DiskCopyReader 1.0.0

DiskCopyReader

A lightweight .NET library for reading Apple Disk Copy 4.2 (.dc42) disk image files. Disk Copy was a utility created by Apple for duplicating floppy disks on classic Macintosh computers.

Features

• Read Disk Copy 4.2 image files
• Support for multiple disk encodings:
  • GCR 400KB (early Macintosh floppies)
  • GCR 800KB (double-sided Macintosh floppies)
  • MFM 720KB (PC-compatible floppies)
  • MFM 1.44MB (high-density floppies)
• Extract raw disk data and tag data
• Verify data and tag checksums
• Access image metadata (name, encoding, sizes)
• Support for .NET 9.0
• Zero external dependencies

Installation

Add the project reference to your .NET application:

dotnet add reference path/to/DiskCopyReader.csproj

Or, if published on NuGet:

dotnet add package DiskCopyReader

Usage

Reading a Disk Copy Image

using DiskCopyReader;

// Open a Disk Copy image file
using var stream = File.OpenRead("disk.image");

// Parse the Disk Copy image
var image = new DiskCopy42Image(stream);

// Get image header information
Console.WriteLine($"Image Name: {image.Header.ImageName}");
Console.WriteLine($"Disk Encoding: {image.Header.DiskEncoding}");
Console.WriteLine($"Data Size: {image.Header.DataSize} bytes");
Console.WriteLine($"Tag Size: {image.Header.TagSize} bytes");

Accessing Disk Data

// Get the raw disk data
byte[] diskData = image.ImageData;

// Get the tag data (used for copy protection/verification)
byte[] tagData = image.TagData;

// Write the raw disk data to a file
File.WriteAllBytes("output.dsk", diskData);

Verifying Checksums

using DiskCopyReader.Utilities;

// Calculate and verify checksums
uint dataChecksum = DiskCopyChecksum.Calculate(image.ImageData);
uint tagChecksum = DiskCopyChecksum.Calculate(image.TagData);

bool dataValid = dataChecksum == image.Header.DataChecksum;
bool tagValid = tagChecksum == image.Header.TagChecksum;

Console.WriteLine($"Data checksum valid: {dataValid}");
Console.WriteLine($"Tag checksum valid: {tagValid}");

API Overview

DiskCopy42Image

The main class for reading Disk Copy 4.2 images.

• DiskCopy42Image(Stream stream) - Opens a Disk Copy image from a stream
• Header - Gets the DC42 header containing image metadata
• ImageData - Gets the raw disk data as a byte array
• TagData - Gets the tag data as a byte array

DC42Header

Contains the Disk Copy 4.2 image header metadata:

• ImageName - Name of the disk image (up to 63 characters)
• ImageNameLength - Length of the image name
• DataSize - Size of the disk data in bytes
• TagSize - Size of the tag data in bytes
• DataChecksum - Checksum of the disk data
• TagChecksum - Checksum of the tag data
• DiskEncoding - Encoding type of the disk (GCR400k, GCR800k, MFM720k, MFM1440k)
• Format - Format byte
• MagicNumber - Magic number for format verification (0x0100)

DiskEncoding

Specifies the disk encoding used in the image:

• GCR400k - 400KB GCR encoding (early Macintosh)
• GCR800k - 800KB GCR encoding (double-sided Macintosh)
• MFM720k - 720KB MFM encoding (PC-compatible)
• MFM1440k - 1.44MB MFM encoding (high-density)

Building

Build the project using the .NET SDK:

dotnet build

Run tests:

dotnet test

DiskCopyDumper CLI

Extract a Disk Copy image to raw disk data using the dumper tool.

Install/Build

dotnet build dumper/DiskCopyDumper.csproj -c Release

Usage

dc-dumper <input> [-o|--output <path>] [--include-tags] [--verify]

• <input>: Path to the Disk Copy image file.
• -o|--output: Destination directory for extracted files (defaults to input filename).
• --include-tags: Also extract tag data to a separate file.
• --verify: Verify data and tag checksums.

Output files:

• <ImageName>.dsk - Raw disk data
• <ImageName>.tags - Tag data (if --include-tags is specified)

Requirements

• .NET 9.0 or later

License

MIT License. See LICENSE for details.

Copyright (c) 2026 Hugh Bellamy

About Disk Copy 4.2

Disk Copy was a disk image utility created by Apple Computer for the classic Macintosh operating system. The DC42 format (Disk Copy 4.2) was widely used for distributing software and system disks. Key characteristics:

• 84-byte header containing metadata
• Support for both GCR (Group Coded Recording) and MFM (Modified Frequency Modulation) encodings
• Optional tag data for copy protection and verification
• Checksums for data integrity verification
• Used extensively for Macintosh floppy disk distribution

Related Projects

• MfsReader - Reader for MFS (Macintosh File System) volumes
• HfsReader - Reader for HFS (Hierarchical File System) volumes