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Overview

CDT (Cass Data Tape) files are tape images for Amstrad CPC that store programs sequentially, like real cassette tapes. DevCPC can automatically generate CDT files alongside DSK images.

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What is a CDT?

CDT is the standard format for Amstrad CPC cassette tapes. Unlike DSK (random access), CDT stores files sequentially:
  • Sequential loading - Files load in order
  • Tape-like behavior - Mimics real cassette tapes
  • Authentic experience - True-to-original loading
  • Variable size - Only uses space needed

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Configuration

Add to your devcpc.conf: 
# Enable CDT generation
CDT="${PROJECT_NAME}.cdt"

# List files in loading order (CRITICAL)
CDT_FILES="loader.bas 8BP0.bin main.bin"

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Important: File Order

The order in CDT_FILES is critical - files are recorded and loaded sequentially: 
# ✅ Correct: loader first, then binaries
CDT_FILES="loader.bas 8BP0.bin game.bin"

# ❌ Wrong: tries to load binaries before loader
CDT_FILES="game.bin 8BP0.bin loader.bas"

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File Types Supported

TypeExtensionDescriptionCPC Load Command
BASIC.basTokenized BASICRUN"
Binary.binMachine codeRUN"FILE"
Screen.scnFull screensRUN"SCREEN"
Raw.txt, etcData filesDirect read

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CDT Creation Process

During devcpc build, if CDT is configured:

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Step 1: Read Map File

DevCPC reads ${PROJECT_NAME}.map to get:
  • File type (BASIC, binary, screen, raw)
  • Load address
  • Execute address

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Step 2: Validate Files

Checks that all files in CDT_FILES:
  • Exist in obj/ directory
  • Are registered in map file
  • Have valid metadata

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Step 3: Create Empty CDT

Initializes the tape image: 
python3 cdt.py --create my-game.cdt

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Step 4: Add Files in Order

Adds each file sequentially: BASIC/ASCII files: 
--put-ascii loader.bas --name "LOADER"
Binary files: 
--put-bin 8BP0.bin --load-addr 0x5C30 --start-addr 0x5C30 --name "8BP0"
Screen files: 
--put-bin title.scn --load-addr 0xC000 --start-addr 0xC000 --name "TITLE"
Raw files: 
--put-raw data.bin --name "DATA"

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Step 5: Show Catalog

Displays tape structure: 
ℹ Catálogo del CDT:
Block 0: LOADER (BASIC)
Block 1: 8BP0 (Binary, 0x5C30)
Block 2: MAIN (Binary, 0x4E20)

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Build Output Example

═══════════════════════════════════════
  Crear Cinta CDT
═══════════════════════════════════════

ℹ CDT: space-game.cdt
ℹ Archivos: loader.bas 8BP0.bin game.bin

→ Creando CDT vacío...
✓ CDT creado: dist/space-game.cdt

→ Añadiendo loader.bas al CDT...
ℹ   Tipo: BASIC
ℹ   Name: LOADER
✓ loader.bas añadido

→ Añadiendo 8BP0.bin al CDT...
ℹ   Tipo: BINARIO
ℹ   Load: 0x5C30
ℹ   Exec: 0x5C30
ℹ   Name: 8BP0
✓ 8BP0.bin añadido

→ Añadiendo game.bin al CDT...
ℹ   Tipo: BINARIO
ℹ   Load: 0x4E20
ℹ   Exec: 0x4E20
ℹ   Name: GAME
✓ game.bin añadido

✓ 3 archivo(s) añadido(s) al CDT

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Loading from CDT

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From CPC (464)

10 REM Start tape
20 RUN"
The tape auto-plays and loads files in order.

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From CPC (664/6128)

Models with disk drives need to switch to tape: 
10 |TAPE        ' Switch from disk to tape
20 RUN"         ' Start loading
DevCPC automatically handles this when using devcpc run --cdt.

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CDT vs DSK Comparison

FeatureCDT (Tape)DSK (Disk)
AccessSequentialRandom
SpeedSlow (~1-5 min)Fast (~instant)
Load commandRUN"RUN"FILE"
File orderCriticalDoesn’t matter
SizeVariableFixed (178KB)
AuthenticityHigh (retro)Moderate
Use caseDemos, nostalgiaDevelopment, games
HardwareAll CPCs664/6128/M4

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When to Use CDT

✅ Use CDT for:
  • Authentic retro experience
  • Demos and presentations
  • Type-in programs
  • Competitions requiring tape
  • CPC 464 (tape-only model)
✅ Use DSK for:
  • Fast development cycle
  • Random file access
  • Multiple programs
  • Frequent updates

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Running CDT Files

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Automatic Mode

devcpc run
With RUN_MODE="auto" in config:
  • If CDT exists and is configured → uses CDT
  • Otherwise → uses DSK

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Force CDT

devcpc run --cdt
Always uses tape, ignoring RUN_MODE.

