tank-vs-ufo-2.0

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tank-vs-ufo-2.0

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Assembly rewrite of the 1981 BASIC type-in program Tank-v-UFO by Duane Later, for the Commodore VIC-20.

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- tank-vs-ufo-2.prg
- tank-vs-ufo-2.d64
- tank-vs-ufo-2.tap
- - screenshot-gameplay-3.png
  - tank-vs-ufo-2.0-assembly-rewrite.gif
- tank-vs-ufo-2.asm
- prg2tap.py
LICENSE
README.md

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# 프로젝트 배지

Assembly
Machine Language
6502
Kick Assembler
Commodore VIC-20

// repository documentation

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<img src="images/screenshots/tank-vs-ufo-2.0-assembly-rewrite.gif" width="80%" alt="Tank vs UFO 2.0 (Assembly Language Rewrite)"> 
 See <a href="https://www.youtube.com/watch?v=hgRPHROP4tU">YouTube</a> video, for recording with sound.

**Tank vs UFO 2.0** is an assembly language rewrite of the 1981 **BASIC** type-in game, **Tank-v-UFO**, by *Duane Later*, from the **Commodore VIC-20** User's Manual. The original game's gameplay mechanics is preserved, with one strafing UFO at a time dropping aimed bombs, a tank trundling along the ground line, endless play with a UFOS/TANKS kill tally. But the engine underneath is brand new, with a non-blocking state machine, clocked by a single 60 Hz frame loop optimised for PAL, so the game never pauses the way the original **BASIC** version did during events, like air explosions or UFO crashes, etc.

**Features**

- **Faithful gameplay:** Like *Duane Later's* original, one UFO at a time, aimed bombs, endless play and score chase (no win condition), the same as the originally published versions of the original.

- **Non-blocking engine:** The tank stays under player control while a UFO explodes, UFOs keep strafing while the tank burns, and bullet-vs-bomb intercepts never pause play.

- **Animated, heat-graded effects:** 3-frame air explosions, ground puffs where bombs miss, and alternating flame glyphs with red tips over a yellow base.

- **Runs on a stock unexpanded VIC-20** A single PRG loading at `$1001`, started with a plain `RUN`.

- **Release:** Ships as a bare `.prg`, a bootable `.d64` disk image, and a `.tap` Datasette tape image.

> ***See also:** [**Tank vs UAP**](https://github.com/rohingosling/tank-vs-uap) An extended arcade-style reimagining of the same 1981 original, also for the unexpanded **Commodore VIC-20**.*

## 📑 Contents

- [🔎 Overview](#-overview)
- [🚀 Quick Start](#-quick-start)
- [🎮 Controls](#-controls)
- [🕒 History](#-history)
- [📂 Project Structure](#-project-structure)
- [💻 Building From Source](#-building-from-source)
- [🙋‍♂️ Acknowledgements](#-acknowledgements)
- [📄 Credits & Licensing](#-credits--licensing)

## 🔎 Overview

A lone UFO strafes back and forth across the sky (character rows 4–16), dropping bombs aimed at your tank. You drive left and right along the ground line and fire straight up. Shoot the UFO and it explodes in the air, then crash-dives diagonally into the ground and burns. Get hit and your tank burns instead. If you die, a fresh tank is spawned and the duel continues. There is no end and no win condition, just a UFO vs tank tally, exactly as *Duane Later* wrote it in 1981.

The rewrite targets a stock unexpanded **Commodore VIC-20**, optimised for PAL, with NTSC also supported by the jiffy-clock timebase. The whole game is one PRG, no expansion RAM, no overlays, no custom IRQ. The KERNAL interrupt keeps running and supplies both the 60 Hz frame timebase and the current-key input, matching the original's `PEEK(197)` semantics.

