Master Boot Record (MBR)

Media for the Atari used to have a root sector in the old MBR format. Up to 4 partitions are defined in the root sector, all other definitions are scattered across the disk in special structures and are inconvenient to handle. This format is described in the AHDI specification. An MBR supports partitions only on the first 2 TiB of a medium, since the sector numbers are limited to 32 bit. Naturally, the MBR format is fully supported by HDDRIVER and HDDRUTIL.

GUID Partition Table (GPT)

With a GUID partition table, all partition definitions are located at the beginning of a medium, which makes handling comparatively simple. At the end there is a backup of these data. The integrity of all data is secured by checksums. Media with a capacity of more than 2 TiB absolutely require 64 bit sector numbers, and therefore a GPT scheme, in order to use their full capacity. Thanks to native GPT support HDDRIVER handles GPT partitions without any problems and can boot from them. With HDDRUTIL you can partition in GPT format, also for other platforms than the Atari.

Advantages and disadvantages of a GPT

Just like any other platform, the Atari naturally also benefits from a GPT.

• Simple data structures, since the data is not scattered across the disk.
• High compatibility with other platforms, since GPTs are platform-independent.
• GPTs are future-proof in case just like Linux also Windows or macOS introduce a a strict validation of the root sector, so that TOS/Windows compatible MBR partitions stop working.
• Linux recognizes TOS partitions immediately. With FAT16 filesystems Linux supports up to 4096 bytes per logical sector, which means that it can directly mount TOS partitions up to 256 MiB.
• Partitions of any operating system can coexist on the same medium.
• Increased data security through a mandatory backup of the complete partition table. HDDRUTIL can restore this table including the root sector, also for other platforms.
• Checksums ensure the integrity of the partition data. This allows HDDRUTIL or other tools to detect corrupted partition tables.
• Tools for other platforms can, like HDDRUTIL, repair a GPT for the Atari, e.g. fdisk.
• Based on standardized attributes, HDDRIVER detects whether partitions are read-only or hidden.
• GPT partitions have a 36-character name, which can be edited with HDDRUTIL or tools on other platforms.
• A quick backup of the partition data can be easily obtained by saving the first 34 sectors to a file (e.g. on a floppy disk) using HDDRUTIL.
• Future versions of HDDRUTIL will allow to conveniently edit properties of GPT partitions. Furthermore, it will be possible to add and remove partition entries, without losing data of unaffected partitions.

• GPTs have been around for over 25 years. They are standard, but not exactly rocket science. With old, unmaintained drivers (AHDI, CBHD, HuSHI, ICDBOOT, ...), one naturally cannot benefit from a GPT.

You should not only consider a GPT for data exchange with other platforms. Even only the higher data security (recovering the complete partition table) alone can speak in favor of a GPT.

Mapping of GPT partition types to MBR and XHDI types

MBR partition types on the Atari consist of three characters, usually uppercase (exception: "F32"). On DOS/Windows each MBR type consists of one byte. GPT partition types are represented by a 16-byte platform independent unique ID (GUID). (This GUID for the type must not be confused with the GUID for the partition as such.) GUIDs for partition types are mapped to the types used on the Atari as follows. Depending on the partition type HDDRIVER supports the standard attributes "legacy BIOS bootable" (corresponds to the MBR flag "active"), "hidden" and "read-only".

When partitioning with a GPT scheme, HDDRUTIL automatically uses the GPT types corresponding to the MBR types, so that you do not have to rethink. Partitioning is done as usual, and the familiar MBR types always appear externally. There are no other visible changes during normal operation either.