FAT file system-FAT Volume Initialization

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.