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137 lines
3.5 KiB
Markdown
137 lines
3.5 KiB
Markdown
[← Home](../../README.md) · [Reverse Engineering](../README.md)
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# Recovering Data Structures
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## Overview
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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.
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---
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## The MOVE.L offset(An),Dm Pattern
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Structure field accesses appear as:
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```asm
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MOVEA.L _DOSBase, A6
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MOVE.L ($17A,A6), A0 ; SysBase->LibList at offset +0x17A
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MOVE.L (A0), A1 ; lh_Head
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```
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The key is the **base register** and **constant offset**. Match the offset against known structure definitions.
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---
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## Common Structures and Key Offsets
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### `struct ExecBase` (at absolute address `$4`)
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| Offset | Field | Type |
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|---|---|---|
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| +0 | `LibNode` | `struct Library` |
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| +0x128 | `TaskReady` | `struct List` |
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| +0x132 | `TaskWait` | `struct List` |
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| +0x17A | `LibList` | `struct List` |
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| +0x182 | `DeviceList` | `struct List` |
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| +0x21E | `ChipRevBits0` | `UWORD` |
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| +0x280 | `MemList` | `struct List` |
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### `struct Node` (8 bytes)
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| Offset | Field |
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|---|---|
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| +0 | `ln_Succ` (next node) |
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| +4 | `ln_Pred` (prev node) |
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| +8 | `ln_Type` (UBYTE) |
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| +9 | `ln_Pri` (BYTE priority) |
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| +10 | `ln_Name` (STRPTR) |
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List traversal:
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```asm
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MOVEA.L lh_Head, A0 ; first node
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.loop:
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TST.L (A0) ; ln_Succ == NULL?
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BEQ.S .done
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; process node at A0
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MOVEA.L (A0), A0 ; A0 = ln_Succ
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BRA.S .loop
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```
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### `struct Process` (extends `struct Task`)
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| Offset | Field |
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|---|---|
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| +0 | `pr_Task` (struct Task) |
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| +92 | `pr_MsgPort` |
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| +128 | `pr_CLI` (BPTR, non-NULL if CLI) |
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| +172 | `pr_SegList` (BPTR to segment list) |
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Detection in disassembly:
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```asm
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MOVE.L ($80,A4), D0 ; pr_CLI at offset +0x80
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BEQ.S .wb_launch ; NULL = Workbench
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```
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### `struct IORequest` / `struct IOStdReq`
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| Offset | Field |
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|---|---|
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| +0 | `io_Message` (struct Message) |
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| +20 | `io_Device` |
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| +24 | `io_Unit` |
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| +28 | `io_Command` (UWORD) |
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| +30 | `io_Flags` (UBYTE) |
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| +32 | `io_Error` (BYTE) |
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| +36 | `io_Length` (ULONG) |
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| +40 | `io_Actual` (ULONG) |
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| +44 | `io_Data` (APTR) |
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| +48 | `io_Offset` (ULONG) |
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---
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## Annotating Structures in IDA Pro
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### Define a structure type:
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1. `View → Open Subviews → Local Types` → `Insert` → paste C struct definition
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2. IDA parses the struct and creates a type entry
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3. Navigate to the base register in disassembly
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4. Press `T` (structure offset) on any `offset(An)` operand
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5. Select the struct type → all accesses auto-annotated
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### Import NDK39 headers:
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Use `File → Load file → Parse C header file` → select `exec/execbase.h`, `exec/tasks.h`, etc. from NDK39.
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---
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## Exec Node Traversal Loops
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A recurring pattern: walking the `LibList` or `DeviceList`:
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```asm
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; Annotated after struct recovery:
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MOVEA.L SysBase, A6
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LEA (LibList,A6), A0 ; &SysBase->LibList
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MOVEA.L (lh_Head,A0), A1 ; first lib node
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.scan:
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MOVEA.L (ln_Succ,A1), A2 ; peek next
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TST.L A2
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BEQ.S .not_found
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; compare ln_Name string
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MOVEA.L (ln_Name,A1), A0
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JSR ___strcmp
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TST.L D0
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BEQ.S .found
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MOVEA.L A2, A1
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BRA.S .scan
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```
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---
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## References
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- NDK39: `exec/execbase.h`, `exec/tasks.h`, `exec/nodes.h`, `exec/io.h`
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- [exec_base.md](../../06_exec_os/exec_base.md) — full ExecBase field listing
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- [lists_nodes.md](../../06_exec_os/lists_nodes.md) — MinList/List traversal
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- IDA Pro: Structure subview, Local Types, T hotkey for struct offset
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