.ADF File Format – Hard disk images

Amiga Disk File

A floppy disk holds one AmigaDOS volume that starts at block 0. An Amiga hard disk can hold several, and describes them in a small table at its start, the Rigid Disk Block (RDB). An image of such a disk begins with these blocks (it is often given the extension .hdf, which is also the extension of unrelated HDF4 and HDF5 files; that naming is our own observation, not from a source read here), and each partition then begins with an ordinary boot block of its own. A hardfile, by contrast, is “a file which contains an Amiga volume” and is described by Clévy’s section 7 as “a big floppy dump file”: it has no RDB; its section was not read in full for this page.

There are five kinds of block. The RDSK block must be in the first 16 blocks of the disk (RDB_LOCATION_LIMIT in the AROS header), counted in 512-byte sectors before the block size is known. It holds the geometry and the heads of three linked lists: partitions (PART), file system code (FSHD, then LSEG blocks) and bad blocks (BADB). Every block has the same first four longs, all big-endian:

OffsetSizeNameDescription
0x004idFour letters: RDSK, PART, FSHD, LSEG or BADB.
0x044size in longsHow many longs the checksum covers: 64 for RDSK, PART and FSHD (256 bytes), the whole block (128 for 512-byte blocks) for LSEG and BADB.
0x084checksumChosen so that the covered longs add up to zero (ignoring overflow), the same rule as the root block, but the sum is at offset 8 and not 0x14.
0x0C4host idSCSI target id of the host controller, 7 for IDE and ZIP disks.

A list pointer of $FFFFFFFF (−1) means “none” or “last”. Block numbers in the RDB are counted from the start of the disk; block numbers inside a partition are counted from the first block of that partition. In PART, FSHD, LSEG and BADB the next block of the list is the long at 0x10.

RDSK: the Rigid Disk Block

OffsetSizeNameDescription
0x104block sizeBytes per block, usually 512; “can be other powers of 2”.
0x144flagsUsually $17. Bit 0 no more disks on this controller, 1 no more LUNs, 2 no more target ids, 3 do not try reselection, 4 disk id valid, 5 controller id valid, 6 synchronous SCSI (dangerous if the drive cannot).
0x184bad block listFirst BADB block, or −1.
0x1C4partition listFirst PART block, or −1.
0x204file system header listFirst FSHD block, or −1.
0x244drive init codeOptional drive-specific initialisation code, called as DriveInit(lun, rdb, ior) (Clévy), or −1.
0x2824reservedSix longs, −1.
0x404cylindersNumber of cylinders.
0x444sectorsSectors per track.
0x484headsNumber of heads.
0x4C4interleave
0x504parkLanding-zone cylinder, soon after the last one.
0x5412reservedThree longs, 0.
0x604write precompensationStarting cylinder.
0x644reduced writeStarting cylinder for reduced write current.
0x684step rate
0x6C20reservedFive longs, 0.
0x804RDB blocks, lowFirst block of the range reserved for these blocks.
0x844RDB blocks, highLast block of that range, inclusive.
0x884low cylinderFirst cylinder of the partitionable area.
0x8C4high cylinderLast cylinder of the partitionable area, inclusive.
0x904blocks per cylinderHeads × sectors.
0x944auto park seconds0 for no auto-park.
0x984highest RDSK blockThe highest block used by these blocks (not counting replacement bad blocks).
0x9C4reserved0.
0xA08 + 16 + 4disk vendor, product, revisionText, such as IOMEGA, ZIP 100, R.41.
0xBC8 + 16 + 4controller vendor, product, revisionText.
0xD840drive init nameAROS lists a 40-character rdb_DriveInitName here; Clévy shows ten reserved longs (0), so it is undefined in practice. (Both sources agree on the position.)

The geometry fields give the size: a cylinder is heads × sectors blocks (Clévy: “the ‘CylBlocks’ field equals ‘heads’ × ‘sectors’”). The reserved area is often the first two cylinders, between the RDB-blocks low and high blocks inclusive; the partitionable area runs from the low to the high cylinder. Clévy’s geometry examples: a ZIP disk 2891 cylinders, 1 head, 68 sectors; an 80 MB IDE disk 980 cylinders, 10 heads, 17 sectors.

