How does memory translation work in the FAT filesystem?

0
votes

I am required to create my own implementation of a filesystem in C. I am planning on creating a system similar to that of the FAT system. We are given one file of size 10MB, which acts as our own "disk." I understand that the FAT table stores cluster numbers, and the Root Directory stores other pertinent information about each file we create (e.g. file name, size, date and time of last modification, start block in FAT, etc.). But I am confused about how the cluster numbers are translated to physical addresses in the data region on the disk.

For example, let's say an entry in the Root Directory says that a file starts in block 100 in the FAT table, and in block 100 of the FAT table is the integer 327, which is where the next cluster of the file is located. How are these addresses translated to physical addresses in the data region of the disk? Where are these physical addresses translated and stored?

2
cfilesystemsfat32
no. the root directories stores information about files in the root directory. the FAT table stores metadata about your files. starting cluster, size, filename, blah blah blah. – Marc B
You might want to find an old DOS 3 or 4 era reference on how FAT worked, if you want to emulate that architecture. I agree with Marc, you don't quite have it right. – Zeiss Ikon

2 Answers

2
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Clusters vary in size between different versions of FAT (FAT12, FAT16, and FAT32), but in general the cluster number points to a consecutively numbered cluster of whatever size is present in the format for the existing file system. As I recall (from long ago) FAT12, at least on hard disks, used 2 kibibyte clusters (made up of four 512-byte sectors each), with a maximum cluster number of 2^11 (12 bits starting with zero), so cluster 327 would be 327 * 2048 bytes from the start of the data area of the disk.

The data area includes the FAT, backup FAT, and all directories. My recollection is that each cluster entry in the FAT contains a pointer to the next cluster in the file that occupies that cluster, length of data if it's the last cluster of the file, and some other information needed in reading or writing the file, while the directory entry contains the file name, first cluster, size/date/etc..

1
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A disk is divided into sectors. A hard disk for example has a sector size of 512 bytes. Addressing data on the disk usually uses these sectors and data is read/written in blocks of this size. The FAT filesystem groups a number of sectors into clusters. For example you could have 8 sectors per cluster. This constant is stored along with other information about the filesystem in the first few sectors of the partition. The FAT driver uses this value to compute the sector number from the cluster number. The formula is something like this:

SectorNumber = SectorsPerCluster * ClusterNumber + Constant

The constant is the sector number of the first sector of the data region of the partition. You can find the exact formula in the FAT Specification.