Apple Partition Map was introduced in 1986. It was developed in keeping intension of handling drives of sizes up to more than a few hundred megabytes. Whereas original objective was to handle a variety of block sizes, in practice only 512-byte blocks are supported. the several address fields are 32 bits unsigned which means the format can handle disks up to two Terabytes in size.
The Apple Partition Map used by disk is divided into logical blocks with 512 bytes. In that first block is Block 0, which contains an Apple- specific data structure called “Drive Descriptor Map” for the Macintosh Toolbox Read Only Memory to load driver updates and patches before loading from a MFS or HFS partition.
In disk, Block 1 is the second block, which contains the Apple Partition Map that describes the partition structure of drive and contains each partition entries. These partition entries can be in any precise order and do not need to correspond to the physical group of the partitions. There is no restriction for number of partition entries but the map could not be enlarged later. The APM is created when drive has been partitioned and the remaining space on the drive belongs to the remaining partitions. Afterwards, no free space is available to enlarge the Apple Partition Map. Moreover, the APM maps out all space used and unused on disk, unlike the least x86 master boot record that only accounts for used non-map partitions only. This means that every block on the disk belongs to a partition except block 0.
The APM partitions are located at the beginning of the disk and described in the partition map structure. This structure is processed by the code of the firmware, so the map does not contain boot code as in the DOS partition able. Each partition map entry describes the starting sector of the partition, the type, the size and the volume name. The data structure also holds values about data inside of the partition, such as the location of any boot code and the location of the data area.
Reasons behind corruption of Apple Partition corruption