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The table of partitions has four 16-byte structures to describe each partition:

Offset

Length

Description

Partition entry locations

0x00

Status

 0x80 = bootable (active),

 0x00 = non-bootable,

 other = invalid

$1BE, $1CE, $1DE, $1EE

0x01

CHS address of first sector in partition (next 3 bytes):

 Head

 Sector is in bits 5–0; bits 9–8 of Cylinder are in bits 7–6

 bits 7–0 of Cylinder

$1BF, $1CF, $1DF, $1EF

0x04

partition type

$1C2, $1D2, $1E2, $1F2

0x05

CHS address of last sector in partition (next 3 bytes):

 Head

 Sector is in bits 5–0; bits 9–8 of Cylinder are in bits 7–6

 bits 7–0 of Cylinder

$1C3, $1D3, $1E3, $1F3

0x08

LBA of first sector in the partition, in little-endian format

$1C6, $1D6, $1E6, $1F6

0x0C

Number of sector in partition, in little-endian format

$1CA, $1DA, $1EA, $1FA

10.4 DOS/FAT Primary Partition

A primary partition contains one FAT file system. The “partition type” code for a primary partition

describes the type of the file system. The FAT file systems have made use of quite a number of

partition type codes over time due to the limits of various DOS and Windows OS versions. Please refer

to FAT Partition Type and Size for a short summary of partition types useful in the context of the Atari

platform.

The following is an overview of the order of the structures in a primary FAT file system partition:

Boot

Sector

FS Information

Sector (FAT32

only)

reserved

sectors

(optional)

File

Allocation

Table #1

File

Allocation

Table #2

Root

Directory

(FAT12/16 only)

Data Region (for files and

directories) ...

(To end of partition or

disk)

size in

sectors

(number of reserved sectors)

(number of FATs) *

(sectors per FAT)

(number of root entries

* 32) / Bytes per sector

NumberOfClusters *

SectorsPerCluster

A FAT file system is therefore composed of these four sections:

 The Boot sectors region, located at the very beginning of the partition: The first reserved sector

(logical sector 0) is the Boot Sector. It includes an area called the BIOS Parameter Block (with

some basic file system information, in particular its type, and pointers to the location of the other

sections) and usually it contains the operating system's boot loader code. The total count of

reserved sectors is indicated by a field inside the Boot Sector. Important information from the

Boot Sector is accessible through a DOS structure called the BIOS Parameter Block (BPB). For

FAT32 file systems, the reserved sectors include a File System Information Sector, usually at

sector 1, and a Backup Boot Sector, usually at Sector 6. The exact location of these two sectors

is specified in the Extended FAT32 BPB.

 The FAT region: This typically contains two copies (may vary) of the File Allocation Table for the

sake of redundancy checking, although the extra copy is rarely used, even by disk repair utilities.

These are maps of the Data region, indicating which clusters are used by files and directories.

 The Root Directory region: This is the Directory Table that stores information about the files and

directories located in the Root Directory. It imposes on the Root Directory a fixed maximum size

which is pre-allocated at creation of this volume.

 The Data region: This is where the actual file and directory data is stored and takes up most of the

partition. The size of files and subdirectories can be increased arbitrarily (as long as there are free

clusters) by simply adding more links to the file's chain in the FAT

1 2 ... 47 48 49 50 51 52 53 54 55 56 57 ... 68 69

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