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FAT32 Directory Entries and Operations Guide. Operating Systems CS3430 Sarah Diesburg. Outline. Directory entries Short-name and long-name entries Project 3 operations. Directory Entries. Directory Entries. Lists names of files and directories in a directory Types - PowerPoint PPT Presentation
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FAT32 Directory Entries and Operations Guide
Operating Systems
CS3430
Sarah Diesburg
Outline
Directory entries Short-name and long-name entries
Project 3 operations
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Directory Entries
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Directory Entries
Lists names of files and directories in a directory
Types Short-name directory entry Long-name directory entry
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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
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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
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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
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Long-name Directory Entry
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Long-name part 1
Long-name part 2
Short-name
Two long-name entries are needed to hold the file name
Long-name Directory Entry
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Long-name part 1
Long-name part 2
Short-nameShort name entry for the file must exist too
Long-name Directory Entry
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Long-name part 1
Long-name part 2
Short-name
Long-name entries for this file immediately precede short-name entry
Directory entries
Long-name entry for “fatgen103.pdf"
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Directory entries
Short-name entry for “fatgen103.pdf"
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Long-name Directory Entries You can ignore the long directory entries
Can just display the short names This makes the project easier
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“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?
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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
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Utility Operations
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FAT32 Utility Oprations
info open close size cd
ls read
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Utility recognizes the following built-in commands:
Precautions
File or directory must exist before performing operations on it
File must be open and flagged for reading before you attempt to read from it
Be sure you are reading from the right location Off by 1 byte can throw whole project off
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Handling Open Files
An open file is a file we allow I/O operations to be performed on read
To handle open files, we must maintain a table of files that are open
Opening Files
/] open FATINFO.TXT rw
If “FATINFO.TXT” is found in the current directory, open “FATINFO.TXT”
In order to find fatinfo.txt, you must be able to interpret the current directory and determine whether fatinfo.txt is part of that directory
Once you open fatinfo.txt, store its name in the open file table
open
1. Check if the file is already open
2. Check that the provided file name exists in the requested directory
3. If it exists, add the file to your open file table
close
1. Check that the file name provided exists in your open file table
2. If it does, remove that entry from your open file table
Closing Files
/] close FATINFO.TXT
Remove this file’s entry from the open file table
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ls
1. Make sure that provided directory name is directory and exists in requested directory
2. Seek first data cluster
3. Iterate through and print each directory entry in the cluster
4. If more directory entries left than first cluster can hold, seek next cluster and repeat 3
size
1. Check that provided file name exists in the requested directory
Can be accomplished by seeking through the clusters of the requested directory
2. If it does, extract the size information Pay attention to endianness!
cd
1. Check that provided directory name is a directory and exists
2. 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
I/O on Open 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
read
1. Make sure file name provided is in open-file table and flagged as read-capable
2. Check that the provided position is valid
3. Check that the requested number of bytes is valid
4. Seek to data cluster corresponding to the requested start position and begin reading
5. If more data to be read, seek the next clusters and repeat 4
Use Cases
Be sure to look at the Project 3 Use Cases document and make sure you code acts the same way.
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