Technical information
© Jean Louis-Guérin – V1.2a – September 2014 Page 8 / 69
Chapter 1. Partitioning Information
This section provides basic information about Atari & PC hard disk partitioning (layout). For technical
details on the TOS and FAT file systems please refer to the second part of this document.
1.1 Hardware Consideration
Atari ST / STE computers provide an external DMA bus connection through the Atari Computer
System Interface (ASCI) connector. This bus is very similar (simplified version) to the standard SCSI
bus and allows connection of different type of devices such as hard disks.
Atari Mega STe, TT, Falcon computers provide direct support for IDE and/or SCSI Hard disk buses.
The disk drives are connected to Atari ST ASCI bus through a Host Adapter. The host adapter acts
as an interface between the Atari DMA bus and the drive controller. For example a SCSI disk can be
connected to the ASCI bus through an ICD AdSCSI Plus host adapter, or an SD card can be
accessed through a CosmosEx Device that acts as a host adapter.
When several devices are connected to an ASCI bus you must assign a unique ID to each of them.
The maximum usable size of a hard disk is limited by several factors:
The size of the hard disk itself,
The Hard Disk Driver software,
The File System limitations, and
The Host Adapter capability.
We can differentiate two families of host adapters:
The host adapters that strictly implement the ASCI-AHDI command set (corresponding to the
SCSI group 0 command set). With this type of host adapter the maximum size of the drive is
limited to 1GB (2^11 sectors of 512 bytes). For examples: The ICD Link I, the Satan Drive.
The host adapters that implement the ICD extended command set (corresponding to the SCSI
group 1 command set). With this type of adapter the maximum size of the drive is lifted to 2TB
(2^32 sectors of 512 bytes). For examples: The ICD Link II, the CosmosEx device.
To access drives bigger than 1GB you need to have a host adapter and a hard disk driver that both
understand the ICD extended command set.
1.2 Hard Disk File System Primer
The Atari ST/STE platform uses natively the TOS file system as defined in the Atari AHDI 3.0
document. The PC platform uses a wide variety of file systems but in this document we will only look
at the DOS/FAT file system that can be used on an Atari platform.
While both of these file systems look similar they
are different. Therefore we need to have a basic
comprehension of both file systems in order to
appreciate their limitations and incompatibilities.
Detailed technical information on TOS, DOS file
systems can be found in Part II of this document.
The picture on the right shows an example of
layout for a partitioned and initialized hard disk
with three primary partitions and an extended
partition that contains two primary partitions (see
9.4.2 for more details).
An already partitioned and initialized disk is
composed of:
The Reserved area: containing the Master
Boot Record (MBR), located at physical
sector 0, and followed by reserved sectors.
The MBR defines the number of partitions
and their positions on the disk.
The Partition area: containing from one to 4 partitions (primary or extended) with the actual data.