Technical information

© Jean Louis-Guérin – V1.2a – September 2014 Page 51 / 69
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
12
-19)
65,517 (2
16
-19)
268,435,437 (2
28
-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
4
Optional Disk signature
0x01BC
2
Usually Nulls; 0x0000
0x01BE
0x01CE
0x01DE
0x01EE
4 * 16
Table of partitions: Four 16-byte entries, IBM Partition Table scheme
0x01FE
2
MBR Signature $AA55 in little-endian format