Apple Partition Map

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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.[1]

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.[2] Because APM allows 32 bits worth of logical blocks, the historical size of an APM formatted disk using small blocks[3] is limited to 2 TiB.[4]

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.

Intel-based Macs[edit]

For more details on this topic, see Apple–Intel transition.

For accessing volumes, both APM and GUID partitions can be used in a standard manner with Mac OS X 10.4 and higher. For starting an operating system, PowerPC-based systems can only boot from APM disks[5] whereas Intel-based systems generally only boot from GUID disks.[1]

However, Intel-based models that came with Mac OS X Tiger or Leopard preinstalled are able to boot from both APM and GUID disks. This is due to the installation media for these versions of Mac OS X, which are APM partitioned in order to be compatible with PowerPC-based systems.[6] Nevertheless, the installation of Mac OS X on an Intel-based Mac demands a GUID partitioned disk or will refuse to continue. Apple never officially supported booting from an internal APM disk on an Intel-based system, but there is an official Apple document describing how to set up a dual bootable external APM disk for use with PowerPC and Intel.[7]

Layout[edit]

Each entry of the partition table is the size of one data block, which is normally 512 bytes.[1][8] Each partition entry on the table is the size of one block or sector of data. Because the partition table itself is also a partition, the size of this first partition limits the number of entries to the partition table itself.

The normal case is that 64 sectors (64 × 512 = 32 KB) are used by the Apple Partition Map: one block for the Driver Descriptor Map as Block 0, one block for the partition table itself and 62 blocks for a maximum of 62 data partitions.[9]

Each partition entry includes the starting sector and the size, but also a name, a type, a position of the data area and possible boot code. It also includes the total number of partitions in that partition table. This ensures that, after reading the first partition table entry, the firmware is aware of how many blocks more to read from the media in order to have processed every partition table entry. All entries are in big-endian byte-order.[citation needed]

Address Size
in bytes
Contents Required?
Decimal Hex
0 0x0000 1 signature1 (ASCII value "P") No
1 0x0001 1 signature2 (ASCII value "M") No
2–3 0x0002 2 reserved No
4–7 0x0004 4 number of partitions (total) Yes
8–11 0x0008 4 starting sector of partition Yes
12–15 0x000C 4 size of partition (in sectors) Yes
16–47 0x0010 32 name of partition (fixed ASCII right-side NULL padded) No
48–79 0x0030 32 type of partition (fixed ASCII right-side NULL padded) No
80–83 0x0050 4 starting sector of data area in partition No
84–87 0x0054 4 size of data area in partition (in sectors) No
88–91 0x0058 4 status of partition No
92–95 0x005C 4 starting sector of boot code No
96–99 0x0060 4 size of boot code (in sectors) No
100–103 0x0064 4 address of bootloader code No
104–107 0x0068 4 reserved No
108–111 0x006C 4 boot code entry point No
112–115 0x0070 4 reserved No
116–119 0x0074 4 boot code checksum No
120–135 0x0078 16 processor type (fixed ASCII right-side NULL padded) No
136–511 0x0088 376 reserved No

Partition identifiers[edit]

Types beginning with "Apple_" are reserved for assignment by Apple, all other custom defined types are free to use. However registration with Apple is encouraged.

