mirror of
https://github.com/alfishe/amiga-bootcamp.git
synced 2026-06-13 00:26:28 +00:00
21751c0025
Comprehensive technical documentation covering: - Hardware: OCS/ECS/AGA custom chip registers, Copper & Blitter deep dives - Boot sequence: cold boot through startup-sequence - Binary format: HUNK executable spec, relocation, debug info - Linking & ABI: .fd files, LVO tables, register calling conventions - Exec kernel: tasks, interrupts, memory, signals, semaphores - AmigaDOS: file I/O, FFS/OFS layout, CLI/Shell scripting - Graphics: planar bitmaps, Copper programming, HAM/EHB modes - Intuition: screens, windows, IDCMP, BOOPSI - Devices: trackdisk, SCSI, serial, timer, audio, keyboard - Libraries: utility, expansion, IFFParse, locale, ARexx - Networking: bsdsocket API, SANA-II, TCP/IP stack comparison - Toolchain: GCC, vasm/vlink, SAS/C, NDK, debugging - Reverse engineering: IDA/Ghidra setup, compiler fingerprints, case studies - CPU & MMU: 68040/060 emulation libs, PMMU, cache management - Driver development: SANA-II, Picasso96/RTG, AHI audio All files include breadcrumb navigation. No local paths or proprietary content.
90 lines
2.7 KiB
Markdown
90 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/
|