Hold back the invaders, their infernal machines.

That was the first verse from Space Invaders, the 1980 song from the Australian group Player One. It was an awfully camp song, and the band that made it was hopefully abducted by angry aliens. But at the time, it sounded like a terrific, thumping soundtrack to many of us who were obsessed with playing the arcade video game Space Invaders.

There is scarcely a person between the ages of 35 and 50 who has not at one time or another tried to save the Earth from a legion of marauding extra-terrestrials. Before these aliens could establish a staging post on the moon, it was our job to blow the b******s to smithereens. We only had one laser cannon as a weapon and four lousy green barriers to hide behind.

It was a futile task. The faster we shot the aliens, the faster they marched towards us. As they got closer, bombs dropped to destroy our stationary defence bunkers. No wonder there is an entire generation of gamers on antidepressants. It's post-traumatic stress disorder. We once were warriors.

All this reminiscing was prompted by the realisation that Space Invaders - created by video-game designer Tomohiro Nishikado in 1978 for erstwhile pachinko machine vendor Taito Corp - turns 30 this year.

My initial close encounter of the third kind was in 1981 at the Centrepoint shopping mall in Sydney, Australia. A couple of mates and I caught the train down to the city after hearing about this new video game about invading aliens.

Growing up, we played pinball machines in the corner shops of Umina Beach, New South Wales. The first video game we played was Pong. It was a Christmas gift in 1974. I thought Pong was hi-tech, with the game console's wires snaking across the living room floor. I relished slamming that dot from one side of the screen to the other.

At Centrepoint, there was a competition to see who could be the grand Space Invaders champion. About 100 arcade machines had been imported from Japan. We had never seen anything like it. A huge room was packed wall-to-wall with upright cabinets, adorned with buttons, joysticks and eye-catching artwork. But it was the noise that was intoxicating; the zaps, buzzes and boomp-boomp-boomp as the alien army advanced relentlessly.

Space Invaders, which featured two-dimensional computer graphics played on a television set, was the first video game to feature a high-score function.

It was a time when our pockets jangled heavily with coins and our synapses were firing on all cylinders, fuelled by salt-and-vinegar crisps and Coca-Cola. We were doing our bit for planet Earth, long before the eco-nazis urged us to say 'No' to plastic bags.

Similar battles were being waged in Melbourne, Brisbane and around the world. I have no idea who became the national Space Invaders grand champion, but it was not one of us.

There are several accounts of how Nishikado had the idea for Space Invaders - which was titled Space Monsters until his boss insisted on a name change. One version says it came to Nishikado in a dream, in which Santa Claus and his flying reindeers suddenly morphed into a bunch of evil aliens. Children, using odd bits and pieces, hastily assembled a laser gun powered by a car battery and blasted the creatures out of the sky.

In an online interview, Nishikado says the targets in his shooting game were initially going to be tanks or aeroplanes. But that was too difficult to simulate using the 8-bit technology of the day. He also considered soldiers as targets but felt that would be politically incorrect.

Another version says Nishikado happened to see a magazine feature about American filmmaker George Lucas' blockbuster Star Wars, which was then slated for release in Japan. He was also inspired by the aliens from HG Wells' The War of the Worlds. The creatures became simplified images of crabs, squid and octopuses.

Space Invaders was launched in Tokyo in June 1978. It was so successful that extra 100-yen coins had to be minted in Japan to cope with the shortage. Taito licensed Space Invaders to the amusement machine maker Midway for production in the United States. It wasn't long before the whole world went crazy over Space Invaders. The machines reached Britain in 1979. Space Invaders swiftly spread from the amusement arcades into laundromats, pubs and corner stores. It reached homes in 1980, when the game became the first arcade title adapted by Atari for its cartridge-based 2600 system.

Over the years, Taito generated about US$500 million revenue from the game. It remains one of the most loved and influential video games in history, spawning a raft of imitators.

That is not bad for a basic software program using a 2-megaHertz Intel 8080 processor and a mere 8 kilobytes of random access memory (Ram). To put that in perspective, a top-of-the-range desktop personal computer specifically designed for hard-core gaming currently uses a 2.4-gigaHertz Intel Core 2 Quad processor (which combines two or more chip cores in a single processor package) and about 4 gigabytes (4 million kilobytes) of Ram.

Space Invaders, Pong and Pac-Man are among the oldies-but-goodies showcased at Game On, a touring exhibition on the evolution of computer games. In Hong Kong last year, the show is a collaboration between the British Council, London's Barbican Art Gallery and Cyberport.

'The history of computer gaming is fascinating but underexposed,' says David Chung Wai-keung, head of information technology operations at Hong Kong Cyberport Management.

There have been several variations of Space Invaders over the years. The original was built into a 'cocktail table' and offered black-and-white graphics. The early upright cabinets used in arcades boasted cellophane overlays to give the impression of colour on screen.

Initially, the machines were sold to clubs and pubs, but people started asking to purchase them for their home. 'I get a lot of women buying them for their partners' 40th birthday,' says James McLennan, of Arcade Tables.

McLennan reckons his cocktail table machines encourage more social interaction than their modern counterparts. 'Blokes might get some mates around to watch a football game and they'll have some beers and challenge each other to a game of Space Invaders before the kickoff. Try having ale and nuts playing an Xbox!'

The Halo series, created by US game developer Bungie and published by Microsoft Game Studios, is one of the world's best-selling video-game franchises. As of October last year, the Halo trilogy had sold more than 20 million copies.

There is little doubt Halo is this generation's Space Invaders. Consider the similarities: both are shooter games and tackle a war against aliens in the future. The game may have changed, but we're still trying to save humanity.

Additional reporting by Bien Perez.