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exec_os: enrich all stubs to bootcamp-quality reference articles
Complete rewrite of 14 exec_os articles from stubs to comprehensive deep-dive technical references with architecture diagrams, pitfalls, and best practices. New: multitasking.md (scheduler, IPC, memory safety, real-world scenarios) Enriched: exec_base, tasks_processes, library_system, library_vectors, interrupts, exceptions_traps, memory_management, message_ports, signals, semaphores, io_requests, lists_nodes, resident_modules Updated indexes: 06_exec_os/README.md, root README.md
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Ilia Sharin
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@ -4,101 +4,240 @@
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## Overview
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`ExecBase` is the root structure of AmigaOS, located at absolute address `$4`. It is a `struct Library` extended with all exec kernel state: memory lists, task queues, interrupt vectors, library lists, and hardware abstraction fields.
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`ExecBase` is the root structure of AmigaOS, located at absolute address `$4`. It is a `struct Library` extended with all exec kernel state: memory lists, task queues, interrupt vectors, library lists, and hardware abstraction fields. Every system call goes through `ExecBase` — it is the single point of truth for the entire running system.
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---
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## Locating ExecBase
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```c
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/* C — standard method */
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struct ExecBase *SysBase = *((struct ExecBase **)4);
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/* Or use the auto-open variable (SAS/C, GCC): */
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extern struct ExecBase *SysBase; /* Linker resolves from startup code */
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```
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In assembly:
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```asm
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MOVEA.L 4.W, A6 ; A6 = SysBase
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; Assembly — canonical method
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MOVEA.L 4.W,A6 ; A6 = SysBase (short absolute addressing)
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; All exec LVO calls use JSR offset(A6)
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```
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> **Why address $4?** The 68000 stores the initial Program Counter at address $4 in the exception vector table. During cold boot, the CPU reads this address to find the ROM entry point. After boot, Exec overwrites it with a pointer to ExecBase. This is the one absolute address every Amiga program can rely on.
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---
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## Structure Layout
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```mermaid
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graph TB
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EB["ExecBase at $4"] --> LIB["Library Header<br/>(struct Library)<br/>Offsets 0–33"]
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EB --> SOFT["Software Config<br/>SoftVer, ColdCapture,<br/>WarmCapture, SysStkUpper"]
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EB --> TASK["Task Scheduling<br/>TaskReady, TaskWait,<br/>ThisTask, IDNestCnt,<br/>TDNestCnt, Quantum"]
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EB --> MEM["Memory Management<br/>MemList, MaxLocMem,<br/>MaxExtMem"]
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EB --> LISTS["System Lists<br/>LibList, DeviceList,<br/>ResourceList, PortList,<br/>IntrList"]
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EB --> HW["Hardware Detection<br/>AttnFlags, ChipRevBits0,<br/>PowerSupplyFrequency,<br/>VBlankFrequency"]
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style EB fill:#e8f4fd,stroke:#2196f3,color:#333
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style TASK fill:#e8f5e9,stroke:#4caf50,color:#333
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style HW fill:#fff3e0,stroke:#ff9800,color:#333
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```
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---
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## Key Field Groups
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### Library Header (offset 0)
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### Library Header (Offset 0–33)
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Every library starts with this — ExecBase is exec.library's own base:
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```c
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struct Library LibNode; /* +0 — ln_Name = "exec.library" */
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/* +20 — lib_Version (40 = OS3.1, 44 = OS3.2) */
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struct Library LibNode; /* ln_Name = "exec.library" */
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/* lib_Version = 40 (OS 3.1), 45 (OS 3.2), 47 (OS 3.2.2) */
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/* lib_OpenCnt = open count */
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```
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### Interrupts (offset +84)
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```c
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UWORD AttnFlags; /* +0x128 — processor capability flags */
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UWORD AttnResched; /* +0x12A — reschedule attention flag */
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```
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### Boot and Configuration
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| Offset | Field | Type | Description |
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|---|---|---|---|
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| `$22` | `SoftVer` | `UWORD` | Kickstart software revision |
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| `$24` | `LowMemChkSum` | `WORD` | Checksum of vectors $0–$200 |
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| `$26` | `ChkBase` | `ULONG` | ExecBase self-checksum |
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| `$2A` | `ColdCapture` | `APTR` | Cold reboot intercept vector |
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| `$2E` | `CoolCapture` | `APTR` | Warm reboot intercept vector (after Diag) |
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| `$32` | `WarmCapture` | `APTR` | Keyboard reset intercept vector |
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| `$36` | `SysStkUpper` | `APTR` | Top of supervisor stack |
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| `$3A` | `SysStkLower` | `APTR` | Bottom of supervisor stack |
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| `$3E` | `MaxLocMem` | `ULONG` | Top of Chip RAM (e.g., `$200000` for 2 MB) |
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| `$42` | `DebugEntry` | `APTR` | Entry point for ROM debugger (Wack) |
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| `$46` | `DebugData` | `APTR` | Data area for ROM debugger |
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| `$4A` | `AlertData` | `APTR` | Last Alert data |
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| `$4E` | `MaxExtMem` | `APTR` | Top of Extended RAM (or NULL) |
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### Task Scheduling
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| Offset | Field | Description |
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|---|---|---|
