ADF (Amiga Disk File) Format¶

The ADF format is a sector-by-sector dump of an Amiga-formatted floppy disk, used by emulators and disk imaging tools. This article documents the complete on-disk structure of Amiga filesystems. Source: Laurent Clévy's ADF FAQ v1.13.

File size¶

Disk type	Blocks	File size
DD (double density)	1760 (11×2×80)	901,120 bytes
HD (high density)	3520 (22×2×80)	1,802,240 bytes

Each block is 512 bytes. Block ordering: sectors → sides → cylinders.

Block 0 = sector 0, side 0, cylinder 0
Block 11 = sector 0, side 1, cylinder 0
Block 22 = sector 0, side 0, cylinder 1
...

MFM encoding¶

Amiga floppies use MFM (Modified Frequency Modulation) encoding. Unlike PC floppy controllers (FDC), the Amiga does MFM encoding/decoding in software via the CPU (sometimes assisted by the Blitter). This is why PC floppy drives can't read Amiga disks even though both use MFM — the sync word and sector format differ.

Bootblock (blocks 0-1, 1024 bytes)¶

Offset  Size  Field       Description
0x00    4     DiskType    'D','O','S' + 3 flag bits
0x04    4     Checksum    Special bootblock checksum
0x08    4     Rootblock   Block number of root (880 for DD)
0x0C    1012  Boot code   Optional executable boot code

DiskType flag bits¶

Value	File system
0	OFS (Old/Original File System)
1	FFS (Fast File System)
2	OFS + International mode
3	FFS + International mode
4	OFS + Directory Cache + International
5	FFS + Directory Cache + International

If the first 3 bytes are PFS, the disk uses the third-party Professional File System.

OFS vs FFS¶

OFS data blocks store 488 bytes of user data (512 − 24 byte header).
FFS data blocks store the full 512 bytes (no header overhead).
FFS is faster and supports directory caching, links, and international mode.

Bootblock checksum (68000 assembly)¶

    lea   bootbuffer,a0
    move.l a0,a1
    clr.l  4(a1)              ;clear checksum field
    move.w #(BOOTBLOCKSIZE/4)-1,d1
    moveq  #0,d0
lp: add.l  (a0)+,d0           ;accumulate longwords
    bcc.s  skip
    add.l  #1,d0              ;add carry back
skip:
    dbra   d1,lp
    not.l  d0                  ;bitwise complement
    move.l d0,4(a1)           ;store as checksum

Rootblock (block 880 for DD, 1760 for HD)¶

Located at the physical middle of the disk to minimize average seek time.

Key fields:

Offset	Size	Field	Description
0x00	4	type	`T_HEADER` (2)
0x0C	4	ht_size	Hash table size (72 longwords for floppy)
0x14	4	checksum	Rootblock checksum
0x18	288	ht[72]	Hash table — block numbers for root directory entries
−0xC8	4	bm_flag	Bitmap flag (−1 = valid)
−0xC4	100	bm_pages[25]	Bitmap block pointers
−0x5C	4	r_days	Last root alteration: days since 1 Jan 1978
−0x58	4	r_mins	Minutes past midnight
−0x54	4	r_ticks	Ticks (1/50s) past minute
−0x50	1	name_len	Volume name length
−0x4F	30	diskname	Volume name
−0x28	12	v_date	Last disk alteration date
−0x1C	12	c_date	Filesystem creation date
−0x04	4	sec_type	`ST_ROOT` (1)

Date format¶

All Amiga dates: days since 1 January 1978, minutes past midnight, ticks (1/50 second). Constraints: 0 ≤ mins < 1440, 0 ≤ ticks < 3000. The Amiga filesystem has no inherent Y2K problem.

Hash table — directory lookup¶

File and directory names are hashed to find their block number:

int HashName(unsigned char *name) {
    unsigned long hash = strlen(name);
    for (int i = 0; i < strlen(name); i++) {
        hash = hash * 13;
        hash = hash + toupper(name[i]);   /* case insensitive */
        hash = hash & 0x7ff;
    }
    return hash % ((BSIZE/4) - 56);       /* 0 <= hash < 72 for floppy */
}

HashTable[hash] gives the first block number. Collisions are resolved by a chained linked list via the next_hash field in each file/directory header block. For example, file_1a, file_24, and file_5u may hash to the same value.

Forbidden characters in names¶

/ and : are forbidden. Allowed: *!@#$%|^+&_()=\-[]{}';",<>.? and accented characters.

File storage¶

Files are stored as a chain of blocks:

File Header Block — contains filename, size, comment, protection bits, and an array of data block numbers.
Data Blocks — OFS: 488 bytes data + 24 byte header. FFS: full 512 bytes data.
Extension Blocks — if the file header can't hold all data block numbers, extension blocks chain additional arrays.

Directory storage¶

Directory blocks have the same structure as the root block but with sec_type = ST_DIR (2) and parent_dir pointing to the parent.

Links¶

Hard link — points directly to the target's file header block number. Same filesystem only.
Soft link — stores the path string to the target. Can cross filesystems.

Hard disk structure (Rigid Disk Block)¶

Hard disks start with a Rigid Disk Block (RDB) containing: - Partition table (one or more partitions) - Drive geometry (cylinders, heads, sectors) - Filesystem information - Bad block list - Park cylinder

The RDB is located before the first partition, typically at cylinder 0. Each partition has its own bootblock, rootblock, and bitmap, independent of other partitions.

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