PART: a partition

OffsetSizeNameDescription
0x104nextThe next PART block, or −1.
0x144flagsBit 0 bootable, bit 1 no automount.
0x188reservedTwo longs.
0x204device flagsPreferred flags for OpenDevice.
0x241 + 31drive nameA length byte and up to 31 characters, such as DH0.
0x4460reservedFifteen longs.
0x80The DOS environment vector (the same numbers as in a MountList), 20 longs from here:
0x804environment sizeLongs in the vector: 16 (11 is the least).
0x844block sizeIn longs: 128 for 512-byte blocks.
0x884sector origin0.
0x8C4surfacesHeads of the drive.
0x904sectors per block1.
0x944blocks per track
0x984reserved blocksAt the start of the partition, usually 2 (at least 1).
0x9C4preallocatedReserved blocks at the end, normally 0; no effect on the root block’s position.
0xA04interleave0.
0xA44low cylinderFirst cylinder of the partition, inclusive.
0xA84high cylinderLast cylinder, inclusive.
0xAC4buffersOften 30.
0xB04buffer memory type0.
0xB44maximum transferOften $7FFFFFFF (Clévy) or $00FFFFFF (amitools’ default).
0xB84maskAddress mask; often $FFFFFFFE (Clévy) or $7FFFFFFE (amitools’ default).
0xBC4boot prioritySigned, for auto-boot.
0xC04DOS typeDOS and a flag byte (see DOS types); also UNI\0 AT&T System V, UNI\1 Unix boot, UNI\2 BSD for System V, resv reserved (swap space) (Clévy). Clévy advises reading the partition’s own boot block, “don’t rely only on the PART and FSHD ‘DosType’ field”.
0xC44baudDefault baud rate for Commodore’s serial handlers, from the A2232 multiserial board.
0xC84controlUsed by Commodore’s AUX handler.
0xCC4boot blocksKickstart 2.0: number of blocks of boot code to load at start-up.
0xD048reservedTwelve longs.

Where the partition is. Its first block is low cylinder × surfaces × blocks per track and it holds (high − low + 1) × surfaces × blocks per track blocks, using the partition’s own environment numbers (our derivation from the fields above and Clévy’s statement that block pointers inside a partition are relative to its first block; the sources do not give the sum in these words). The partition’s root block is in the middle of it, by the rule on the boot block page.

FSHD: file system header, and LSEG: its code

An FSHD block says how to start the task for a file system that is stored on the disk itself, so an Amiga can use a file system its ROM does not contain. Clévy: “You don’t need it to reach partitions.”

OffsetSizeNameDescription
0x104nextThe next FSHD block, or −1.
0x144flags
0x188reservedTwo longs.
0x204DOS typeThe file system this code is for, such as DOS\1 or a third-party id.
0x244versionHigh 16 bits the version, low 16 the revision: $0027001B is 39.27 (Clévy’s example).
0x284patch flagsBits for those of the following fields that must be substituted into a standard device node for this file system; for example $180 substitutes the segment list and the global vector.
0x2C4typeDevice node type, 0.
0x304task0.
0x344lockNot used, 0.
0x384handlerFile name to load, 0.
0x3C4stack size0.
0x404prioritySigned, 0.
0x444startupSigned, 0.
0x484segment list blockThe first LSEG block (needs processing before substitution, says Clévy).
0x4C4global vectorBCPL global vector, −1.
0x5092 + 84reservedTwenty-three longs reserved “by PatchFlags” (0x50–0xAB), then twenty-one more (0xAC–0xFF). AROS names the last 84 bytes fhb_FileSysName, which Clévy lists as reserved; the sources agree on where they are.

LSEG blocks carry the code: a block of id LSEG, the covered longs (the whole block), checksum, host id, the next LSEG block at 0x10 (−1 for the last) and then 123 longs (492 bytes on a 512-byte block) of load data. Concatenated in order, the data is the file system “stored like an executable, with relocation hunks” (Clévy), that is an Amiga HUNK file. The viewer lists these blocks but does not join or decode the code.

BADB: bad blocks

Id BADB, the covered longs (the whole block), checksum, host id, the next BADB block at 0x10, a reserved long, then pairs of block numbers from 0x18: the bad block and the replacement block it is remapped to (((BSIZE / 4) − 6) / 2 pairs, 61 for 512-byte blocks).

Reading a disk image

  1. Look for RDSK at the start of blocks 0 to 15. If it is not there the image is not an RDB disk (a hardfile or a floppy-style image).
  2. Check the checksum; take the block size from the RDSK block.
  3. Follow the partition list from 0x1C; each PART block gives a partition’s name, DOS type and cylinder range.
  4. For each partition, read its boot block at its first block; the first four bytes name the actual file system, as for a floppy.

The explorer here opens such an image when the file is named .adf: it lists the RDB blocks and the partitions. It does not yet open the partitions as volumes, and a file named .hdf is not routed to it (that extension belongs to HDF4 and HDF5 in the explorer’s registry).

Sources

The three agree on every offset above. Where they differ (the usual maximum transfer and mask, the last bytes of RDSK and FSHD) the page says so.

Not described

Known, not yet covered

  • Reference: no Commodore text (hardblocks.h from the developer kit, dos/filehandler.h) was read; the AROS header is a reimplementation and Clévy says he took his from the kit. No real RDB disk image was available to check the layout on, only images built from it.
  • Reference: the rest of the DosEnvec beyond 0xCC, the meaning of the FSHD flags and device-node fields in practice, how a driver uses the drive-init code and the bad-block list, and the standard names of other file system ids.
  • Viewer: the partitions are listed but not opened as volumes; the FSHD code is not joined or decoded; images with a block size other than 512 are read for the RDB blocks only; a .hdf name is not routed here.

Unknown

  • Reference: RDBs on disks with a block size other than 512: the sources say “can be other powers of 2” and nothing more.