mirror of
https://github.com/alfishe/amiga-bootcamp.git
synced 2026-06-13 00:26:28 +00:00
91 lines
2.7 KiB
Markdown
91 lines
2.7 KiB
Markdown
|
[← Home](../README.md) · [AmigaDOS](README.md)
|
|||
|
|
|
|||
|
|
# 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.
|
|||
|
|
|
|||
|
|
---
|
|||
|
|
|
|||
|
|
## 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 |
|
|||
|
|
|
|||
|
|
---
|
|||
|
|
|
|||
|
|
## 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 |
|
|||
|
|
|
|||
|
|
---
|
|||
|
|
|
|||
|
|
## Hash Function
|
|||
|
|
|
|||
|
|
File/directory names are hashed into the 72-slot table:
|
|||
|
|
|
|||
|
|
```c
|
|||
|
|
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.
|
|||
|
|
|
|||
|
|
---
|
|||
|
|
|
|||
|
|
## 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 |
|
|||
|
|
|
|||
|
|
---
|
|||
|
|
|
|||
|
|
## Checksum Algorithm
|
|||
|
|
|
|||
|
|
```c
|
|||
|
|
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 */
|
|||
|
|
}
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
---
|
|||
|
|
|
|||
|
|
## 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/
|