I am trying to write bare metal FAT32 file system driver on RPI 3B. I am able to read FAT sectors and root directory sectors using emmc driver.
I have doubt on how to follow FAT entries linked list when next entry pointer (next cluster number) doesn't fit in the current FAT sector? Should I read the FAT sector each time I get to the new cluster number?
My current understanding is as follows: Get first cluster number (cluster_number) of directory/file Read the FAT sector which contains first_cluster_number entry. Say I read FAT sector as
uint8_t fat_sector[512] = { 0 };
uint32_t this_fat_sector_num, this_fat_entry_offset;
this_fat_sector_num = unusedSectors + reservedSectorCount + ((cluster_number * 4)/ bytesPerSector);
this_fat_entry_offset = (cluster_number * 4)/ bytesPerSector;
read_fat_sector(this_fat_sector_num, & fat_sector[0]);
// Calculate next cluster in chain
uint32_t next_cluster_number = ((uint32_t * fat_sector[this_fat_entry_offset])) & 0x0fffffff;
// Calculate next cluster in chain 1 more time, is below code correct ?
uint32_t next_next_cluster_number = ((uint32_t * fat_sector[next_cluster_number])) & 0x0fffffff;
What happens when the next cluster number is not present in already read fat_sector buffer (512 bytes)? If cluster number = index of next entry in fat_sector, Do I need to multiply it by 4 given that fat 32 entries span 4 bytes. If anyone could give some clarity, that will be helpful. Thanks in advance.
Implement a cache (in RAM) of the FAT. Let's say that the cache has enough RAM for 20 sectors and starts out empty.
Next write a "getFATentry" function that checks if the sector is in the cache and finds the right entry in the cache if it is; or (if necessary) evicts something from the cache to make room, fetches the right sector from disk into the cache, then finds the right entry in the cache.
Once that's done you can next_cluster = getFATentry(previous_cluster); without worrying about the cache or any disk IO (but will want to do something when the FAT is modified - e.g. adopt a "write-through" or "write-back" policy).
Note: By adjusting the size of the "FAT cache" you can improve performance or reduce RAM consumption. It'd be nice to allow the cache to grow/shrink dynamically (e.g. grow to be as large as the whole FAT if nothing else needs RAM, but shrink to bare minimum if all the RAM is needed for something else).