PyCAS - Python CAS File Reader

A pure Python library for reading and converting Atari 8-bit cassette (CAS) files. This library provides comprehensive support for the CAS file format as defined in the A8CAS specification.

Features

• Pure Python: Compatible with Python 3.x, no external dependencies
• Complete CAS format support: Handles all chunk types (FUJI, baud, data, fsk, pwms, pwmc, pwmd, pwml)
• Binary conversion: Extract data chunks to binary (.bin) files
• Flexible dumping: View chunk contents in hex, ASCII, or both formats
• Chunk selection: Dump specific chunks or ranges
• Metadata extraction: Read tape descriptions and baudrate information
• CLI tool: Command-line interface for file inspection and conversion

Installation

From PyPI

pip install atari-cas-reader

From source

git clone https://github.com/yourusername/pycas.git
cd pycas
pip install -e .

Quick Start

As a Library

from pycas import CASReader, read_cas_file

# Simple usage - get data and metadata
byte_array, metadata = read_cas_file('tape.cas')
print(f"Description: {metadata['description']}")
print(f"Baudrate: {metadata['baudrate']}")
print(f"Data size: {len(byte_array)} bytes")

# Advanced usage with CASReader class
reader = CASReader('tape.cas')
reader.read()

# Get metadata
metadata = reader.get_metadata()
print(f"Chunks: {metadata['chunk_count']}")

# Get chunk information
for chunk_info in reader.get_chunk_info():
    print(f"[{chunk_info['index']}] {chunk_info['type']}: {chunk_info['length']} bytes")

# Extract data to byte array
data = reader.to_byte_array()

# Convert to binary file
reader.to_bin_file('output.bin')

# Get individual data blocks
for i, block in enumerate(reader.get_data_blocks()):
    print(f"Block {i}: {len(block)} bytes")

# Dump chunks in hex format
print(reader.dump_chunks(chunk_indices=[0, 1], show_hex=True, show_ascii=True))

Command-Line Interface

Show file information (default)

python -m pycas tape.cas

Output:

CAS File: tape.cas

Metadata:
  description: My Atari Program
  baudrate: 600
  chunk_count: 5
  data_block_count: 3

Chunks:
  [0] FUJI: 16 bytes (aux: 0)
  [1] baud: 2 bytes (aux: 0)
  [2] data: 132 bytes (aux: 0)
  [3] data: 132 bytes (aux: 0)
  [4] data: 64 bytes (aux: 0)

Total data bytes: 328

Convert to binary file

python -m pycas tape.cas --to-bin output.bin

Dump chunks in hex format

# Dump all chunks (hex only, default)
python -m pycas tape.cas dump

# Dump all chunks with both hex and ASCII
python -m pycas tape.cas dump --hex --ascii

# Dump specific chunk
python -m pycas tape.cas dump --chunk 0 --hex --ascii

# Dump chunk range
python -m pycas tape.cas dump --chunk 2-4 --hex --ascii

# Dump multiple chunks and ranges
python -m pycas tape.cas dump --chunk 0,2,5-7 --hex --ascii

# Dump ASCII only
python -m pycas tape.cas dump --ascii

Example dump output:

Chunk [2] Type: data, Length: 132, Aux: 0
--------------------------------------------------------------------------------
0000: 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 | UUUUUUUUUUUUUUUU
0010: 7f 7f 7f 00 01 02 03 04 05 06 07 08 09 0a 0b 0c | ................
0020: 48 45 4c 4c 4f 20 57 4f 52 4c 44 21 9b 00 00 00 | HELLO WORLD!....