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| +0x128 | `TaskReady` | `struct List` — tasks ready to run |
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| +0x132 | `TaskWait` | `struct List` — tasks waiting on signals |
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| +0x126 | `IDNestCnt` | Interrupt disable nesting count |
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| +0x127 | `TDNestCnt` | Task disable nesting count |
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| Offset | Field | Type | Description |
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|---|---|---|---|
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| `$126` | `IDNestCnt` | `BYTE` | Interrupt disable nesting (−1 = enabled) |
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| `$127` | `TDNestCnt` | `BYTE` | Task disable nesting (−1 = enabled) |
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| `$128` | `ThisTask` | `APTR` | Pointer to currently running Task |
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| `$12C` | `Quantum` | `UWORD` | Time slice for equal-priority round-robin |
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| `$12E` | `Elapsed` | `UWORD` | Ticks elapsed in current quantum |
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| `$130` | `SysFlags` | `UWORD` | Internal scheduler flags |
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| `$132` | `TaskReady` | `List` | Tasks ready to run (sorted by priority) |
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| `$146` | `TaskWait` | `List` | Tasks blocked on `Wait()` |
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### Memory
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| Offset | Field | Description |
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|---|---|---|
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| +0x130 | `MemList` | `struct List` of `MemHeader` regions |
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| +0x134 | `ResourceList` | Resources list |
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### Library and Device Lists
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| Offset | Field | Description |
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|---|---|---|
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| +0x17A | `LibList` | `struct List` — loaded libraries |
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| +0x182 | `DeviceList` | `struct List` — loaded devices |
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| +0x18A | `IntrList` | Interrupt server list |
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| +0x192 | `PortList` | Public message ports |
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| +0x19A | `TaskList` | All tasks (not just ready/waiting) |
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| Offset | Field | Type | Description |
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|---|---|---|---|
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| `$15A` | `MemList` | `List` | All memory regions (`MemHeader` chain) |
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### Vectors and ROM
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| Offset | Field | Description |
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|---|---|---|
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| +0x26 | `SoftVer` | Kickstart software revision |
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| +0x10 | `ChkBase` | Checksum of library header |
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| +0x222 | `PowerSupplyFrequency` | 50 or 60 Hz |
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| +0x21E | `ChipRevBits0` | Chip revision detection flags |
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### System Lists
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### Chip Revision Flags (`ChipRevBits0`)
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| Offset | Field | Type | Description |
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|---|---|---|---|
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| `$17A` | `LibList` | `List` | Loaded libraries (`NT_LIBRARY`) |
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| `$18E` | `DeviceList` | `List` | Loaded devices (`NT_DEVICE`) |
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| `$1A2` | `IntrList` | `List` | Interrupt server lists |
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| `$1B6` | `ResourceList` | `List` | System resources |
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| `$1CA` | `PortList` | `List` | Public message ports |
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| `$1DE` | `SemaphoreList` | `List` | Public semaphores |
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| Bit | Constant | Chip |
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|---|---|---|
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| 4 | `ATNF_68010` | 68010 or better |
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| 5 | `ATNF_68020` | 68020 or better |
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| 6 | `ATNF_68030` | 68030 |
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| 7 | `ATNF_68040` | 68040 |
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| 10 | `ATNF_FPU40` | 68040 internal FPU |
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### Hardware Detection
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| Offset | Field | Type | Description |
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|---|---|---|---|
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| `$128` | `AttnFlags` | `UWORD` | CPU and FPU capability flags |
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| `$212` | `VBlankFrequency` | `UBYTE` | VBL rate: 50 (PAL) or 60 (NTSC) |
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| `$213` | `PowerSupplyFrequency` | `UBYTE` | Mains: 50 or 60 Hz |
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| `$21E` | `ChipRevBits0` | `UBYTE` | Chip revision detection |
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---
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## Detecting CPU and Chipset
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## AttnFlags — CPU Detection
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| Bit | Constant | Meaning |
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| 0 | `AFF_68010` | 68010 or better detected |
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| 1 | `AFF_68020` | 68020 or better |
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| 2 | `AFF_68030` | 68030 or better |
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| 3 | `AFF_68040` | 68040 or better |
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| 4 | `AFF_68881` | 68881 FPU detected |
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| 5 | `AFF_68882` | 68882 FPU detected |
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| 6 | `AFF_FPU40` | 68040 internal FPU |
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| 7 | `AFF_68060` | 68060 detected (OS 3.1+) |
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| 10 | `AFF_PRIVATE` | Exec private — do not use |
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### Usage
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```c
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/* CPU: */
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if (SysBase->AttnFlags & AFF_68020) { /* 020+ */ }
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if (SysBase->AttnFlags & AFF_68040) { /* 040 */ }
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/* Check for 68020+ */
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if (SysBase->AttnFlags & AFF_68020)
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{
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/* Can use CACHE instructions, 32-bit multiply, etc. */
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}
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/* Chipset (via graphics.library): */
