FAT: Difference between revisions

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If "table_value" is greater than or equal to (>=) 0xFF8 then there are no more clusters in the chain. This means that the whole file has been read. If "table_value" equals (==) 0xFF7 then this cluster has been marked as "bad". "Bad" clusters are prone to errors and should be avoided. If "table_value" is not one of the above cases then it is the cluster number of the next cluster in the file.

Since FAT12 uses an entry size that is not evenly divisible by 8 bits, figuring out how to interpret the FAT ~~on a little-endian machine~~ can be slightly confusing. Consider two entries of 0x123 and 0x456 ~~back-to-back~~. ~~On a little-endian machine, the~~ first byte of the first entry is the bottom two nibbles (0x23) and the highest nibble goes into the bottom nibble of the second byte (0x?1). Since the next entry is now starting mid-byte, only the lowest nibble can fit in the byte (0x6?) and the two highest nibbles go into the next byte (0x45). Therefore the 2 entries back-to-back look like this: 0x23 0x61 0x45.

Since FAT12 uses an entry size that is not evenly divisible by 8 bits, figuring out how to interpret the FAT can be slightly confusing. Consider two successive entries with values 0x123 and 0x456. The first byte of the first entry is the bottom two nibbles (0x23) and the highest nibble goes into the bottom nibble of the second byte (0x?1). Since the next entry is now starting mid-byte, only the lowest nibble can fit in the byte (0x6?) and the two highest nibbles go into the next byte (0x45). Therefore the 2 entries back-to-back look like this: 0x23 0x61 0x45.

This placement might be confusing, but if we consider a little-endian machine it will start make more sense. If you load the WORD value at offset zero, the resulting value will be 0x6123. Now the nibbles are in correct order, so to get the value of the first of the two entries you just have to AND the value with 0xfff. For the second entry, you have to first load WORD at offset 1, resulting in the value of 0x4561 and shifting it down by 4 bits (effectively removing the bottom nibble).

==== FAT 16 ====

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 If "table_value" is greater than or equal to (>=) 0xFF8 then there are no more clusters in the chain. This means that the whole file has been read. If "table_value" equals (==) 0xFF7 then this cluster has been marked as "bad". "Bad" clusters are prone to errors and should be avoided. If "table_value" is not one of the above cases then it is the cluster number of the next cluster in the file.
-Since FAT12 uses an entry size that is not evenly divisible by 8 bits, figuring out how to interpret the FAT on a little-endian machine can be slightly confusing. Consider two entries of 0x123 and 0x456 back-to-back. On a little-endian machine, the first byte of the first entry is the bottom two nibbles (0x23) and the highest nibble goes into the bottom nibble of the second byte (0x?1). Since the next entry is now starting mid-byte, only the lowest nibble can fit in the byte (0x6?) and the two highest nibbles go into the next byte (0x45). Therefore the 2 entries back-to-back look like this: 0x23 0x61 0x45.
+Since FAT12 uses an entry size that is not evenly divisible by 8 bits, figuring out how to interpret the FAT can be slightly confusing. Consider two successive entries with values 0x123 and 0x456. The first byte of the first entry is the bottom two nibbles (0x23) and the highest nibble goes into the bottom nibble of the second byte (0x?1). Since the next entry is now starting mid-byte, only the lowest nibble can fit in the byte (0x6?) and the two highest nibbles go into the next byte (0x45). Therefore the 2 entries back-to-back look like this: 0x23 0x61 0x45.
+This placement might be confusing, but if we consider a little-endian machine it will start make more sense. If you load the WORD value at offset zero, the resulting value will be 0x6123. Now the nibbles are in correct order, so to get the value of the first of the two entries you just have to AND the value with 0xfff. For the second entry, you have to first load WORD at offset 1, resulting in the value of 0x4561 and shifting it down by 4 bits (effectively removing the bottom nibble).
 ==== FAT 16 ====

FAT: Difference between revisions

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