CAS File Format

The CAS format stores Atari 8-bit cassette data as a series of chunks. Each chunk has:

• Header (8 bytes):
  • Chunk type (4 bytes, ASCII)
  • Length (2 bytes, little-endian)
  • Auxiliary data (2 bytes, little-endian)
• Data (variable length)

Supported Chunk Types

• FUJI: Tape description (UTF-8 text)
• baud: Transmission baudrate (default: 600 baud)
• data: Standard SIO record (typically 132 bytes)
• fsk : Non-standard FSK signal lengths
• pwms: Turbo transmission settings
• pwmc: Turbo synchronization signals
• pwmd: Turbo data block
• pwml: Raw PWM state sequence

API Reference

CASReader Class

Methods

• read(): Parse the CAS file and load all chunks
• to_byte_array() -> bytearray: Extract all data chunks into a single byte array
• to_bin_file(output_path: str) -> int: Write data chunks to binary file, returns bytes written
• get_all_chunks_as_bytes() -> bytearray: Convert entire CAS file (including headers) to byte array
• get_metadata() -> dict: Extract metadata (description, baudrate, chunk counts)
• get_data_blocks() -> List[bytes]: Get list of all data chunk contents
• get_chunk_info() -> List[dict]: Get information about all chunks
• dump_chunks(chunk_indices=None, show_hex=True, show_ascii=False) -> str: Format chunk contents for display

Properties

• filepath: Path to the CAS file
• chunks: List of parsed Chunk objects
• data_blocks: List of data chunk contents

Chunk Class

Dataclass representing a CAS chunk:

• header: ChunkHeader: Chunk header information
• data: bytes: Chunk data payload

ChunkHeader Class

Dataclass representing chunk header:

• chunk_type: bytes: 4-byte chunk type identifier
• length: int: Length of chunk data
• aux_data: int: Auxiliary/metadata value

Utility Functions

• read_cas_file(filepath: str) -> Tuple[bytearray, dict]: Convenience function returning data and metadata
• parse_chunk_selection(chunk_spec: str, max_chunks: int) -> List[int]: Parse chunk selection string

Examples

Extract program data

from pycas import CASReader

reader = CASReader('program.cas')
reader.read()

# Save just the data chunks to binary
reader.to_bin_file('program.bin')
print(f"Extracted {len(reader.to_byte_array())} bytes")

Analyze tape structure

from pycas import CASReader

reader = CASReader('tape.cas')
reader.read()

print("Tape Analysis")
print("=" * 60)

metadata = reader.get_metadata()
print(f"Description: {metadata['description']}")
print(f"Baudrate: {metadata['baudrate']}")
print(f"\nChunk Structure:")

for info in reader.get_chunk_info():
    print(f"  [{info['index']:2d}] {info['type']:4s} - "
          f"{info['length']:4d} bytes (aux: {info['aux_data']})")

total_size = sum(chunk.header.length for chunk in reader.chunks)
print(f"\nTotal payload: {total_size} bytes")

Dump specific chunks

from pycas import CASReader, parse_chunk_selection

reader = CASReader('tape.cas')
reader.read()

# Parse user input like "0,2-5,7"
chunks = parse_chunk_selection("0,2-5,7", len(reader.chunks))

# Dump with hex and ASCII
output = reader.dump_chunks(chunks, show_hex=True, show_ascii=True)
print(output)

Development

Building from source

# Clone repository
git clone https://github.com/yourusername/pycas.git
cd pycas

# Install in development mode
pip install -e .

# Run tests (if available)
python -m pytest

Building for PyPI

# Install build tools
pip install build twine

# Build distribution
python -m build

# Upload to PyPI (requires credentials)
python -m twine upload dist/*

Requirements

• Python >= 3.6
• No external dependencies

License

MIT License - see LICENSE file for details

Contributing

Contributions are welcome! Please feel free to submit pull requests or open issues for bugs and feature requests.

References

• A8CAS File Format Specification
• Atari 8-bit Cassette System

Author

Claude.ai Sonnet 4.5 - Prompted by Aykut Kılıç

Changelog

0.1.0 (2025-01-XX)

• Initial release
• CAS file reading and parsing
• Binary conversion support
• Hex/ASCII dump functionality
• Command-line interface
• Complete chunk type support