A Lifetime Quest For Learning Machine Code On The ZX Spectrum Next

Background

The ZX Spectrum is a classic microcomputer from 1982 that captivated a generation with its unique charm and simplicity. It was a revolution in personal computing whose impact is still felt to this very day, 40 years after its original launch by Sinclair. It was the first personal computer I owned and it opened my eyes to the world of 8-bit gaming and programming and I still cherish it to this day.

Fast forward to 2023, and the ZX Spectrum Next emerges, a modern reincarnation fuelled by a successful Kickstarter campaign. This new iteration retains the nostalgic essence while boasting enhanced capabilities.

The heart of Spectrum programming lies in two languages: BASIC, known for its simplicity, and machine code, celebrated for its performance and precision. And back in the day, machine code was the way to go by developers for building games for the Spectrum, although it was a lot more complex than programming in BASIC which for all I knew was the only way to program back then.

So naturally, I always wondered why programmers almost always programmed the Spectrum using machine code. It seemed complicated to learn. And yes, there were lots of great tutorials and examples I could tinker with in magazines and books despite the lack of Internet in the 80s. Still most of it was inaccessible to me and frankly over my 14 year old brain at the time.

It always boggled my mind how anyone can build a game entirely in machine code, and the Kickstarter campaign motivated me to demystify this once and for all.

So fast forward 40 years later, in anticipation of receiving my new ZX Spectrum Next, I felt prepared to scratch this itch and finally learn how to program the Spectrum using machine code.

I wrote this guide to delve into why machine code is a game-changer for Spectrum enthusiasts and to document all that I found in one place. And also because I thought it could be helpful for people in my shoes who’ve been as curious as I was.

Advantages of Assembly Language in Game Development

Traditionally, BASIC was the go-to language for the original Spectrum. However, its limitations in speed and efficiency were evident, especially for game development. Speed and compact data storage are where assembly language shines in the context of the Spectrum. It also allows for efficient graphics rendering and complex effects, which are difficult to achieve with BASIC. This makes assembly language an invaluable skill for developing visually stunning and user responsive games.

One of the significant advantages of using assembly language for ZX Spectrum programming, particularly in game development, is the ability to utilize ROM interrupts and functions combined with access to advanced graphics effects. Programmers can create more complex and visually appealing games. They can handle multiple tasks simultaneously, like animating sprites while checking for user input, all without slowing down the game. Let’s break down what this means and why it’s beneficial:

ROM Interrupts: These are signals that temporarily halt the normal execution flow of the main program to handle specific tasks or event like screen refresh or input handling, then return control back to the main program. In the context of the ZX Spectrum, they re used for managing various tasks like screen refresh, reading keyboard input, and handling sound and music playback.

Using ROM interrupts in games allows for smoother, more responsive gameplay. For instance, a game can use an interrupt to ensure that a character’s movement on the screen is synchronized with the screen’s refresh rate, preventing flickering or tearing.

ROM Functions: These are predefined routines stored in the Spectrum’s firmware (ROM) and can be accessed via assembly to perform specific tasks saving valuable time and memory. Examples include character rendering, handling I/O operations, or even complex mathematical calculations.

By using these ROM functions, assembly programmers can save time and memory, as they don’t need to write these routines from scratch. It allows for more efficient use of the Spectrum’s limited resources where every byte counts.

Advanced Graphics Effects: In addition to leveraging ROM interrupts and functions, assembly language greatly enhances the capability to create sophisticated graphics effects on the ZX Spectrum through direct access to the ZX Spectrum’s hardware. This allows programmers to manipulate the display at a very low level, controlling aspects like pixel plotting, colour attributes, and screen buffering. And the efficient memory management can lead to more detailed sprite animations and backgrounds.

Unlike BASIC, which relies on built-in commands for graphics, assembly allows developers to write custom routines. This means they can create unique graphical effects, like smooth scrolling, sprite multiplexing (displaying more sprites than the hardware limit), and custom screen layouts that wouldn’t be possible otherwise.

In game development, the speed of redrawing graphics on the screen is crucial. Assembly language enables optimized drawing with techniques like dirty rectangle redraws or character-based screen updates, which are far more efficient than the full-screen redraws typical in BASIC programs.

In Summary, the ability to use ROM interrupts and functions is a key reason why assembly language is preferred for developing sophisticated and smooth-running games on the ZX Spectrum. It opens up a realm of possibilities that go far beyond the capabilities of BASIC. By exploiting the full graphical potential of the ZX Spectrum, assembly language allows developers to push the ZX Spectrum to its limits gaining higher sprite counts, detailed environments, and smoother animations. Allowing for the creation of games that are both technically impressive and engaging to play.

The resulting advantages in game development are tremendous. Meaning that the utilization of assembly language in ZX Spectrum game development is not merely a choice; it’s a strategic decision that unlocks the Spectrum’s full potential resulting in games that are not only technically superior but also more engaging to play. For anyone aspiring to develop high-quality games on the ZX Spectrum, mastering assembly language is inevitable.

Starting with NextBASIC

Beginners are encouraged to start with NextBASIC to grasp the fundamentals before diving into the more complex world of machine code. The ZX Spectrum Next changes the game with NextBASIC, which is eight times faster and supports advanced hardware features. Despite these improvements, machine code remains the gold standard for creating high-performance games, mirroring the quality of early 1980s arcade games.

A Stepping Stone

For those aspiring to move from BASIC to machine code, Boriel ZX Basic offers a balanced approach, simplifying the coding process while maintaining speed. The ‘next build’ libraries further empower developers by unlocking the Spectrum Next’s advanced features. And BBC Basic for the ZX Spectrum Next is another project worth checking out.

Transitioning to Machine Code

But as we saw in the previous section, learning Z80 assembly is inevitable for tackling performance issues that BASIC cannot efficiently resolve.

A great starting point for those looking to start their journey with machine code, is Spectrum Machine Language for the Absolute Beginner by William Tang which is great for learning the fundamentals. I highly recommend this book as a starting point.

From there, for a comprehensive understanding, Jonathan Cauldwell’s guide, “How to Write ZX Spectrum Games,” and Darryl Sloan’s YouTube series provide excellent insights and remain beginner friendly.

To deepen your understanding, “Programming the Z80” by Rodney Zaks is an essential read, offering insights into the Z80 processor used in the Spectrum. This book clearly shows which status flags are used and when they get modified.

For a modern perspective in the sea of resources for learning ZX Spectrum programming, “ZX Spectrum Assembly. Let’s Make a Game?” by Juan Antonio Rubio García stands out as a notable mention. This book is for experienced programmers who already have a solid grounding in programming concepts and familiarity with the hardware aspects of the ZX Spectrum. This book dives into the intricacies of the machine, which can be overwhelming for complete novices. A lack of understanding of the ZX Spectrum’s workings can make the content of this book hard to grasp. It’s a valuable resource for those who meet its prerequisites in terms of experience and dedication. It’s not a starting point for everyone, but for the right audience, it offers a deep and insightful journey into the world of ZX Spectrum game programming. It is appreciated for its practicality and the inclusion of QR codes for code examples.

For those who find “ZX Spectrum Assembly. Let’s Make a Game?” too advanced, an alternative is ‘Cracking The Code‘ which might be more suitable for beginners or those who have struggled with assembly language in the past.

Conclusion

The journey from BASIC to machine code in the world of ZX Spectrum Next programming is both challenging and rewarding. This guide is the rabbit hole for aspiring programmers wanting to embark on this exciting journey. Remember, the path to mastering machine code is demanding, but the rewards of creating sophisticated and efficient Spectrum games are immeasurable.