VIS),
+    .bg   = BG_TILE_RAM(BG_SLOT),
+    .fg   = BG_TILE_RAM(FG_SLOT),
+    .map  = BG_MAP_RAM(MAP_SLOT),
+    .palv = BG_PALETTE,
+};
+Screen scr_sub = {
+    .bgv  = BG_TILE_RAM_SUB(BG_SLOT_VIS),
+    .fgv  = BG_TILE_RAM_SUB(FG_SLOT_VIS),
+    .bg   = BG_TILE_RAM_SUB(BG_SLOT),
+    .fg   = BG_TILE_RAM_SUB(FG_SLOT),
+    .map  = BG_MAP_RAM_SUB(MAP_SLOT),
+    .palv = BG_PALETTE_SUB,
+};
+
+void scr_make_main(ScreenDevice *scr) {
+    scr->nds = &scr_main;
+}
+
+void scr_make_sub(ScreenDevice *scr) {
+    scr->nds = &scr_sub;
+}
+
 void init_screens(void) {
     // Allocate VRAM for screens
     videoSetMode(DISPLAY_BG0_ACTIVE | DISPLAY_BG1_ACTIVE | MODE_0_2D);
@@ -127,6 +152,9 @@ void erase_screen(Screen *nds) {
 void flip_buffer(Screen *nds) {
     dmaCopyWords(0, nds->bg, nds->bgv, TILES_MEM);
     dmaCopyWords(0, nds->fg, nds->fgv, TILES_MEM);
+    for (int i=0; i<16; i++) {
+        nds->palv[i] = nds->pal[i];
+    }
 }
 
 void black_screen(Screen *nds) {
diff --git a/arm9/source/devices/screen.h b/arm9/source/devices/screen.h
index a406051..3e75334 100644
--- a/arm9/source/devices/screen.h
+++ b/arm9/source/devices/screen.h
@@ -25,9 +25,10 @@
 
     typedef struct {
         u16 *bgv, *fgv;       // visible tile memory
-        u16 *bg, *fg;         // tile memory
-        u16 *map;             // tile map
-        u16 *pal;             // colour palette
+        u16 *bg, *fg;         // buffered tile memory
+        u16 *map;             // tile map (never changes)
+        u16 *palv;            // visible colour palette
+        u16 pal[16];          // buffered colour palette
     } Screen;
 
     typedef struct {
@@ -43,6 +44,8 @@
     } ScreenDevice;
 
     void init_screens(void);
+    void scr_make_main(ScreenDevice *scr);
+    void scr_make_sub(ScreenDevice *scr);
 
     void set_palette_high(ScreenDevice *scr, u8 high);
     void set_palette_low(ScreenDevice *scr, u8 low);
diff --git a/arm9/source/main.c b/arm9/source/main.c
index 8be7837..095e089 100644
--- a/arm9/source/main.c
+++ b/arm9/source/main.c
@@ -37,23 +37,6 @@ void receive_input(void) {
 }
 
 
-Screen scr_main = {
-    .bgv = BG_TILE_RAM(BG_SLOT_VIS),
-    .fgv = BG_TILE_RAM(FG_SLOT_VIS),
-    .bg  = BG_TILE_RAM(BG_SLOT),
-    .fg  = BG_TILE_RAM(FG_SLOT),
-    .map = BG_MAP_RAM(MAP_SLOT),
-    .pal = BG_PALETTE,
-};
-Screen scr_sub = {
-    .bgv = BG_TILE_RAM_SUB(BG_SLOT_VIS),
-    .fgv = BG_TILE_RAM_SUB(FG_SLOT_VIS),
-    .bg  = BG_TILE_RAM_SUB(BG_SLOT),
-    .fg  = BG_TILE_RAM_SUB(FG_SLOT),
-    .map = BG_MAP_RAM_SUB(MAP_SLOT),
-    .pal = BG_PALETTE_SUB,
-};
-
 int main(void) {
     #define ALIVE(br)  (br && br->alive)
     #define AWAKE(br)  (br && br->alive &&  br->awake)
@@ -79,8 +62,8 @@ int main(void) {
     start_br(&br[1], keyboard_program, sizeof(keyboard_program));
     br_main = &br[0];
     br_sub = &br[1];
-    br_main->scr.nds = &scr_main;
-    br_sub->scr.nds = &scr_sub;
+    scr_make_main(&br_main->scr);
+    scr_make_sub(&br_sub->scr);
 
     while (1) {
         if (AWAKE(br_main)) run_br(br_main);