amiga-bootcamp/07_dos/filesystem.md

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Filesystem — FFS/OFS Block Structure

Overview

AmigaOS supports two native filesystem types: OFS (Old File System, OS 1.x) and FFS (Fast File System, OS 2.0+). Both use a block-based layout with 512-byte blocks. FFS differs by storing data blocks without headers, improving throughput.

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Block Types

Block	Type ID	Description
Boot block	"DOS\0" / "DOS\1"	Blocks 0–1; OFS=DOS\0, FFS=DOS\1
Root block	T_HEADER (2)	Always at middle of partition; directory root
File header	T_HEADER (2)	Metadata for one file
Directory header	T_HEADER (2)	Metadata for one directory
Data block	T_DATA (8)	OFS: header + data; FFS: pure data
File extension	T_LIST (16)	Overflow pointer table for large files
Hash chain	—	Root/dir blocks have a 72-entry hash table

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Root Block Layout (Simplified)

Offset	Field	Description
0	type	Always 2 (T_HEADER)
4	header_key	Own block number
8	high_seq	Number of data blocks in hash table
12	ht_size	Hash table size (usually 72)
16	first_data	Unused
20	checksum	Block checksum
24–312	ht[72]	Hash table: block pointers for directory entries
420	bm_flag	Bitmap valid flag (-1 = valid)
424–472	bm_pages[25]	Pointers to bitmap blocks
484	last_altered_days	Modification date
504	disk_name	BSTR: volume name

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Hash Function

File/directory names are hashed into the 72-slot table:

ULONG hash_name(const char *name, int table_size) {
    ULONG hash = strlen(name);
    for (int i = 0; name[i]; i++) {
        hash = hash * 13 + toupper(name[i]);
        hash &= 0x7FF;
    }
    return hash % table_size;
}

Collisions are resolved by chaining: each file/dir header has a hash_chain pointer to the next entry in the same slot.

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OFS vs FFS

Feature	OFS (DOS\0)	FFS (DOS\1)
Data blocks	24-byte header + 488 bytes data	Pure 512 bytes data
Max filename	30 chars	30 chars
International	No	DOS\2 (INTL OFS), DOS\3 (INTL FFS)
Dir cache	No	DOS\4 (FFS + dir cache)
Throughput	~488/512 = 95% efficiency	100% efficiency

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Checksum Algorithm

LONG compute_checksum(ULONG *block, int longs) {
    LONG sum = 0;
    for (int i = 0; i < longs; i++) sum += block[i];
    return -sum;  /* stored in the checksum field to make total = 0 */
}

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References

• NDK39: dos/filehandler.h
• Ralph Babel: The AmigaDOS Manual (3rd edition) — definitive FFS reference
• Laurent Clevy: The Amiga Filesystem — http://lclevy.free.fr/adflib/