Atari MEGA STe Technical Information download pdf (Page 42)

Chapter 9. Information about TOS Partitions

In this chapter we will describe the layout and various information concerning the Atari Hard Disks TOS

partitioning as defined in the AHDI 3.00 specification.

Compared to the initial Atari AHDI specification, AHDI 3.00 adds support for hard disks with more than four

partitions, and for partitions of size greater or equal to 32 MB (16 MB if TOS < 1.04).

9.1 TOS Hard Disk Layout

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

 The Root Sector (RS) defines the number of partitions and their positions on the disk.

 The optional Bad Sector List contains the list of bad sectors detected during low level formatting on

the disk. This is not used anymore on “modern” drive (SCSI / IDE / SD Card…).

 One or up to 4 partitions. There are two kinds of partitions defined in AHDI 3.0 specification:

standard partitions and extended partitions:

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

partitions, but most of its content is the actual data. AHDI defines two types of standard

partitions: regular partition (GEM) or big partition (BGM a partition whose size is ≥ 32MB).

 An extended partition is a special partition that contains standard partitions.

9.2 TOS Root Sector

The Root Sector (RS) of a TOS File System is always the first 512-byte sector (Physical Sector 0) of

a partitioned data storage device such as a hard disk. This is equivalent to the Master Boot Record in

the FAT file System. The Root Sector contains:

 The disk's primary partition table, with one or several entries (up to 4) for the standard partitions.

This partition table may also contain one entry for an extended partition.

 And eventually some bootstrapping code (also called the IPL).

By definition, there are exactly four possible entries in the primary partition table of the Root Sector.

The partition size and the partition start address are stored as 32-bit quantities. Because the physical

sector size is always 512 bytes, this implies that neither the maximum size of a partition nor the

maximum start address (both in bytes) can exceed 2^32 * 512 bytes, or 2 TB.

The content of the Root sector is described in the following table:

Offset

Length

Description

$0000

440

Boot loader code for a bootable disk.

Not used and usually filled with 0 for a non-bootable disk

$1B6

Cylinders

$1B8

Heads

$1B9

$00 = SASI

$FF = SCSI

$1BA

Write pre-compensation cylinder

$1BC

Reduced write current cylinder

$1BE

Parking cylinder offset

$1BF

Step rate

$1C0

Interleave

$1C1

Sectors per track

$01C2

Hard Disk Size in number of physical (512 bytes) sectors

$01C6

4 * 12

Table for the 4 possible partitions described by four 12-byte partitions entry

(described below) starting at location $01C6 , $01D2, $01DE, $01EA

$01F6

Bad sectors list offset from beginning of disk. Specified in number of physical sectors.

$01FA

Bad sectors count in number of physical sectors

$01FE

Checksum

The grayed information is historical for very old drive, and is not used on “modern” drives.

The last word in the Root Sector (at offset $1FE) is reserved for the sector checksum. To be

executable a Root Sector checksum must be equal to the magic number $1234.

1 2 ... 37 38 39 40 41 42 43 44 45 46 47 ... 68 69

Comments to this Manuals

No comments

Atari MEGA STe Technical Information Page 42

Comments to this Manuals

Related products and manuals for PC/workstation barebones Atari MEGA STe