amiga-bootcamp/01_hardware/common/m68k_cpu.md

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M68k CPU on the Amiga

Overview

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.

Privilege Modes

Mode	SR[13] (S-bit)	Stack Pointer	Access
Supervisor	1	SSP (A7')	Full hardware access
User	0	USP (A7)	Restricted — no privileged instructions

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.

Register Set

D0–D7   Data registers (32-bit)
A0–A6   Address registers (32-bit)
A7      Stack pointer (USP in user mode, SSP in supervisor mode)
PC      Program counter
SR      Status register (CCR + supervisor bits)

CCR (Condition Code Register) — lower byte of SR:

bit 4: X  (extend)
bit 3: N  (negative)
bit 2: Z  (zero)
bit 1: V  (overflow)
bit 0: C  (carry)

SR upper byte (supervisor only):

bit 15: T1 (trace mode — single-step)
bit 14: T0
bit 13: S  (supervisor state)
bit 10-8: I2-I0 (interrupt mask level, 0–7)

Exception Vector Table

Located at $000000 (physical), shadowed from Kick ROM into Chip RAM on boot.

Offset	Vector	Description
$000	Reset SSP	Initial supervisor stack pointer
$004	Reset PC	Initial program counter (ROM entry)
$008	Bus Error	Access fault
$00C	Address Error	Odd-address word/long access
$010	Illegal Instruction	Undefined opcode
$014	Divide by Zero	DIVS/DIVU by zero
$018	CHK	CHK instruction bound check fail
$01C	TRAPV / cpTRAPcc	Overflow trap
$020	Privilege Violation	User-mode privileged instruction
$024	Trace	Single-step trap
$028	Line 1010 (A-line)	Opcode $Axxx — OS trap dispatch
$02C	Line 1111 (F-line)	Opcode $Fxxx — FPU / emulation
$060	Spurious Interrupt	No response to interrupt acknowledge
$064	Level 1 Autovector	CIA-B timer / serial / disk
$068	Level 2 Autovector	CIA-A, software
$06C	Level 3 Autovector	VBL, copper, blitter
$070	Level 4 Autovector	Audio, disk DMA
$074	Level 5 Autovector	Serial port
$078	Level 6 Autovector	CIA / ext interrupt
$07C	Level 7 Autovector	NMI (not maskable)
$080–$0BC	TRAP #0–#15	Software traps

AmigaOS Use of Exception Vectors

• A-Line ($028): AmigaOS uses this for its internal library call dispatch on some early titles; modern code uses direct JSR via library base.
• Level 3 ($06C): Vertical Blank interrupt — exec's AddIntServer(INTB_VERTB, ...) chains here.
• Level 6 ($078): CIA interrupts chain here; exec dispatches to INTB_EXTER servers.
• TRAP #0 ($080): Used by Supervisor() to enter supervisor mode from user code.

Interrupt Priority Levels

AmigaOS maps hardware interrupt levels to internal interrupt bits (INTENA/INTREQ):

IPL	Source	INTENA bits
1	Serial TX, disk block	INTB_TBE, INTB_DSKBLK
2	Software (PostIntServer)	INTB_SOFTINT
3	VBlank, copper, blitter	INTB_VERTB, INTB_COPPER, INTB_BLIT
4	Audio ch 0–3, disk DMA	INTB_AUD0–INTB_AUD3, INTB_DSKSYNC
5	Serial RX	INTB_RBF
6	External / CIA	INTB_EXTER
7	NMI (rarely used)	—

CPU Detection at Runtime

AmigaOS stores CPU capability flags in ExecBase->AttnFlags (offset $128):

#include <exec/execbase.h>

struct ExecBase *SysBase = *((struct ExecBase **)4);
UWORD attn = SysBase->AttnFlags;

if (attn & AFF_68020) { /* 68020+ present */ }
if (attn & AFF_68030) { /* 68030+ present */ }
if (attn & AFF_68040) { /* 68040+ present */ }
if (attn & AFF_68060) { /* 68060 present  */ }
if (attn & AFF_FPU)   { /* FPU present    */ }
if (attn & AFF_68881) { /* 68881/68882    */ }

AttnFlags bit definitions (exec/execbase.h):

#define AFF_68010   (1<<0)
#define AFF_68020   (1<<1)
#define AFF_68030   (1<<2)
#define AFF_68040   (1<<3)
#define AFF_68881   (1<<4)  /* on-chip FPU or external 68881 */
#define AFF_FPU     (1<<4)
#define AFF_68882   (1<<5)
#define AFF_FPU40   (1<<6)  /* 040/060 on-chip FPU */
#define AFF_68060   (1<<7)

Key Instruction Subsets by CPU

Instruction	68000	68020	68030	68040
MULS/MULU 16×16	✓	✓	✓	✓
MULS/MULU 32×32	✗	✓	✓	✓
Bit-field (BFEXTU etc.)	✗	✓	✓	✓
CALLM/RTM	✗	✓	✗	✗
CAS/CAS2	✗	✓	✓	✓
LPSTOP	✗	✗	✓	✓
CINV/CPUSH	✗	✗	✓	✓

References

• Motorola M68000 Family Programmer's Reference Manual (M68000PM/AD)
• ADCD 2.1 Hardware Manual: http://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_guide/node0000.html
• NDK39: exec/execbase.h — AttnFlags definitions