Microsoft KB Archive/65541

= How to Interpret 12-Bit FATs =

Article ID: 65541

Article Last Modified on 11/26/2003

-

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

-



This article was previously published under Q65541



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



MORE INFORMATION
=======================================================================

DESCRIPTION OF 12-BIT FAT FIELD ENTRIES

=
========================================================== 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.</ol>

<pre class="fixed_text">=======================================================================

HOW TO LOCATE AND INTERPRET A FAT FIELD ENTRY

=
========================================================== 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.

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: <pre class="fixed_text">  (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

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

Additional query words: 3.20 3.21 3.30 3.30a 4.00 4.01 5.00

Keywords: KB65541

-

[mailto:TECHNET@MICROSOFT.COM Send feedback to Microsoft]

© Microsoft Corporation. All rights reserved.