Identifier / type Contents / file system Name (typical) Remarks
Apple_Boot bootloader MOSX_OF3_Booter, eXternal booter This boot partition is used by Mac OS X on New World Macs (Open Firmware 3.0 and greater) when the file system on the main partition is not supported by Open Firmware, like in a software RAID configuration or when using a HFS+ case-sensitive or a UFS file system. It contains BootX on an HFS filesystem.
Apple_Boot_RAID bootloader Raid Partition
Apple_Bootstrap NewWorld bootblock Although it is a general Open Firmware (New World) boot partition, it is specifically used by yaboot. It must be HFS formatted, so that it can be accessed by Open Firmware.
Apple_Driver device driver Macintosh Mac OS classic drivers partition
Apple_Driver43 SCSI Manager 4.3 device driver Macintosh Mac OS classic drivers partition
Apple_Driver43_CD SCSI CD-ROM device driver Macintosh Mac OS classic drivers partition
Apple_Driver_ATA ATA device driver Macintosh Mac OS classic drivers partition
Apple_Driver_ATAPI ATAPI device driver Macintosh Mac OS classic drivers partition
Apple_Driver_IOKit I/O Kit driver Macintosh Mac OS classic drivers partition
Apple_Driver_OpenFirmware Macintosh
Apple_Extra unused This identifier masks an unused partition map entry.
Apple_Free free space Extra This identifier masks free space as a partition map entry.
Apple_FWDriver FireWire device driver Macintosh Mac OS classic drivers partition
Apple_HFS Hierarchical File System Apple_HFS While normally a HFS or HFS+ volume for Mac OS and Mac OS X, it can also contain an MS-DOS formatted file system (File Allocation Table, which can be accessed by Mac OS and Mac OS X).
Apple_HFSX HFS Plus This partition contains a HFS+ volume without a HFS wrapper. HFSX was introduced with Mac OS X 10.3 and is only used in special cases, like case sensitive HFS+. HFSX is the standard partition type on Intel-based Macs (which use GUID instead of APM).
Apple_Loader SecondaryLoader Like Apple_Boot but on Old World Macs, it is used when Mac OS X is installed on a file system not readable by Open Firmware. This partition does not contain a filesystem—instead it contains the BootX machine code in XCOFF format. This partition type was discontinued with Mac OS X 10.3.
Apple_MFS Macintosh File System This partition is used by Mac OS for the Macintosh File System (MFS), which was introduced with the Macintosh 128K in 1984.
Apple_Partition_Map partition map Apple The partition map is also a partition of its own. It can vary in size depending on how many partitions it may contain.
Apple_Patches patches Patch Partition Mac OS classic patch partition
Apple_PRODOS ProDOS ProDOS file system
Apple_RAID RAID Apple_RAID_OfflineV2 This identifier marks a Mac OS X partition used in a software RAID configuration. It normally contains the same filesystems a regular Mac OS X installation would have, like HFS/HFS+ or UFS. The separate boot partition Apple_Boot is mandatory.
Apple_Rhapsody_UFS Unix File System This partition contains a Unix File System (UFS) used by the Apple Rhapsody operating system (a development name marking the transition from OPENSTEP to Mac OS X) and is also used by Mac OS X Server 1.0 through 1.2 v3.
Apple_Scratch empty This identifier marks an empty partition.
Apple_Second Second stage bootloader
Apple_UFS Unix File System This partition contains a Unix File System (UFS) and is used by Mac OS X, Mac OS X Server (Version 10.0 and newer) and various Unix-like operating systems.
Apple_UNIX_SVR2 A/UX, Unix Originally introduced for A/UX (Apple Unix operating system based on System V Release 2, hence SVR2) on the 68k, it was later reused for MkLinux which used the Extended file system. It is the standard partition identifier for many Unix-like operating systems, including Linux and NetBSD. It may contain any file system suitable for the installed operating system. If bootable, a file system that can be read by the Open Firmware bootloader from Apple_Bootstrap (e.g. yaboot) must be used.
Apple_Void ISO9660 padding A dummy partition map entry to ensure correct partition alignment on bootable media.
Be_BFS Be File System This partition contains a Be File System (BFS) and is normally used by BeOS.

Partition status[edit]

Value Description System
0x00000001 entry is valid A/UX
0x00000002 entry is allocated A/UX
0x00000004 entry in use A/UX
0x00000008 entry contains boot information A/UX
0x00000010 partition is readable A/UX
0x00000020 partition is writable A/UX, Macintosh
0x00000040 boot code is position independent A/UX
0x00000100 partition contains chain-compatible driver Macintosh
0x00000200 partition contains a real driver Macintosh
0x00000400 partition contains a chain driver Macintosh
0x40000000 automatically mount at startup Macintosh
0x80000000 the startup partition Macintosh

See also[edit]

References[edit]

  1. ^ a b c "Technical Note TN2166 - Secrets of the GPT". Apple. Retrieved 2013-02-24. "…a standard block size of 512 bytes… […] Apple did consider extending APM to support larger disks. However, as such a change would break all existing partitioning tools…" 
  2. ^ [1]
  3. ^ [2]
  4. ^ MacTech Magazine: Apple's Transition from Apple Partition Map to the GUID Partition Table by Criss Myers
  5. ^ http://lists.apple.com/archives/Macos-x-server/2006/Apr/msg01335.html
  6. ^ http://eshop.macsales.com/articles/booting-your-intel-mac-to-an-apm-formatted-drive
  7. ^ [3]
  8. ^ Rebe, René; Klaus, Susanne (r396). "Creating custom Linux solutions - Apple Partition Map". T2 System Development Environment. Retrieved 2013-02-24. "While the original intent was to handle various block sizes, in practice only 512 byte blocks are supported."  Check date values in: |date= (help)
  9. ^ "Yellow Dog Solutions - Attaching Firewire Disks to a Linux Box". Fixstars Corporation. Retrieved 2013-02-24. "Comment: The command pdisk -l /dev/sda shows a size of 63 blocks for the Apple_partition_map. There is a multitude of examples like this to be found on the internet." 


External links[edit]