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amiga-bootcamp/05_reversing/static/struct_recovery.md
Ilia Sharin 21751c0025 docs(amiga): complete AmigaOS 3.1/3.2 developer reference — 172 files across 17 sections 
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.
2026-04-23 12:17:35 -04:00

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← Home · Reverse Engineering

Recovering Data Structures

Overview

Amiga executables use OS structures extensively — ExecBase, Node, Process, IORequest, etc. This document describes how to recover and annotate these structures in disassembly by matching field access patterns against NDK39 header offsets.

The MOVE.L offset(An),Dm Pattern

Structure field accesses appear as:

MOVEA.L  _DOSBase, A6
MOVE.L   ($17A,A6), A0     ; SysBase->LibList at offset +0x17A
MOVE.L   (A0), A1           ; lh_Head

The key is the base register and constant offset. Match the offset against known structure definitions.

Common Structures and Key Offsets

struct ExecBase (at absolute address $4)

Offset Field Type
+0 LibNode struct Library
+0x128 TaskReady struct List
+0x132 TaskWait struct List
+0x17A LibList struct List
+0x182 DeviceList struct List
+0x21E ChipRevBits0 UWORD
+0x280 MemList struct List

struct Node (8 bytes)

Offset Field
+0 ln_Succ (next node)
+4 ln_Pred (prev node)
+8 ln_Type (UBYTE)
+9 ln_Pri (BYTE priority)
+10 ln_Name (STRPTR)

List traversal:

MOVEA.L  lh_Head, A0    ; first node
.loop:
    TST.L   (A0)        ; ln_Succ == NULL?
    BEQ.S   .done
    ; process node at A0
    MOVEA.L (A0), A0    ; A0 = ln_Succ
    BRA.S   .loop

struct Process (extends struct Task)

Offset Field
+0 pr_Task (struct Task)
+92 pr_MsgPort
+128 pr_CLI (BPTR, non-NULL if CLI)
+172 pr_SegList (BPTR to segment list)

Detection in disassembly:

MOVE.L  ($80,A4), D0    ; pr_CLI at offset +0x80
BEQ.S   .wb_launch      ; NULL = Workbench

struct IORequest / struct IOStdReq

Offset Field
+0 io_Message (struct Message)
+20 io_Device
+24 io_Unit
+28 io_Command (UWORD)
+30 io_Flags (UBYTE)
+32 io_Error (BYTE)
+36 io_Length (ULONG)
+40 io_Actual (ULONG)
+44 io_Data (APTR)
+48 io_Offset (ULONG)

Annotating Structures in IDA Pro

Define a structure type:

  1. View → Open Subviews → Local Types → Insert → paste C struct definition
  2. IDA parses the struct and creates a type entry
  3. Navigate to the base register in disassembly
  4. Press T (structure offset) on any offset(An) operand
  5. Select the struct type → all accesses auto-annotated

Import NDK39 headers:

Use File → Load file → Parse C header file → select exec/execbase.h, exec/tasks.h, etc. from NDK39.

Exec Node Traversal Loops

A recurring pattern: walking the LibList or DeviceList:

; Annotated after struct recovery:
MOVEA.L  SysBase, A6
LEA      (LibList,A6), A0     ; &SysBase->LibList
MOVEA.L  (lh_Head,A0), A1    ; first lib node
.scan:
    MOVEA.L (ln_Succ,A1), A2 ; peek next
    TST.L   A2
    BEQ.S   .not_found
    ; compare ln_Name string
    MOVEA.L (ln_Name,A1), A0
    JSR     ___strcmp
    TST.L   D0
    BEQ.S   .found
    MOVEA.L A2, A1
    BRA.S   .scan

References

  • NDK39: exec/execbase.h, exec/tasks.h, exec/nodes.h, exec/io.h
  • 06_exec_os/exec_base.md — full ExecBase field listing
  • 06_exec_os/lists_nodes.md — MinList/List traversal
  • IDA Pro: Structure subview, Local Types, T hotkey for struct offset