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Chapter 10. Information about DOS/FAT Partitions

In this chapter I describe the layout and various information concerning the DOS/FAT Hard Disks partitioning.

PC hard disk partitioning is a vast subject and I will only present here information that can be useful in the

context of its usage on Atari.

 The layout of PC DOS hard disks is similar but not identical to layout of Atari hard disks.

10.1 DOS/FAT File System Information

This information is taken from Wiki File Allocation Table Article

FAT

Full Name

File Allocation Table

(12-bit version)

(16-bit version)

(32-bit version)

Partition identifier

0x01

0x04, 0x06, 0x0E, 0x0F

0x0B, 0x0C

Directory contents

Table

File allocation

Linked List

Max file size

4 GB minus 1 byte (or volume size if smaller)

Max cluster count

4,077 (2

-19)

65,517 (2

-19)

268,435,437 (2

-19)

Max filename size

8.3 filename, or 255 UTF-16 characters when using LFN

Max volume size

32 MB

2 GB (up to 4GB)

2 TB (up to 8 TB)

Dates recorded

Creation, modified, access (accuracy today only)

Date range

January 1, 1980 - December 31, 2107

File Attributes

Read-only, hidden, system, volume label, subdirectory, archive

10.2 DOS/FAT Hard Disk Layout

Partitioning and Initialization of the disk write information that defines the layout of the disk:

 The Master Boot Record (MBR) defines the number of partitions and their positions on the disk.

 The Reserved Sectors is optional. However, for historical reason, a partition on a FAT file system is

aligned on a cylinder boundary (Cylinder 0, head 1, Sector 1 in CHS notation). The 62 (usual value)

sectors gap between them is left unused. This is not required with LBA drives, but we need to

follow this rule in order to make happy old software (MS-DOS for example).

 One or several partitions.

There are two types of partitions: primary partitions and extended partitions:

 A primary partition contains a number of control structures, necessary to describe the partitions,

but most of its content is the actual data.

 An extended partition is a special kind of partition which itself is subdivided into primary

partitions.

10.3 DOS/FAT Master Boot Record (MBR)

The first physical sector 0 on a disk on a hard disk contains the Master Boot Record structure (this

equivalent to the Atari Root Sector):

Offset

Length

Description

0x0000

440

Boot loader code. Filled with zero if disk is not bootable.

0x01B8

Optional Disk signature

0x01BC

Usually Nulls; 0x0000

0x01BE

0x01CE

0x01DE

0x01EE

4 * 16

Table of partitions: Four 16-byte entries, IBM Partition Table scheme

0x01FE

MBR Signature $AA55 in little-endian format

1 2 ... 46 47 48 49 50 51 52 53 54 55 56 ... 68 69

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