Lab 11

Project 3 Requests

The FAT

How to get value of cluster N=4?

Formula from the whitepaper:

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FATOffset = 4*N;

ThisFATSecNum = BPB_ResvdSecCnt + (FATOffset / BPB_BytsPerSec);

ThisFATEntOffset = REM(FATOffset / BPB_BytsPerSec);

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Translated to functions of N

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ThisFATSecNum(N) = BPB_ResvdSecCnt + ((4*N) / BPB_BytsPerSec);

ThisFATEntOffset(N) = REM((4*N) / BPB_BytsPerSec);

The FAT

ReadFAT(4)

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int sector = ThisFATSecNum(4);

The FAT

ReadFAT(4)

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int sector = ThisFATSecNum(4);

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int offset = sector*BytsPerSec;

The FAT

ReadFAT(4)

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int sector = ThisFATSecNum(4);

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int offset = sector*BytsPerSec;

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offset += ThisFATEntOffset(4);

The FAT

ReadFAT(4)

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int sector = ThisFATSecNum(4);

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int offset = sector*BytsPerSec;

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offset += ThisFATEntOffset(4);

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fseek(...,offset, SEEK_SET);

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uint32_t someint;

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int value = fread(&someint, 4, 1, ...);

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return value;

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The FAT

WriteFAT(4, Val = 12345)

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int sector = ThisFATSecNum(4);

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int offset = sector*BytsPerSec;

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offset += ThisFATEntOffset(4);

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fseek(...,offset, SEEK_SET);

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fwrite(&val, 4, 1, ...)

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What's the point?

• Every file has a cluster associated with it in its directory entry
• When creating a file, how do you know which cluster's aren't used
• Solution: If ReadFAT(N) = 0, cluster N is free.
• Every cluster has M sectors (See BPB)
• Every sector has N bytes (See BPB)
• Max size of a file is M*N bytes?
• Obviously not, so what happens when the cluster is full
• Start writing to another empty cluster!

What's the point

• Start writing to another empty cluster!
• ... So if each directory entry only has one cluster associated with it, when reading the file, how d you know where the second cluster is?
• or third...or fourth...etc.
• When you pick a new (empty) cluster to continue a file in, record it in the fat

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FAT Examples

• If a file's first cluster is 147, and you need more space
• pick the first empty cluster you see
• N s.t. ReadFAT(N) == 0
• Assume you get 27
• Continue writing in 27...repeat until you write everything needed
• If you just wrote to cluster 97, and you do not need more space, WriteFAT(97,EOC)
• If you just deleted a file whose first cluster is C
• Set DirEntry.Name[0] = EMPTY_ENTRY (or 0)
• RecursiveDelete(C):
• int value = ReadFAT(C);
• WriteFAT(C, 0);
• if (value != EOC) RecursiveDelete(C);

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Next Week...

Forget about all of this for your holiday break

Questions/Requests?