Submitted by twovests in technology
iPods had a special operating system, not iOS and even based on MacOS. Neat:
The operating system is entirely implemented in a single binary, and there is no memory space separation between the kernel, user interface, and other components. While eApps (games) are implemented as separate binaries that are installed to the iPod's storage, they are executed within the same memory space as the kernel. iPod Software can therefore be seen as having equivalent monolithic architecture to Classic Mac OS.
More reading on a Jailbreaking wiki: https://freemyipod.org/wiki/RetailOS