Moons of Jupiter (Vic 20)


1982 Romik
Code and design – David Byrden

As you have probably guessed by now and from my blog, I have particular fond memories of games from my childhood, none more so than on the Commodore 64, Atari 2600 and also the Vic 20. One such Vic 20 game is yet another along with Chariot Race which is firmly en-grained into my memories, the classic ‘Moons of Jupiter’.

Cover art for the game

‘Moons of Jupiter’ was the product of David Byrden, who came over from Ireland to work for Romik Software back in the early 80’s. Romik were particularly stand out compared to most companies at the time. Most companies would hire a talented artist to create an amazing fantasy inlay – which very often covered up a game which was to be very poor once you loaded it up. Romik wanted to be open about what you were buying, so proudly presented screenshots on the front cover (Although earlier covers were actual carefully handdrawn screenshots).

David Byrden was caught up in the buzz of creating games when starting up with Romik, though the flirtation with developing games in a new era was to be sadly short lived:

“My technical skill in hardware was matched by my complete lack of business acumen and so I didn’t enjoy much success from gaming. I went to work for Romik, wrote some games-to-order which were greatly inferior, then the company folded in 1985 and I returned to Ireland. In retrospect I should have honed my coding skills and sold my services to a more long-lasting company.”

Very modest words from David, which is especially apparent when you first load up ‘Moons of Jupiter’. Back in 1982, games on the Vic 20 were fairly standard and many were coded in BASIC and were poor clones of arcade games, a handful at that time (The years that followed saw vast improvements) stood out from the crowd, and most were Commodore’s own output. ‘Moons of Jupiter’ was different though, and was a spectacular Asteroids clone with full bit-mapped graphics (replacing the vectors of the original).

The game in full action

Not only were the graphics particularly striking, but the amount of action, the speed and smoothness of it all was breathtaking. It even posed a significant challenge, with tracking bombs called Gologs that occasionally popped out and which were hard to try and shake off (You cannot shoot them, but must guide them into the path of a moon to destroy). In addition to this, the ability to hyperspace was missing – which ramped things up a notch too.

Overall the game felt like the work of someone who had been working with the hardware for years, though David had in fact only really just started properly programming with the hardware. David enlightened us with his secret behind the game’s engine…

“The smooth graphics was achieved by getting the specs for the graphics hardware and the microprocessor, and doing everything at the level of assembly code. Fractional pixel positioning was the key to the smoothness; I think that I used 2-byte
accuracy for X and Y positions, so each pixel was divided into 256 parts. And of course, timing was determined by the screen refresh, not by whatever speed the code could run at.

The next trick was to put those large objects on-screen when the Vic had a limited number of bit-mapped characters. I had to reserve the necessary characters to cover each object, place them on the screen where the object would appear, and draw the object within them (at the correct internal offset) with correct handling of overlaps, i.e. drawing some objects partially within the character blocks of other ones. It was complex, but I could see from the start exactly what I wanted to do, so there were no head-scratching moments, just lots of careful coding.”

In the early days of gaming, releases from companies often started out as  submissions sent in from bedroom programmers hoping to get published.   This was exactly how David got his foot in the door and and signed up with Romik:

“I wrote the game entirely on my own and sent it to Romik on spec.  It was after accepting that game
that they hired me. In fact Mike Barton flew to Ireland in order to close the deal.  Mike Barton chose the name (Moons of Jupiter). I have no musical talent, so if there’s music in the game it was provided by Romik. The packaging is theirs too. Everything else is by me.”

Overall the game was a great success for Romik, scoring very highly in all the magazines at the time and became one of their most popular selling titles. What was next though for David?

“My second big effort was a graphics editing program for the C64 which was picked up by Commodore and packaged as Tony Hart’s Art Master. But the lack of background knowledge hit me, and although the program had some really efficient code in it, it wasn’t a usable graphics editor. I didn’t know that you ought to use a tablet for input! So that didn’t sell. Then I dropped out of game writing altogether.”

Sadly we were never to see any more of David’s promising coding talent. It would have been fantastic to see his work transfer over to the C64 and see more excellent games, but it just wasn’t to be.

“I did hope to write C64 games, and if the industry hadn’t collapsed I think that I would have. I do also recall experimenting with 3D first-person graphics for a game that simulated flight – but, sadly, I don’t have many more memories of what I did, and I don’t have source code.”.

If you like your asteroid clones and want to check out one of the best clones out there, then you can do no wrong by taking a look at the excellent ‘Moons of Jupiter’.

Download game (TAP format)

Gallery of scans and screenshots


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3 thoughts on “Moons of Jupiter (Vic 20)

  1. Robinson Mason June 1, 2011 / 2:11 am

    Really enjoying these posts and the inlay scans are great. Thanks!

  2. Jason Mitchell February 7, 2012 / 12:45 pm

    Tony Hart’s Art Master was awesome!! I bought that and found it very easy to use!! No tablet was needed.

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