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Force DSK

devcpc run --dsk
Always uses disk, ignoring RUN_MODE.

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Execution Modes

Configure in devcpc.conf: 
# Auto-detect (default)
RUN_MODE="auto"

# Always use disk
RUN_MODE="dsk"

# Always use tape
RUN_MODE="cdt"
ModeBehaviorOverride
autoCDT if configured, else DSK--dsk / --cdt
dskAlways DSK--cdt
cdtAlways CDT--dsk

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Complete Example

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Project Configuration

# devcpc.conf
PROJECT_NAME="platformer"
BUILD_LEVEL=2

# Generate both formats
DSK="${PROJECT_NAME}.dsk"
CDT="${PROJECT_NAME}.cdt"

# Files on tape (in order)
CDT_FILES="intro.bas title.scn loader.bas 8BP2.bin"

# Default to tape
RUN_MODE="cdt"

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File Structure

platformer/
├── bas/
│   ├── intro.bas      # First: intro animation
│   └── loader.bas     # Third: load game
├── assets/
│   └── screen/
│       └── title.png  # Second: title screen
├── asm/
│   └── make_all_mygame.asm  # Fourth: game code
└── devcpc.conf

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Build and Run

# Build both DSK and CDT
devcpc build

# Output:
# - dist/platformer.dsk (all files, random access)
# - dist/platformer.cdt (ordered files, sequential)

# Test with tape
devcpc run --cdt

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Loading Sequence

What happens on CPC:
  1. User types: RUN"
  2. Tape starts playing
  3. Loads intro.bas → runs animation
  4. Loads title.scn → shows title (from intro.bas)
  5. Loads loader.bas → sets up memory
  6. Loads 8BP2.bin → game starts

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File Order Best Practices

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Typical Game Order

# Pattern: intro → screens → loader → code
CDT_FILES="intro.bas title.scn loader.bas 8BP0.bin"
  1. Intro - Quick BASIC animation/message
  2. Title screen - Visual while loading
  3. Loader - Sets up memory and palette
  4. Game binary - Main game code

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Simple Game Order

# Pattern: loader → code
CDT_FILES="loader.bas 8BP0.bin"
  1. Loader - Basic setup
  2. Game - Game code

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Demo Order

# Pattern: screen → screen → code
CDT_FILES="logo.scn title.scn demo.bin"
  1. Logo - Company/group logo
  2. Title - Demo title
  3. Demo - Demo code

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Advanced CDT Features

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Multiple Programs

CDT can contain completely separate programs: 
CDT_FILES="game1.bas 8BP0.bin game2.bas 8BP1.bin"
User can load any program by positioning tape.

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Data Files

Include data files in sequence: 
CDT_FILES="loader.bas 8BP0.bin levels.dat music.dat"
Load from BASIC: 
100 OPENIN "levels.dat"
110 INPUT#9, level$
120 CLOSEIN

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Screens as Loaders

Screens can auto-execute: 
CDT_FILES="autoload.scn loader.bas 8BP0.bin"

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Troubleshooting

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CDT Not Created

Check configuration: 
# Both must be set
CDT="${PROJECT_NAME}.cdt"
CDT_FILES="loader.bas 8BP0.bin"
Check files exist: 
ls obj/loader.bas obj/8BP0.bin

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Files Not Loading

Wrong order:
  • Loader must come before binaries
  • Screens before code that displays them
Missing map entry:
  • Files must be in ${PROJECT_NAME}.map
  • Build generates this automatically
Wrong file type:
  • Check extension (.bas, .bin, .scn)
  • Verify type in map file

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Tape Errors on CPC

“Read error”:
  • Try |TAPE first (664/6128)
  • Check emulator tape speed
  • Verify CDT file not corrupted
“File not found”:
  • CDT loads sequentially
  • Can’t skip files
  • Must load in order

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Manual CDT Operations

# View CDT catalog
python3 ~/.DevCPC/tools/abasm/src/cdt.py dist/my-game.cdt --dir

# Create empty CDT
python3 ~/.DevCPC/tools/abasm/src/cdt.py --create custom.cdt

# Add BASIC file
python3 ~/.DevCPC/tools/abasm/src/cdt.py custom.cdt \
  --put-ascii loader.bas --name "LOADER"

# Add binary file
python3 ~/.DevCPC/tools/abasm/src/cdt.py custom.cdt \
  --put-bin game.bin --load-addr 0x4000 --start-addr 0x4000

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Best Practices

  1. Order matters - Plan loading sequence carefully
  2. Test early - Use devcpc run --cdt frequently
  3. Keep it simple - Fewer files = faster loading
  4. Document order - Comment CDT_FILES in config
  5. Provide DSK too - Give users both options

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See Also

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