How to run a city builder on 16 MHz

Do you feel spoiled by modern hardware? I know I do. Even the cheapest CPUs have at least 4 cores nowadays, running billions of operations per second. The idea that you could run any software on less than a GHz almost seems like fiction. Yet, there was a time when whole games ran at hardware a thousand times less powerful than that. When thinking about impressive technical achievements in retro games, most people will first point to graphical milestones set by action games like Doom or Quake. Rightfully so, but at least I am personally far more impressed by the early examples of the city builder genre. Simulating whole economies on a large scale with a fraction of today’s power seems impossible if modern builder games even struggle on modern machines. Of course this didn’t stop developers in the past to develop and run city builder games on significantly weaker hardware. The original SimCity for example lists Intel’s 386SX as the CPU in it’s system requirements. A CPU which at the time of it’s launch ran at magnificent 16 MHz. (Note: an earlier version of this text mistakenly assumed that it ran on 12.5 MHz, which was the clock rate of base version of CPU, not the SX version. Whoops.)The idea of running a full game, especially one in the traditionally CPU-heavy genre of city builders? Pure magic from a modern point point of view. So, why could Maxis run a full city simulation on this ancient hardware, when nowadays even simple idle games require multiple gigahertz of CPU power? Of course there are a lot of reason for this, most of them external (OS ecosystem etc.) but this doesn’t mean that there are no clever optimizations done in the game code itself. Luckily, this source code was made public and released under the GPL license in 2008, under the name Micropolis. Don’t be confused by the name, this is still the code of the original name, but since only the code was made open source, not the trademark, it has to be called differently. But even under a different name, this is a precious resource, since we can exactly see how the game managed to run so well. As is often the case when analyzing software performance, we can ignore 90 percent of the code. Our focus is on the core of the city simulation, the components that update systems such as traffic, fire, power, and crime. And there is indeed one single function responsible for doing all of this: The Simulate() function in s_sim.c As the comment implies, this function is called every frame and the …