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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.
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01_hardware/common/m68k_cpu.md
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[← Home](../../README.md) · [Hardware](../README.md)
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# M68k CPU on the Amiga
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## Overview
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AmigaOS runs on the Motorola 68000 family exclusively. The CPU operates in two privilege modes and interacts with the custom chips via the shared 16/32-bit bus arbitrated by Agnus/Alice.
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## Privilege Modes
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| Mode | SR[13] (S-bit) | Stack Pointer | Access |
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|---|---|---|---|
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| **Supervisor** | 1 | SSP (A7') | Full hardware access |
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| **User** | 0 | USP (A7) | Restricted — no privileged instructions |
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AmigaOS runs entirely in **supervisor mode**. User-mode tasks switch to supervisor via `Supervisor()` or trap instructions. The OS does not enforce user-mode isolation — all tasks share one address space.
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## Register Set
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```
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D0–D7 Data registers (32-bit)
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A0–A6 Address registers (32-bit)
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A7 Stack pointer (USP in user mode, SSP in supervisor mode)
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PC Program counter
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SR Status register (CCR + supervisor bits)
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```
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**CCR (Condition Code Register) — lower byte of SR:**
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```
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bit 4: X (extend)
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bit 3: N (negative)
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bit 2: Z (zero)
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bit 1: V (overflow)
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bit 0: C (carry)
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```
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**SR upper byte (supervisor only):**
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```
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bit 15: T1 (trace mode — single-step)
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bit 14: T0
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bit 13: S (supervisor state)
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bit 10-8: I2-I0 (interrupt mask level, 0–7)
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```
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## Exception Vector Table
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Located at **$000000** (physical), shadowed from Kick ROM into Chip RAM on boot.
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| Offset | Vector | Description |
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|---|---|---|
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| $000 | Reset SSP | Initial supervisor stack pointer |
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| $004 | Reset PC | Initial program counter (ROM entry) |
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| $008 | Bus Error | Access fault |
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| $00C | Address Error | Odd-address word/long access |
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| $010 | Illegal Instruction | Undefined opcode |
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| $014 | Divide by Zero | DIVS/DIVU by zero |
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| $018 | CHK | CHK instruction bound check fail |
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| $01C | TRAPV / cpTRAPcc | Overflow trap |
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| $020 | Privilege Violation | User-mode privileged instruction |
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| $024 | Trace | Single-step trap |
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| $028 | Line 1010 (A-line) | Opcode $Axxx — OS trap dispatch |
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| $02C | Line 1111 (F-line) | Opcode $Fxxx — FPU / emulation |
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| $060 | Spurious Interrupt | No response to interrupt acknowledge |
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| $064 | Level 1 Autovector | CIA-B timer / serial / disk |
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| $068 | Level 2 Autovector | CIA-A, software |
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| $06C | Level 3 Autovector | VBL, copper, blitter |
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| $070 | Level 4 Autovector | Audio, disk DMA |
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| $074 | Level 5 Autovector | Serial port |
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| $078 | Level 6 Autovector | CIA / ext interrupt |
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| $07C | Level 7 Autovector | NMI (not maskable) |
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| $080–$0BC | TRAP #0–#15 | Software traps |
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## AmigaOS Use of Exception Vectors
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- **A-Line ($028):** AmigaOS uses this for its internal library call dispatch on some early titles; modern code uses direct JSR via library base.
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- **Level 3 ($06C):** Vertical Blank interrupt — exec's `AddIntServer(INTB_VERTB, ...)` chains here.
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- **Level 6 ($078):** CIA interrupts chain here; exec dispatches to `INTB_EXTER` servers.
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- **TRAP #0 ($080):** Used by `Supervisor()` to enter supervisor mode from user code.
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## Interrupt Priority Levels
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AmigaOS maps hardware interrupt levels to internal interrupt bits (`INTENA`/`INTREQ`):
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| IPL | Source | INTENA bits |
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|---|---|---|
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| 1 | Serial TX, disk block | `INTB_TBE`, `INTB_DSKBLK` |
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| 2 | Software (PostIntServer) | `INTB_SOFTINT` |
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| 3 | VBlank, copper, blitter | `INTB_VERTB`, `INTB_COPPER`, `INTB_BLIT` |
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| 4 | Audio ch 0–3, disk DMA | `INTB_AUD0`–`INTB_AUD3`, `INTB_DSKSYNC` |
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| 5 | Serial RX | `INTB_RBF` |
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| 6 | External / CIA | `INTB_EXTER` |
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| 7 | NMI (rarely used) | — |
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## CPU Detection at Runtime
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AmigaOS stores CPU capability flags in `ExecBase->AttnFlags` (offset $128):
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```c
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#include <exec/execbase.h>
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struct ExecBase *SysBase = *((struct ExecBase **)4);
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UWORD attn = SysBase->AttnFlags;
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if (attn & AFF_68020) { /* 68020+ present */ }
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if (attn & AFF_68030) { /* 68030+ present */ }
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if (attn & AFF_68040) { /* 68040+ present */ }
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if (attn & AFF_68060) { /* 68060 present */ }
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if (attn & AFF_FPU) { /* FPU present */ }
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if (attn & AFF_68881) { /* 68881/68882 */ }
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```
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**AttnFlags bit definitions** (`exec/execbase.h`):
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```c
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#define AFF_68010 (1<<0)
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#define AFF_68020 (1<<1)
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#define AFF_68030 (1<<2)
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#define AFF_68040 (1<<3)
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#define AFF_68881 (1<<4) /* on-chip FPU or external 68881 */
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#define AFF_FPU (1<<4)
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#define AFF_68882 (1<<5)
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#define AFF_FPU40 (1<<6) /* 040/060 on-chip FPU */
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#define AFF_68060 (1<<7)
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```
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## Key Instruction Subsets by CPU
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| Instruction | 68000 | 68020 | 68030 | 68040 |
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|---|---|---|---|---|
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| MULS/MULU 16×16 | ✓ | ✓ | ✓ | ✓ |
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| MULS/MULU 32×32 | ✗ | ✓ | ✓ | ✓ |
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| Bit-field (BFEXTU etc.) | ✗ | ✓ | ✓ | ✓ |
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| CALLM/RTM | ✗ | ✓ | ✗ | ✗ |
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| CAS/CAS2 | ✗ | ✓ | ✓ | ✓ |
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| LPSTOP | ✗ | ✗ | ✓ | ✓ |
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| CINV/CPUSH | ✗ | ✗ | ✓ | ✓ |
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## References
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- Motorola *M68000 Family Programmer's Reference Manual* (M68000PM/AD)
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- ADCD 2.1 Hardware Manual: http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node0000.html
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- NDK39: `exec/execbase.h` — `AttnFlags` definitions
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