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Sunday, February 13, 2011 | Permalink
This demo is a 1K demo, that is, the final binary is no more than 1024 bytes in size. The main secrets to creating such a tiny executable is first to throw out the C runtime, and secondly to link with a compressing linker, such as Crinkler
The first thing you learn when you learn to code C/C++ is that the program execution starts at main(). This is of course a bunch of lies. The program typically starts in a startup function provided by the C runtime, which after doing a whole bunch of initialization eventually calls main(). When main() returns the program doesn't terminate, instead it just returns to the C runtime which will close down its stuff and eventually ask the OS to terminate the process. If you create a simple project with an empty main() and compile as usual you will likely get an executable that is something like 30KB or so. So you've already excluded yourself from the 4K demos, let alone 1K, even before writing any code. That overhead is mostly the C runtime. You can throw it out by specifying a custom entry point and handling process termination yourself. Throwing out the C runtime has the disadvantage of losing a whole bunch of features, including some of the most basic things like printf(), but you probably won't need so much of that anyway. With this you can easily get down to only a couple of kilobytes, depending on optimization flags used.
Using Crinkler you can get even smaller executables than with the standard linker. In addition to stripping unneccesary overhead from the executable it also compresses the code and data. It basically includes a small decompressor in the exe that unpacks everything before it runs. Compression rate depends on your code and content. Writing compressible code is essential for good compression ratio. For instance, in this demo the chosen coordinates have lots of zeros in the lower bits, which might not be immediately obvious when looking at the human readable values in the code.
Then there are of course the obvious steps such as compiling for size and setting all the compiler options to generate as small code as possible, and passing good flags to crinkler of course.
In addition to this, it's just a bunch of hard work trimming bytes here and there until you get down to your target size. This demo employs a few tricks, but there are probably more that could be used to trim it even further.
There are two variants of the demo, one compiled to be as tiny as possible and fits within the 1KB limit, and one that's somewhat more fully featured and interesting to watch but slightly larger than 1KB. The latter kind has "-Full-" in the filename. For the tiny version, run the one that matches your desktop resolution as it is not setting the display mode for you. The full version sets the display mode, so you can run any resolution of your preference. The tiny version runs through a limited set of predefined coordinates to zoom to. The full version searches random "interesting" coordinates and thus will appear different every time you run it.