### Features

Deliberate improvements over the 1981 original:

| # | Change |
|---|--------|
| 1 | **Non-blocking events.** The original ran every explosion, crash dive, and fire as a blocking **BASIC** subroutine, freezing the whole game. In this assembly version, every entity is a state machine stepped from a single 60 Hz frame loop. |
| 2 | While the tank is burning, UFOs keep flying but drop no bombs until the new tank spawns. |
| 3 | Tank bullets only collide with a UFO that is still flying — exploding and crash-diving UFOs ignore bullets. |
| 4 | A hit UFO begins its crash dive immediately while the air explosion animates independently at the point of bullet contact. This keeps the game play feeling more responsive. |
| 5 | **Animated effects** At 150 ms per frame: a 3-frame heat-graded air explosion, a 3-frame ground puff where bombs miss, and heat-graded burning-tank fire. |
| 6 | **Colour changes:** - Fire purple → red - Score text yellow → Blue - Tank yellow → blue - Air explosion black → red/yellow - Muzzle flash purple → yellow |
| 7 | The lowest UFO strafing altitude is raised one character row (flight rows 4–16). |
| 8 | **Symmetric tank travel:** I'm not sure if the original version had a bug, or if it was perhaps a intentional strategic decision to prevent scrolling when characters are printed to the bottom-right cell of the screen. Either way, the original left the rightmost column unreachable by the tank. In this version, the column clamp is 0–16 making horizontal travel by the tank symmetrical across the screen. |
| 9 | **Q quits at any time** A stub in the cassette buffer wipes the game's RAM and resets cleanly to **BASIC**. The original **BASIC** version could be escaped easily with `RUN/STOP` or `RUN/STOP + RESTORE` to return to **BASIC**. However, because this rewrite is a machine language program, the `RUN/STOP` key won't work. So the game implements a machine reset and binds it to the `Q` key to give a user a way to return to **BASIC**. |
| 10 | **Event durations re-tuned:** While the idea with this project was to keep the game mechanics faithful to the original, I took the liberty of speeding up game pace a bit by tuning event timings as follows. - Tank burn 1.0 s - Ground fire 1.0 s - Shot fade 0.5 s - Air explosion 3 × 150 ms |

Original-edition bugs fixed (bugs, not behavior):

- The score reprint no longer eats the ground line.
- A bullet hitting a bomb no longer destroys the tank (the original's `PEEK` collision sent any non-space cell to the tank-hit routine). The bullet and bomb now annihilate each other and play continues.
- Explosion and crash-dive cell writes are clamped to the screen edges (the original's address arithmetic wrapped across rows).

## 🚀 Quick Start

Want to just play **Tank vs UFO 2.0**? Download what you need from the v2.0 release:

| File | Download | Use case |
|------|----------|----------|
| `tank-vs-ufo-2.prg` | [download](https://github.com/rohingosling/tank-vs-ufo-2.0/releases/download/v2.0/tank-vs-ufo-2.prg) | Run on **VICE**, or load on a real **VIC-20** via **SD2IEC** / 1541 |
| `tank-vs-ufo-2.d64` | [download](https://github.com/rohingosling/tank-vs-ufo-2.0/releases/download/v2.0/tank-vs-ufo-2.d64) | Bootable 1541 disk image |
| `tank-vs-ufo-2.tap` | [download](https://github.com/rohingosling/tank-vs-ufo-2.0/releases/download/v2.0/tank-vs-ufo-2.tap) | Load on a real **VIC-20** via **TAPuino**, or record onto a cassette |

### Run on VICE

```bash
xvic -memory none -autostart tank-vs-ufo-2.d64
```

The `-memory none` flag is required. **VICE** defaults to a 3 KiB RAM expansion, which moves the **BASIC** program area and silently breaks the `$1001` PRG load — the game targets a stock unexpanded **VIC-20**. If you configure **VICE** through the GUI instead, set the RAM expansion to **none** before launching.

### Run on real hardware

| Loading device | File | Load command |
|----------------|------|--------------|
| **TAPuino**, or `tank-vs-ufo-2.tap` recorded onto a cassette | `tank-vs-ufo-2.tap` | `LOAD "TANK-VS-UFO-2"` |
| **SD2IEC** / **Pi1541** / 1541 Ultimate / real 1541 floppy | `tank-vs-ufo-2.d64` or `.prg` | `LOAD "TANK-VS-UFO-2",8` |

Then type `RUN`.

## 🎮 Controls

| Key | Action |
|-----|--------|
| `Z` | Move tank left |
| `C` | Move tank right |
| `B` | Fire |
| `Q` | Quit — Wipes the game's RAM and resets cleanly to **BASIC** |

Input uses the original's one-key-at-a-time `PEEK(197)` semantics, and the tank cannot be controlled while it is burning — both faithful to the 1981 game.

