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102 lines
2.6 KiB
Markdown
102 lines
2.6 KiB
Markdown
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[← Home](../README.md) · [Boot Sequence](README.md)
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# Cold Boot — Power-On to Kickstart
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## Overview
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When the Amiga powers on or is reset (Ctrl-Amiga-Amiga), the 68000 CPU begins execution from the ROM. The boot process progresses from raw hardware init through to a fully running AmigaOS desktop in approximately 3–8 seconds.
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---
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## Hardware Initialisation (Pre-ROM)
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### 1. CPU Reset Vector
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The 68000 reads two longwords from address `$000000`:
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```
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$000000: Initial SSP (Supervisor Stack Pointer)
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$000004: Initial PC (Program Counter) → ROM entry point
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```
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On Amiga, these locations map to Kickstart ROM at `$FC0000` (256 KB) or `$F80000` (512 KB). The ROM image contains:
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- Word 0–1: SSP value (typically `$000400`)
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- Word 2–3: PC value → ROM entry point
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### 2. ROM Checksum
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First code executed: compute checksum of entire ROM. If it fails → **red screen of death** (solid red background, no further boot).
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```
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Checksum: simple 32-bit additive checksum of all ROM longwords.
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Result must equal $FFFFFFFF (complement to zero).
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The last longword of the ROM is the complement value.
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```
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### 3. Chip Register Reset
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```
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- Write $7FFF to INTENA ($DFF09A) — disable all interrupts
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- Write $7FFF to INTREQ ($DFF09C) — clear all pending interrupts
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- Write $7FFF to DMACON ($DFF096) — disable all DMA
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- CIA chips: reset timer, serial, port registers
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```
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### 4. Memory Detection
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The ROM probes for available memory:
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```
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1. Test Chip RAM at $000000 by writing test patterns
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2. Size Chip RAM: 256 KB, 512 KB, 1 MB, or 2 MB
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3. Probe for Fast RAM at $C00000 (Ranger), $200000 (Slow/Ranger)
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4. Probe for Zorro II auto-config space at $E80000
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5. Probe for 32-bit fast RAM at $07000000+ (Zorro III)
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```
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### 5. Display Diagnostic Colours
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During boot, the background colour indicates progress:
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| Colour | Stage |
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|---|---|
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| Dark grey | ROM checksum passed |
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| Light grey | Chip RAM sized |
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| White | ExecBase initialised |
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| Green flash | DOS boot starting |
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| **Red** | ROM checksum fail |
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| **Yellow** | Chip RAM test fail |
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| **Blue** | Alert (Guru Meditation) |
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---
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## ROM Layout
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### 256 KB ROM (Kickstart 1.x)
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```
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$FC0000–$FFFFFF (256 KB)
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$FC0000: Reset vectors (SSP + PC)
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$FC0008: ROM header (version, checksum)
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...
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$FC0020: First resident module (exec.library)
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...
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$FFFFFC: Checksum complement word
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```
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### 512 KB ROM (Kickstart 2.0+)
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```
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$F80000–$FFFFFF (512 KB)
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$F80000: Reset vectors
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$F80008: ROM header
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...
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$F80020: exec.library resident tag
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...
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$FFFFFC: Checksum complement
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```
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---
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## References
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- RKRM: *Hardware Reference Manual* — reset chapter
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