Customizing the boot flash memory error screen

original source: Anon822's pokecommunity post

Normally if you load the game without a flash chip present on the cart (on an emulator with the wrong save type setting), the game immediately crashes on a white screen.
This modification replaces that with an error screen.

---

src/save_failed_screen.c

add the following code at the end of the file

void CB2_FlashNotDetectedScreen(void){
    static const struct WindowTemplate textWin[] = {
        {
            .bg = 0,
            .tilemapLeft = 3,
            .tilemapTop = 2,
            .width = 24,
            .height = 16,
            .paletteNum = 15,
            .baseBlock = 1,
        }
    };
	
    static const char errorMsg[] = _(
        "{COLOR RED}ERROR!\n"
        "{COLOR DARK_GRAY}Flash memory not detected.\n\n"
        "Set your emulator's save type\nsetting to Flash 1Mb / 128K\n"
        "and reload the rom."
    );

    if (!gMain.state){
        SetGpuReg(REG_OFFSET_DISPCNT, 0);
	SetGpuReg(REG_OFFSET_BLDCNT, 0);
        SetGpuReg(REG_OFFSET_BG0CNT, 0);
        SetGpuReg(REG_OFFSET_BG0HOFS, 0);
        SetGpuReg(REG_OFFSET_BG0VOFS, 0);
        DmaFill16(3, 0, VRAM, VRAM_SIZE);
        DmaFill32(3, 0, OAM, OAM_SIZE);
        DmaFill16(3, 0, PLTT, PLTT_SIZE);
	ResetBgsAndClearDma3BusyFlags(0);
        InitBgsFromTemplates(0, sBgTemplates, ARRAY_COUNT(sBgTemplates));
        LoadBgTiles(0, gTextWindowFrame1_Gfx, 0x120, 0x214);
        DeactivateAllTextPrinters();
        ResetTasks();
        ResetPaletteFade();
	LoadPalette(gTextWindowFrame1_Pal, 0xE0, 0x20);
        LoadPalette(gStandardMenuPalette, 0xF0, 0x20);
	InitWindows(textWin);
	DrawStdFrameWithCustomTileAndPalette(0, TRUE, 0x214, 0xE);
        SaveFailedScreenTextPrint(errorMsg, 1, 0);
	TransferPlttBuffer();
	*(u16*)PLTT = RGB(17, 18, 31);
	ShowBg(0);
        gMain.state++;
    }
}

---

src/main.c

add this after the include section (I added after static void IntrDummy(void); at line 33)

extern void CB2_FlashNotDetectedScreen(void);

Then change SetMainCallback2(NULL); (at line 123)
To

    SetMainCallback2(CB2_FlashNotDetectedScreen);