## 🕒 History

**Tank-v-UFO** appeared in 1981 as a type-in **BASIC** listing by **Duane Later** in the **Commodore VIC-20** User's Manual. **Tank-vs-UFO** was one of the first games many **VIC-20** owners ever ran, typed in line by line from the book that came in the box. Slight variations exist between the 1981 and 1983 editions of the VIC-20 user manual, but the core game mechanics remained the same.

Over the years a number of fan made variations of the game have appeared, including this novel web based variation, [A Tribute to Tank-V-UFO](https://michaeldipperstein.github.io/tankvufo.html), written in **C**, by **Michael Dipperstein**.

**Tank vs UFO 2.0** is an assembly re-write of the original **BASIC** game, intended to keep the core game play mechanics the same, but with slightly enhanced game play pacing and animations.

## 📂 Project Structure

```
tank-vs-ufo-2.0/
├── src/
│   └── tank-vs-ufo-2.asm        The complete game (single Kick Assembler source).
├── build/
│   └── tank-vs-ufo-2.prg        Pre-built VIC-20 binary (loads at $1001).
├── dist/
│   ├── tank-vs-ufo-2.d64        Bootable 1541 disk image.
│   └── tank-vs-ufo-2.tap        Datasette tape image.
├── tools/
│   └── prg2tap.py               PRG -> TAP converter (Python 3).
├── images/
│   └── screenshots/             Gameplay captures used in this README.
├── LICENSE
└── README.md
```

## 💻 Building From Source

Assemble with [**Kick Assembler**](http://www.theweb.dk/KickAssembler/) (requires JRE 8+):

```bash
java -jar KickAss.jar src/tank-vs-ufo-2.asm -odir build
```

Package the disk image with `c1541` (ships with [**VICE**](https://vice-emu.sourceforge.io/)):

```bash
c1541 -format "tank vs ufo 2,t2" d64 dist/tank-vs-ufo-2.d64 -write build/tank-vs-ufo-2.prg "tank-vs-ufo-2"
```

Package the tape image with the bundled converter (requires Python 3.8+):

```bash
python tools/prg2tap.py build/tank-vs-ufo-2.prg dist/tank-vs-ufo-2.tap "TANK-VS-UFO-2"
```

Test builds: assembling with `-define AUTOPILOT` holds the fire key forever (for headless soak tests), `-define FLAMETEST` fabricates a burning tank beside a crashed UFO at boot, and `-define BLASTTEST` fabricates an air blast and muzzle flash at boot. Direct each one's output to a separate PRG with `-o` so the release binary is never overwritten.

## 🙋‍♂️ Acknowledgements

| Tool / Work | Author&nbsp;/&nbsp;Maintainer | Role in this project |
|-------------|-------------------------------|----------------------|
| **Tank-v-UFO** (1981) | Duane&nbsp;Later | The original **BASIC** type-in game, from the **Commodore VIC-20** User's Manual. The behavioural reference for this rewrite. |
| [Kick&nbsp;Assembler](http://www.theweb.dk/KickAssembler/) | Mads&nbsp;Nielsen | 6502 cross-assembler. Builds `tank-vs-ufo-2.prg` from `tank-vs-ufo-2.asm`. |
| [VICE](https://vice-emu.sourceforge.io/) | The&nbsp;**VICE**&nbsp;Team | Commodore emulator suite. `xvic` for development and testing; `c1541` for disk packaging. |

## 📄 Credits & Licensing

The original work of authorship in this repository — the 6502 assembly source (`src/tank-vs-ufo-2.asm`), the non-blocking engine design, the build tooling, and this documentation — is © 2021 Rohin Gosling and released under the [MIT License](LICENSE). The MIT grant is permissive: it allows use, modification, and redistribution (including commercial use), provided the copyright notice and licence text are preserved.

**Tank vs UFO 2.0** is an independent reimplementation of the 1981 type-in game **Tank-v-UFO** by *Duane Later*, originally published in the **Commodore VIC-20** User's Manual. The design, rules, and presentation of that original game are the work of their respective copyright holder(s); this project claims no ownership over them, and the MIT licence above applies only to the new code and material created for this rewrite — **not** to the underlying 1981 work.

This is a personal retrocomputing project, shared for historical, educational, and preservation purposes.

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