Erm...a bunch of 1's and 0's? Seriously: I guess you're talking about Waninkoko's rev20 (the "b" is just to differentiate from the original rev20 which had a fault in the installer...the cIOS themselves contain the same code). It is a cIOS...in full: "custom Input Output System". It is a way so the wii can handle input and output from various sources like the wiimote, the DVD drive and USB hard drives. Waninkoko updates those from time to time to keep up with nintendo's latest official IOS releases, USB drive handling, nand emulation and more of that stuff. The "contents" is a bunch of computer code that makes your wii able to do things like read discs from USB instead of from DVD, or fool the wii into thinking that it reads/writes to the nand while it actually does that from the USB drive. The wii's internal hard drive (the nand) has a large open section for things like savegames, channels and all that junk...and a part where all the IOS'es of the wii are stored (you know the BIOS of your computer? It's the same thing...except that the wii has a whole bunch instead of just 1). These IOS'es have numbers. In addition, there are more numbers available than there are IOS'es installed, which is why a cIOS can be installed to the empty numbers (I usually refer to them as "slots"). If you install rev20b, it'll ask you 2 things: 1. the number to where to install. Either go for an empty slot, or overwrite a cIOS you no longer need. It is ill-advised to choose a number below 200 (if that is even possible with the installer), as overwriting official IOS'es may cause nasty things like system menus not booting (the system menu relies on a working IOS as well). 2. the base IOS. Basically, a cIOS isn't made from scratch, but by making modifications to already existing IOS'es. Like retail games use different IOS'es to work correct, recent cIOS'es must use a different base IOS to allow backups of said games to be played on eg. an USB drive. So...that's all I can think of. If you still don't get an answer to your question...DO A BETTER JOB OF PHRASING IT! (important note: I'm not a programmer...someone please correct or add things if I'm wrong).
