successfully call f_mkfs(). it's a start I guess.

src/FATStorage.cpp

@@ -15,3 +15,106 @@
     You should have received a copy of the GNU General Public License along
     with melonDS. If not, see http://www.gnu.org/licenses/.
 */
+
+#include "FATStorage.h"
+#include "Platform.h"
+
+
+FATStorage::FATStorage()
+{
+    printf("FATStorage begin\n");
+    bool res = Build(".", 0x40000000, "melonDLDI.bin");
+    printf("FATStorage result: %d\n", res);
+}
+
+FATStorage::~FATStorage()
+{
+    //
+}
+
+
+FILE* FATStorage::FF_File;
+u64 FATStorage::FF_FileSize;
+
+UINT FATStorage::FF_ReadStorage(BYTE* buf, LBA_t sector, UINT num)
+{
+    if (!FF_File) return 0;
+
+    u64 addr = sector * 0x200ULL;
+    u32 len = num * 0x200;
+
+    if ((addr+len) > FF_FileSize)
+    {
+        if (addr >= FF_FileSize) return 0;
+        len = FF_FileSize - addr;
+        num = len >> 9;
+    }
+
+    fseek(FF_File, addr, SEEK_SET);
+
+    u32 res = fread(buf, 0x200, num, FF_File);
+    if (res < num)
+    {
+        if (feof(FF_File))
+        {
+            memset(&buf[0x200*res], 0, 0x200*(num-res));
+            return num;
+        }
+    }
+
+    return res;
+}
+
+UINT FATStorage::FF_WriteStorage(BYTE* buf, LBA_t sector, UINT num)
+{
+    if (!FF_File) return 0;
+
+    u64 addr = sector * 0x200ULL;
+    u32 len = num * 0x200;
+
+    if ((addr+len) > FF_FileSize)
+    {
+        if (addr >= FF_FileSize) return 0;
+        len = FF_FileSize - addr;
+        num = len >> 9;
+    }
+
+    fseek(FF_File, addr, SEEK_SET);
+
+    u32 res = fwrite(buf, 0x200, num, FF_File);
+    return res;
+}
+
+
+bool FATStorage::Build(const char* sourcedir, u64 size, const char* filename)
+{
+    filesize = size;
+
+    FF_File = Platform::OpenLocalFile(filename, "wb");
+    if (!FF_File)
+        return false;
+
+    FF_FileSize = size;
+    ff_disk_open(FF_ReadStorage, FF_WriteStorage, (LBA_t)(size>>9));
+
+    FRESULT res;
+
+    MKFS_PARM fsopt;
+    fsopt.fmt = FM_FAT | FM_FAT32;
+    fsopt.au_size = 0;
+    fsopt.align = 1;
+    fsopt.n_fat = 1;
+    fsopt.n_root = 512;
+
+    BYTE workbuf[FF_MAX_SS];
+    res = f_mkfs("0:", &fsopt, workbuf, sizeof(workbuf));
+    printf("MKFS RES %d\n", res);
+
+    //
+
+    ff_disk_close();
+    fclose(FF_File);
+    FF_File = nullptr;
+
+    return true;
+}