1. FAT32 Directory Entries and Operations Guide
Operating Systems
CS3430
Sarah Diesburg

2. Outline Directory entries Project 3 operations
Short-name and long-name entries
Project 3 operations

3. Directory Entries

4. Directory Entries Lists names of files and directories in a directory
Types
Short-name directory entry
Long-name directory entry

5. Short-name Directory Entry
Limits name size to 8 bytes with additional 3 bytes after “.”
Compatible with previous FAT versions
32 bytes total size
Holds important information about file or dir:
Attributes, timestamp, last access date, first cluster number, size

6. Short-name Directory Entry
If DIR_Name[0] == 0xE5, then the directory entry is free (no file or directory name in this entry)
If DIR_Name[0] == 0x00, then the directory entry is free (same as for 0xE5), and there are no allocated directory entries after this one

7. Long-name Directory Entry
Backwards-compatible way to allow longer names to be displayed
Each long-name directory entry is 32 bytes
A long file name can cover a set of long-name directory entries
Each set of long-name directory entries must correspond to a short-name directory entry
Long-name entries must immediately precede corresponding short-name entry

8. Long-name Directory Entry
Long-name part 1
Two long-name entries are needed to hold the file name
Long-name part 2
Short-name

9. Long-name Directory Entry
Long-name part 1
Long-name part 2
Short name entry for the file must exist too
Short-name

10. Long-name Directory Entry
Long-name part 1
Long-name entries for this file immediately precede short-name entry
Long-name part 2
Short-name

11. Directory entries
Long-name entry for “fatgen103.pdf"

12. Directory entries
Short-name entry for “fatgen103.pdf"

13. Long-name Directory Entries
You can ignore the long directory entries
Can just display the short names
This makes the project easier

14. “Dot” Entries
All directories (except root directory of entire system) have “.” and “..” directory entries
“.” means “this directory”
“..” means “the parent directory”
Why do you think the root directory does not have these entries?

15. Directory Entry Attributes
Offset 11 in directory entry, 1 byte long
ATTR_READ_ONLY 0x01
ATTR_HIDDEN 0x02
ATTR_SYSTEM 0x04
ATTR_VOLUME_ID 0x08
ATTR_DIRECTORY 0x10
ATTR_ARCHIVE 0x20
ATTR_LONG_NAME
ATTR_READ_ONLY | ATTR_HIDDEN | ATTR_SYSTEM | ATTR_VOLUME_ID

16. Checking the Image…
You can tell your OS to “mount” the image just like a real file system
$> sudo mount –o loop fat32.img /mnt
$> cd /mnt
$> sudo umount /mnt
Be sure you cd out of the /mnt directory first

17. Checking the Image Mac computer automatically mount the image
They also *modify* it
Add Trash can directory
Add other files
Your test image may become different than other people’s images

18. Utility Operations

19. FAT32 Utility Oprations
Utility recognizes the following built-in commands:
info
stat
volume cd ls
read

20. Precautions
File or directory must exist before performing operations on it
Be sure you are reading from the right location
Off by 1 byte can throw whole project off

21. Volume Volume ID string exists in two places Can get it either place
Boot sector
Root directory
Can get it either place

22. stat Check that provided file name exists in the requested directory
Can be accomplished by seeking through the clusters of the requested directory
If it does, extract the size information
Pay attention to endianness!

23. ls
Make sure that provided directory name is directory and exists in requested directory
Seek first data cluster
Iterate through and print each directory entry in the cluster
If more directory entries left than first cluster can hold, seek next cluster and repeat 3

24. cd Check that provided directory name is a directory and exists
Alter your current working directory to reflect the change
For ease of debugging and use, you may want to alter your prompt to show current working directory

25. I/O Files /] read FATINFO.TXT 0 100
Only allow the read if fatinfo.txt is open
In order to read fatinfo.txt, you must look into the open file table, look up the address of fatinfo.txt (or store it with its name), and read enough of the data clusters to satisfy the read request

26. read
Make sure file name provided is in open-file table and flagged as read-capable
Check that the provided position is valid
Check that the requested number of bytes is valid
Seek to data cluster corresponding to the requested start position and begin reading
If more data to be read, seek the next clusters and repeat 4

27. Other Things
Be sure to look at the Project 3 Use Cases document and make sure you code acts the same way.
You must read the Microsoft FAT32 Specification to understand what’s going on
Be sure your code works on the class virtual machine
You may need to boot into the original kernel