amiga-bootcamp/01_hardware/aga_a1200_a4000/cpu_030_040.md

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# 68030/040 on the Amiga (A3000/A4000)

## Overview

The A3000 ships with a **Motorola 68030** at 16 or 25 MHz. The A4000 ships with either a 68030 or **68040** at 25 MHz. Later accelerator cards bring the 68060. This document covers CPU-specific concerns for AmigaOS 3.1/3.2 on these platforms.

## 68030 (A3000)

### On-Chip Caches
- **Instruction cache**: 256 bytes, direct-mapped
- **Data cache**: 256 bytes, direct-mapped
- Both enabled by default on AmigaOS 3.1 (`CacheControl()` call)

### On-Chip PMMU
- Full 68030 PMMU: ATC (Address Translation Cache), TT registers
- AmigaOS does **not** use the MMU by default on A3000
- Third-party tools (e.g., VMM, mmu.library) use it for virtual memory

### Cache Control (exec)
```c
#include <proto/exec.h>

/* Enable instruction and data caches */
CacheControl(CACRF_EnableI | CACRF_EnableD, CACRF_EnableI | CACRF_EnableD);

/* Flush caches (required before self-modifying code) */
CacheClearU();          /* clear all caches */
CacheClearE(addr, len, CACRF_ClearI | CACRF_ClearD);  /* targeted flush */
```

### Cache Coherency with DMA
The 68030 data cache is **not snooped** by the Amiga chip bus. If the CPU writes to a buffer that the DMA engine (Blitter, audio, custom chips) will read, the cache must be flushed first:

```c
/* Before handing a buffer to DMA: */
CacheClearE(buf, size, CACRF_ClearD);
```

Similarly, after DMA writes to a buffer that the CPU will read:
```c
CacheClearE(buf, size, CACRF_ClearD);
```

This is a common source of bugs in audio/video programming on 68030+ Amigas.

---

## 68040 (A4000 / Accelerators)

### On-Chip FPU — Partial Implementation
The 68040 has an on-chip FPU but omits many instructions present in the 68881/68882:

**Missing from 68040 FPU:**
- `FSIN`, `FCOS`, `FTAN`, `FASIN`, `FACOS`, `FATAN`
- `FETOX`, `FETOXM1`, `FLOGN`, `FLOG10`
- `FSINH`, `FCOSH`, `FTANH`
- `FATANH`, `FASINH`, `FACOSH`
- `FSCALE`, `FREMX`, `FREMX`, `FINTRZ`

AmigaOS provides `68040.library` which installs Line-F exception handlers to emulate the missing instructions in software.

```c
/* 68040.library is opened automatically by exec at boot */
/* Software should check that it is open before using FP */
struct Library *MathLib = OpenLibrary("68040.library", 0);
```

> [!WARNING]
> If `68040.library` is not installed and software uses a missing 68040 FPU instruction, the system will crash with a Line-F exception. Always ensure `68040.library` is present on A4000.

### 4 KB Instruction + Data Caches
- 68040 has **4 KB instruction cache** and **4 KB data cache**, both 4-way set-associative
- Cache coherency is more complex: DMA writes may not be visible to the CPU without invalidation
- `CacheClearE()` / `CacheClearU()` remain the correct API

### 68040 Memory Model
```
CACR (Cache Control Register) accessed via MOVEC:
  bit 15: EDC   — enable data cache
  bit 14: NAD   — no allocate data (streaming mode)
  bit 13: ESB   — enable store buffer
  bit 10: DPI   — disable push-inhibit
  bit  7: EIC   — enable instruction cache
  bit  3: CINV  — cache invalidate
```

### Bus Error Stack Frame (68040)
The 68040 generates a **different bus error stack frame** from the 68000:
- 68000: 14-byte frame
- 68040: 104-byte frame with pipeline state

This matters for exception handlers and debuggers targeting both platforms.

---

## 68060 (Accelerator Cards)

Available via Blizzard 060, CyberStorm 060, etc. Key differences:

- **Superscalar**: two integer pipelines (in-order, no OOO)
- **Branch prediction**: static and dynamic
- **No MOVE16 snooping** on some Amiga implementations
- On-chip FPU: missing same transcendentals as 68040 → needs `68060.library`
- Separate `68060.library` for the additional missing instructions vs 68040

```c
struct Library *Lib060 = OpenLibrary("68060.library", 0);
```

---

## AmigaOS exec CacheControl() API

```c
#include <exec/execbase.h>
#include <proto/exec.h>

/* SysBase->CacheFlags reflect current cache state */
ULONG flags = CacheControl(0, 0);   /* query without changing */

/* Flag bits: */
#define CACRF_EnableD   (1L<<3)   /* data cache enable */
#define CACRF_FreezeD   (1L<<4)   /* data cache freeze */
#define CACRF_ClearD    (1L<<5)   /* clear data cache */
#define CACRF_EnableI   (1L<<8)   /* instruction cache enable */
#define CACRF_FreezeI   (1L<<9)   /* instruction cache freeze */
#define CACRF_ClearI    (1L<<10)  /* clear instruction cache */
#define CACRF_CopyBack  (1L<<31)  /* data cache write-back mode */
```

## References

- Motorola 68030 User's Manual (M68030UM/AD)
- Motorola 68040 User's Manual (M68040UM/AD)
- NDK39: `exec/execbase.h`, `proto/exec.h` — CacheControl, CacheClearE
- ADCD 2.1: `Libraries_Manual_guide/` — exec CacheControl autodoc
- Commodore A3000/A4000 Technical Reference Manuals