Apple Partition Map

Apple Partition Map (APM) is a partition scheme used to define the low-level organization of data on disks formatted for use with 68k and PowerPC Macintosh computers. It was introduced with the Macintosh II.

Disks using the Apple Partition Map are divided into logical blocks, with 512 bytes usually belonging to each block. The first block, Block 0, contains an Apple-specific data structure called “Driver Descriptor Map” for the Macintosh Toolbox ROM to load driver updates and patches before loading from a MFS or HFS partition. Because APM allows 32 bits worth of logical blocks, the historical size of an APM formatted disk using small blocks is limited to 2 TiB.

The Apple Partition Map maps out all space used (including the map) and unused (free space) on disk, unlike the minimal x86 master boot record that only accounts for used non-map partitions. This means that every block on the disk (with the exception of the first block, Block 0) belongs to a partition.

Some hybrid discs contain both an ISO 9660 primary volume descriptor and an Apple Partition Map, thus allowing the disc to work on different types of computers, including Apple systems.

For accessing volumes, both APM and GUID partitions can be used in a standard manner with Mac OS X Tiger (10.4) and higher. For starting an operating system, PowerPC-based systems can only boot from APM disks whereas Intel-based systems generally boot from GUID disks. Nevertheless, Intel-based Macs are able to boot from APM, GPT (GUID Partition Table) and MBR (Master Boot Record, using the BIOS-Emulation called EFI-CSM i.e. the Compatibility Support Module provided by EFI).

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