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How to Interpret 12-Bit FATs

Article ID: 65541 - View products that this article applies to.

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SUMMARY

The following is a description of 12-bit FAT field entries, followed by an explanation of how to locate and interpret them.

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               DESCRIPTION OF 12-BIT FAT FIELD ENTRIES

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The following are some definitions and explanations of FAT field entries:

Twelve-bit FAT entries were used exclusively in DOS versions 1.x and 2.x. They are also used in versions 3.00 and later if the medium contains less than 4087 clusters. Media containing 4087 or more clusters use 16-bit FAT fields in DOS versions 3.00 and later. Each field corresponds directly to an assignable cluster on the disk.

Several FAT field entries have fixed meanings. They are defined as follows:

      12-Bit Entry     Meaning
      ------------     -------

        000h           Free cluster
        001h           Unused code
      FF0-FF6h         Reserved values
        FF7h           Bad cluster
      FF8-FFFh         Last cluster of file

(Any FAT field entry not found in the table above should be considered the next FAT field number in the chain of the file to be examined.)

Also, the first two fields or first three bytes (FAT field numbers 0 and 1) in the FAT are always reserved. The first byte contains a copy of the media descriptor byte and the two remaining bytes contain 0FFh.
A broad view of a 12-bit FAT is as follows: if the FAT were divided into groups of three bytes each, each group would contain two FAT field entries.
Finally, a FAT field number (n) refers to the nth field in the FAT, not the nth byte in the FAT. A FAT entry is the value stored in the field being examined.

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           HOW TO LOCATE AND INTERPRET A FAT FIELD ENTRY

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Twelve-bit FATs are set up so that if the FAT field number is even, it must be interpreted differently than if it is an odd field number.

HOW TO INTERPRET AN EVEN FAT FIELD NUMBER

Formula

Multiply the FAT field number by 1.5. Read the next two bytes at the resulting offset in the FAT. Strip off the last four bits from the second byte and shift the result left 8 bits. OR the resulting value with the first byte. This will give you the entry stored in the FAT field. This entry could be a link or a last cluster marker depending on the value.

Example

You are given the FAT field number 306 (132h).

First, perform the following calculations:

(FAT field number) * 1.5 = FAT byte offset (N)

132h * 1.5 = 1CBh or 459

Read the 459th (N) and 460th (N + 1) bytes in the FAT table. Suppose they contain the values 33h and 41h, respectively. The next field in the chain would be as follows:

   (byte N) OR ((strip last 4 bits of byte N + 1) shift left 8 bits) =
                                                 Next FAT field number

   33h | ((41h & 0Fh) << 8) = 133h or the 307th field in the FAT

HOW TO INTERPRET AN ODD FAT ENTRY NUMBER

Formula

Once again, multiply the FAT field number by 1.5 and floor it down to the next integer less than the result. Read two bytes at the resulting offset. Take the second byte and shift it left four bits. Then, shift the first byte right four bits and AND the two together. The result is the entry for this FAT field.

Example

You are given the FAT field number 307 (133h).

Multiply and floor it:

floor ((FAT field number) * 1.5) = FAT byte offset (N)

floor (307 * 1.5) = 1CCh or 460

Read bytes 460 (N) and 461(N + 1) from the FAT. In this example, they contain the values 41h and 14h, respectively. To interpret the entry perform the following:

(byte N >> 4) OR (byte N + 1 << 4) = FAT Entry

(41h >> 4) | (14h << 4) = 144h or the 324th field in the FAT

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Properties

Article ID: 65541 - Last Review: November 26, 2003 - Revision: 3.0

APPLIES TO

Microsoft MS-DOS 3.1
Microsoft MS-DOS 3.2 Standard Edition
Microsoft MS-DOS 3.21 Standard Edition
Microsoft MS-DOS 3.3 Standard Edition
Microsoft MS-DOS 3.3a
Microsoft MS-DOS 4.0 Standard Edition
Microsoft MS-DOS 4.01 Standard Edition
Microsoft MS-DOS 5.0 Standard Edition

KB65541

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This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.

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