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Chapter 2. Types of Partition used on Atari

Before we detail the partitioning procedures for several hard disk drivers, we need to understand the

basic types of partitions used on Atari and their limitations.

2.1 TOS Partitions

This is the “native” type of partition used an Atari as described in the Atari AHDI documentation. It is

supported by all the Atari hard disk drivers and in fact some of the old drivers only support this type of

partition. This is the type of partitions that you want to use on Atari unless you plan to use the media

(HD or SD card) to transfer data between Atari and PC computers.

Remember that the maximum size for a partition depends on: the hard disk size, the hard disk driver,

the host adapter capability, and the TOS version.

With recent hard disk driver and host adapters (that is supporting the ICD extended commands set)

the maximum size for a partition is:

 Up to 256MB for TOS < 1.04

 Up to 512MB for TOS ≥ 1.04

 Up to 2GB for TOS ≥ 4.x (Falcon)

Contrary to widely spread belief, the boot partition (usually the first partition on disk) can be a big

partition (BGM) and therefore can use the maximum size supported by the TOS (For example 512MB

for 1.04 ≤ TOS < 4.x). The actual limitation to 32MB (GEM) for the boot partition does not come from

TOS/GEMDOS file system but from some hard disk driver. Only very old hard disk driver like SCSITools

and AHDI hard disk driver exhibit this limitation.

2.2 DOS/FAT Partitions

2.2.1 Type and Limit of DOS/FAT Partitions

The following table summarizes the characteristics of different types of DOS/FAT partitions that are of

interest for Atari users:

Partition Type

Fdisk

Size

Fat Type

Version

PRI DOS

0-15 MB

12 bits (FAT12)

MS-DOS 2.0

PRI DOS

16-32 MB

16 bits (FAT16A)

MS-DOS 3.0

EXT DOS

0-2 GB

n/a

MS-DOS 3.3

PRI DOS

32 MB-2 GB

16 bits (FAT16B)

MS-DOS 4.0

PRI DOS

32 MB-2 GB

16 bits (FAT16B)

Windows 95

EXT DOS

0-2 GB

n/a

Windows 95

PRI DOS

512 MB - 2 TB

32 bits (FAT32)

OSR2

EXT DOS

512 MB - 2 TB

32 bits (FAT32)

OSR2

2.2.2 Small DOS/FAT Partitions

By Small DOS partition we mean partitions with a size < 32MB. These partitions are referred as:

 Type $04 (aka FAT-16A) with a size of < 32MB

As we have seen previously, the TOS and FAT file systems do not handle large logical sector the

same way. As a result it is only possible to use a logical sectors size of 1024 for partitions that can be

accessed directly on both platforms. This results to a maximum size of 32MB (65536 * 512 bytes) for

compatible partitions. However we will see later that some solutions exist to overcome this limitation.

Some of the Atari hard disk drivers directly recognize the DOS/FAT partitions. This type of partition

can therefore be useful to transfer data between an Atari and a PC. But as the partition size is very

small (32MB) it is not well suited for large disks.

2.2.3 Large DOS/FAT Partitions

By large DOS partition we mean partitions with a size ≥ 32MB and < 2GB. These partitions are

referred as:

 Type $06 or $0E (aka FAT-16B) with a size range of 32MB – 2GB

 Type $05 or $0F (aka Extended FAT-16B) with a size range of 32MB – 2GB

Type 0x0E and 0x0F forces usage of LBA addressing instead of CHS addressing (recognized by Windows 95 and above).

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