RetailOS
The stock operating system running on non-iOS iPods. It runs everything from device drivers to the clickwheel user interface.
Contents
Naming
There doesn't seem to be a well-known public name for the iPod operating system. The earlier stages of the boot chain call it osos, but that might just be os repeated twice to fill the 4 characters required for payload names.
The iPodLinux folks call it RetailOS.
Architecture
OSOS is a small, embedded, single-user, single-binary, purpose-built RTOS. With time it acquire more and more complex functionality, like PowerVR drivers and being able to load external applications ('eApps') which are used for games.
A part of the operating system (likely the user-facing part) was developed by a company called Pixo [1], and traces of this pedigree can still be found in the OSOS binary (for example in strings Non Pixo Task %d, navigator.PopToPixoMainScreen). It's not exactly clear how much of the underlying RTOS functionality is based on the Pixo product vs. built from scratch by Apple.
Security
As evidenced by the success of the Notes vulnerability, at least up to Nano 4G there was no kind of security hardening, and in fact all processes, including games, seem to be running in ARM system mode. This should make exploitation of newer OSOS bugs trivial.
Boot chain
OSOS is loaded by the second-stage bootloader (stored on NOR/NAND depending on the device generation), from NAND into DRAM.
While other stages of the boot chain (eg. the bootloader, WTF mode, the diagnostics tool) are based around EFI firmware volumes and an EFI runtime, OSOS is a single binary blob without any built-in modularity.
