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”. Because APM allows 32 bits worth of logical blocks, the size of an APM formatted disk is limited to 2 TiB.
The Apple partition map is unusual in that it defines itself as one of the partitions on the disk, and it defines free space as a partition in the map as well. 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.
Read more about Apple Partition Map: Intel-based Macs, Partition Identifiers
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