IOS

iOS, formerly known as iPhone OS (and just Mac OS X), is the operating system for the iPhone, iPod touch, iPad, iPad mini 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 (iPhone3,1) 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 Rev A/A5X and ~1.2GB for A6/A6X.

In iOS 7.0, it takes up ~1.2GB of space A4/A5/A5 Rev A/A5X, ~1.3GB for A6/A6X and ~1.5GB for A7/A7 variant.

In iOS 8.0, it takes up ~1.7GB of space A5/A5 Rev A/A5X, ~1.8GB for A6/A6X, 2.0GB for A7 and ~2.0-2.3GB for A8.