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struct GfxBase *gfx = (struct GfxBase *)OpenLibrary("graphics.library", 36);
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if (gfx->ChipRevBits0 & GFXB_AA_ALICE) { /* AGA Alice chip */ }
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/* Check for FPU */
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if (SysBase->AttnFlags & (AFF_68881 | AFF_68882 | AFF_FPU40))
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{
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/* Floating-point hardware available */
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}
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```
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---
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## ExecBase in IDA Pro
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## Detecting Chipset Revision
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```c
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/* via graphics.library */
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struct GfxBase *gfx = (struct GfxBase *)OpenLibrary("graphics.library", 36);
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if (gfx)
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{
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if (gfx->ChipRevBits0 & GFXB_AA_ALICE) /* AGA Alice */
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if (gfx->ChipRevBits0 & GFXB_AA_LISA) /* AGA Lisa */
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if (gfx->ChipRevBits0 & GFXB_HR_AGNUS) /* ECS Agnus */
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if (gfx->ChipRevBits0 & GFXB_HR_DENISE) /* ECS Denise */
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}
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```
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---
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## Enumerating System Lists
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### Walking the Library List
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```c
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Forbid();
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struct Node *node;
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for (node = SysBase->LibList.lh_Head;
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node->ln_Succ != NULL;
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node = node->ln_Succ)
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{
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struct Library *lib = (struct Library *)node;
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Printf(" %s V%ld.%ld (open: %ld)\n",
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lib->lib_Node.ln_Name,
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lib->lib_Version,
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lib->lib_Revision,
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lib->lib_OpenCnt);
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}
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Permit();
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```
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### Walking the Task Lists
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```c
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Forbid();
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/* Currently running */
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Printf("Running: %s\n", SysBase->ThisTask->tc_Node.ln_Name);
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/* Ready to run */
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for (node = SysBase->TaskReady.lh_Head;
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node->ln_Succ; node = node->ln_Succ)
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{
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Printf(" Ready: %s (pri %ld)\n",
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node->ln_Name, node->ln_Pri);
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}
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/* Waiting */
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for (node = SysBase->TaskWait.lh_Head;
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node->ln_Succ; node = node->ln_Succ)
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{
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Printf(" Wait: %s (pri %ld)\n",
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node->ln_Name, node->ln_Pri);
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}
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Permit();
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```
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---
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## ExecBase Safety Rules
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| Rule | Reason |
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| **Never write to ExecBase fields** | Corrupts kernel state for all tasks |
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| **Use `Forbid()` when walking lists** | Lists change as libraries open/close, tasks start/stop |
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| **Don't cache pointers from lists** | Nodes may be removed between accesses |
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| **Always use LVO functions** | Direct field manipulation bypasses safety checks |
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| **Verify `SysBase` after warm reboot** | `ColdCapture`/`CoolCapture` may have altered it |
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---
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## ExecBase in IDA Pro / Ghidra
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After loading Kickstart ROM:
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1. Create a segment at `$4` containing a pointer
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2. Follow the pointer to the ExecBase (in ROM)
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3. Apply `struct ExecBase` type (from NDK39 headers parsed via `File → Parse C header`)
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4. All `N(A6)` offsets auto-annotate as field names
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1. **Create a segment** at `$000000–$000400` (exception vectors)
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2. **Mark `$4` as a pointer** — follow it to the ExecBase in ROM
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3. **Apply `struct ExecBase` type** from NDK39 headers
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- IDA: `File → Parse C header` with `exec/execbase.h`
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- Ghidra: Import C headers via Data Type Manager
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4. **All `N(A6)` offsets** in exec code auto-annotate as field names
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5. **Label the system lists** — `LibList`, `DeviceList`, `PortList` are entry points for understanding boot order
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---
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@ -106,5 +245,6 @@ After loading Kickstart ROM:
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- NDK39: `exec/execbase.h` — authoritative field definitions
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- ADCD 2.1: exec.library autodoc
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- See also: [Library System](library_system.md) — how libraries relate to ExecBase
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- See also: [Multitasking](multitasking.md) — TaskReady/TaskWait and scheduler internals
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- *Amiga ROM Kernel Reference Manual: Exec* — ExecBase chapter
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- http://amigadev.elowar.com/read/ADCD_2.1/Libraries_Manual_guide/node0072.html
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