Thread: Some details about the exFAT

Microsoft informs us, by a release of some information relating to the sale of a license around the exFAT (Extended File Allocation Table), a new version of its file system. Therefore, we learn that manufacturers of storage media or products using these memory devices can implement the exFAT, a few conditions however. It will in fact adhere to the program of Microsoft, whose duties are set at $ 300 000, while an additional fee will be calculated based on the volume of units produced.

Successor of the older FAT32 and its annoying limitations, the exFAT theoretically supports such files size up to 46 GB It is also compatible with ACL (Access Control Lists). Sony, Canon, Sanyo Sandisk or via the SD Association are all partners exFAT while the consortium responsible for developing memory cards SDXC has access for some time to exFAT.

The file system limits are 128 PiB, although MS first siad 64ZiB and now says 256TiB, but I calculated it at 128 PiB. The FAT is 32 bits with allow up to 2**32 clusters with sector sizes betwee 512 and 4096 bytes with a max cluster size of 32MiB. The maximum number of sectors is 2**64 becasue it is a 8 byte number. The FAT is not used like FAT32, it is only used for fragmented files, exFAT has a bitmap for allocation. There is NO ACL in the current version, it exists on Windows CE and will be in future released of exFAT. That also applies to TexFAT, transactional fat. The subdirectories are limited to 256MiB in size, each entry is 32 bytes and requires a minimum of 3 directory records (0x85, 0xC0, and a 0xC1) for each file/subdirectory

If the file is not fragmented, then the FAT is not used and that speeds up read and writes because that is one less table to update.