IOS

iOS, formerly known as iPhone OS, is the operating system for the iPhone, iPod touch, iPad and Apple TV. iOS has four abstraction layers: the Core OS layer, the Core Services layer, the Media layer, and the Cocoa Touch layer. The OS takes ~350MB of the device's total memory storage, this means that users will not get their full storage space.

In iOS 4.0+, it takes up ~800MB of space, changing the root partition from 500MB to 1000MB, due to the 2x graphics that have been added for the iPhone 4 (GSM) and iPod touch 4G retina displays.

In iOS 5.0, Apple began pushing towards a computer-less experience for iOS. (e.g.- Devices can be activated and updated over the air.) In addition, multitasking gestures are now enabled by default.

In iOS 6.0, it takes up ~850MB of space A4/A5/A5 A/A5X and ~1.2GB for A6/A6X.