basic doubts in FAT32 structure

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Rushikesh Patil - 09 Sep 2003 15:35 GMT

Hello,
Recently I read the structure of FAT32. I am cleared with few things
about it.
1. What is cluster ? it says cluster is an allocation unit and has size of 4
bytes per cluster.
There are two fields BPB_SecPerClus and BPB_BytsPerSec. Usually
BPB_BytsPerSec has value of 512. And as stated above if cluster size is only
4 bytes what is this BPB_SecPerClus ? because sector (512 bytes) is larger
than cluster(4 bytes) so this value turns out to be 0. What exactly cluster
and sector relates to each other and to disk space. Can someone please
explain me this in terms of their bytes on the disk(locations on disk).

2. Location of root directory entry is now obtained using BPB_RootClus value
that indicates the cluster no that contains root directory entry. Again here
I am not clear how can this entry be stated in terms of disk location.

I might be asking very basic/stupid questions but please tolerate with
this.
Thanks in Advance,
Rushikesh.

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Rushikesh Patil.
Persistent Systems Pvt. Ltd.
cell: +(91)98226-39056
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Tim Slattery - 09 Sep 2003 17:36 GMT

>Hello,
> Recently I read the structure of FAT32. I am cleared with few things
>about it.
>1. What is cluster ? it says cluster is an allocation unit and has size of 4
>bytes per cluster.

A cluster is the smallest amount of disk space that the file system
can deal with. Even a one-byte file will have one cluster allocated
for it. In FAT32, clusters are usually 4KB. FAT32 partitions larger
than 8GB will have larger clusters (because the File Allocation Table
- the FAT - can keep track on only so many clusters).

>There are two fields BPB_SecPerClus and BPB_BytsPerSec. Usually
>BPB_BytsPerSec has value of 512. And as stated above if cluster size is only
>4 bytes what is this BPB_SecPerClus ?

As I explained above, cluster size is *never* 4 bytes. This field is
telling you how many 512-byte sectors are in each cluster. For a 4KB
cluster this number would be 8.

>2. Location of root directory entry is now obtained using BPB_RootClus value
>that indicates the cluster no that contains root directory entry. Again here
>I am not clear how can this entry be stated in terms of disk location.

The cluster number is the address within the partition. All files are
located by their cluster numbers.

Signature

Tim Slattery
MS MVP(DTS)
Slattery_T@bls.gov

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