FAT file system-FAT Volume Initialization

FAT16与FAT32磁盘配置解析

最新推荐文章于 2024-09-20 18:48:39 发布

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#initialization #file #table #up #disk #microsoft

本文介绍了FAT16和FAT32文件系统的磁盘配置参数及其计算方法，包括不同磁盘大小对应的扇区数和簇大小。详细解释了如何确定合适的磁盘大小及扇区数，以确保文件系统正常运行。

struct DSKSZTOSECPERCLUS {

DWORD DiskSize;

BYTE SecPerClusVal;

};

*This is the table for FAT16 drives. NOTE that this table includes

* entries for disk sizes larger than 512 MB even though typically

* only the entries for disks < 512 MB in size are used.

* The way this table is accessed is to look for the first entry

* in the table for which the disk size is less than or equal

* to the DiskSize field in that table entry. For this table to

* work properly BPB_RsvdSecCnt must be 1, BPB_NumFATs

* must be 2, and BPB_RootEntCnt must be 512. Any of these values

* being different may require the first table entries DiskSize value

* to be changed otherwise the cluster count may be to low for FAT16.

DSKSZTOSECPERCLUS DskTableFAT16 [] = {

{ 8400, 0}, /* disks up to 4.1 MB, the 0 value for SecPerClusVal trips an error */

{ 32680, 2}, /* disks up to 16 MB, 1k cluster */

{ 262144, 4}, /* disks up to 128 MB, 2k cluster */

{ 524288, 8}, /* disks up to 256 MB, 4k cluster */

{ 1048576, 16}, /* disks up to 512 MB, 8k cluster */

/* The entries after this point are not used unless FAT16 is forced */

{ 2097152, 32}, /* disks up to 1 GB, 16k cluster */

{ 4194304, 64}, /* disks up to 2 GB, 32k cluster */

{ 0xFFFFFFFF, 0} /* any disk greater than 2GB, 0 value for SecPerClusVal trips an error */

};

* This is the table for FAT32 drives. NOTE that this table includes

* entries for disk sizes smaller than 512 MB even though typically

* only the entries for disks >= 512 MB in size are used.

* The way this table is accessed is to look for the first entry

* in the table for which the disk size is less than or equal

* to the DiskSize field in that table entry. For this table to

* work properly BPB_RsvdSecCnt must be 32, and BPB_NumFATs

* must be 2. Any of these values being different may require the first

* table entries DiskSize value to be changed otherwise the cluster count

* may be to low for FAT32.

DSKSZTOSECPERCLUS DskTableFAT32 [] = {

{ 66600, 0}, /* disks up to 32.5 MB, the 0 value for SecPerClusVal trips an error */

{ 532480, 1}, /* disks up to 260 MB, .5k cluster */

{ 16777216, 8}, /* disks up to 8 GB, 4k cluster */

{ 33554432, 16}, /* disks up to 16 GB, 8k cluster */

{ 67108864, 32}, /* disks up to 32 GB, 16k cluster */

{ 0xFFFFFFFF, 64}/* disks greater than 32GB, 32k cluster */

};

RootDirSectors = ((BPB_RootEntCnt * 32) + (BPB_BytsPerSec – 1)) / BPB_BytsPerSec;
TmpVal1 = DskSize – (BPB_ResvdSecCnt + RootDirSectors);
TmpVal2 = (256 * BPB_SecPerClus) + BPB_NumFATs;
If(FATType == FAT32)
    TmpVal2 = TmpVal2 / 2;
FATSz = (TMPVal1 + (TmpVal2 – 1)) / TmpVal2;
If(FATType == FAT32) {
    BPB_FATSz16 = 0;
    BPB_FATSz32 = FATSz;
} else {
    BPB_FATSz16 = LOWORD(FATSz);
    /* there is no BPB_FATSz32 in a FAT16 BPB */
}

Do not spend too much time trying to figure out why this math works. The basis for the computation is complicated; the important point is that this is how Microsoft operating systems do it, and it works. Note, however, that this math does not work perfectly. It will occasionally set a FATSz that is up to 2 sectors too large for FAT16, and occasionally up to 8 sectors too large for FAT32. It will never compute a FATSz value that is too small, however. Because it is OK to have a FATSz that is too large, at the expense of wasting a few sectors, the fact that this computation is surprisingly simple more than makes up for it being off in a safe way in some cases.