Building on the basic SD code in the tuturials in this repo, I have implemented basic FAT32 filesystem support. There are some corners cut but nothing that should affect usage with SD cards.

Example program

See test_dumpfile.s for an example of usage.

It initializes things, then searches the root directory for a folder called "subfoldr", then searches that subfolder for a file called "deepfile.txt", then loads it and prints its contents to the LCD.

This covers most things you'd need to do - of the APIs provided, it just skips enumerating files in a directory one by one, and reading a file byte by byte instead of all at once.

Setup

Mostly you just need to .include the library code in your source and call the right functions.

Your code also needs to provide some memory for the SD and FAT32 libraries to use, by defining some specific symbols. See the example program for more details.

High-level API

fat32_init

This initializes the library. The SD library needs to be initialized first.

On return, the carry is set if there was an error, and in that case you can read an error code from fat32\_errorstage that may help diagnosing what the problem was (probably some issue with the formatting of the SD card).

fat32_openroot

Opens the root directory, preparing to enumerate or search its contents.

fat32_finddirent

With a directory open, scans for an entry matching a given filename.

X and Y should point to the filename in memory, with the high byte of the address in Y.

The carry will be clear on success, or set if the directory entry was not found.

Note that the filename data you provide should match the format used in FAT32 directory entries, i.e. 8 bytes of capitalized filename padded with spaces, followed by 3 bytes of capitalized extension padded with spaces. Note especially that it's not null-terminated, mustn't be lower case, and doesn't include an explicit dot.

fat32_readdirent

Advances to the next entry in the directory, allowing for listing directories without knowing in advance what files they contain.

On success the carry is clear and zp\_sd\_address points at the directory entry in memory, in raw byte format used by FAT32.

Otherwise, if there are no more entries in the directory, then the carry is set.

fat32_opendirent

After finddirent or readdirent, this opens the active directory entry as referenced by zp\_sd\_address.

If the object is a directory, then subsequent calls to finddirent or readdirent will iterate over the subdirectory.

If the object is a file, the file access APIs can then be used to read its contents, and directory iteration APIs won't work any more.

fat32_file_read

Reads an entire file into memory, after it has been opened via opendirent above.

Pass the target address at fat32\_address.

The file size is rounded up to the next multiple of 512 bytes, so data in memory beyond the strict end of the file may also be overwritten.

Caveats

This is a minimal implementation to support loading files from SD cards, and a lot of corners have been cut:

• Only supports FAT32 - not earlier revisions
• The partition must be a type 12 primary partition
• The FAT32 sector size must be 512 bytes
• Long filenames are ignored
• Not much consistency checking is done

Note on exfat

I also have an exfat implementation but my understanding is that exfat is patented by Microsoft and cannot be freely shared. Exceptions were made for Linux implementations and members of the SD Consortium, but in general it is still not free to implement, so I can't share that code.