FAT: Difference between revisions
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m FAT12/Little endian doesn't affect the storage, only it's interpretation. That distinction is made clearer, and the example was provided |
→FAT 12: add information about entries 0 and 1 |
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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. |
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. |
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The entries under index 0 and 1 are reserved. The zeroth entry is reserved because index 0 is used as value of other entries signifying that the given cluster is free. Zeroth entry has to hold value of the BPB_Media field from in the low 8 bits, and the rest of the bits have to be set to zero. For example, if BPB_Media is 0xF8, then the zeroth entry should hold the value 0xFF8. The first entry is reserved for the future and must to hold the value 0xFFF. |
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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. |
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. |
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