iOS, formally known as iPhone OS, is the operating system for all iDevices.


iPhone OS began in and was announced alongside the original iPhone at the Macworld Conference & Expo on . Later that year, on , the original iPhone was released to the world with (what is now) iPhone OS 1.0.


Initially, iOS didn't even have a name; It was initially just said to run a trimmed down version of Mac OS X (now macOS), presumably due to the usage of the XNU kernel and Darwin OS core. With the release of the iPhone OS 2 and iPhone SDK, Apple dubbed it iPhone OS. Initially chosen as it was what ran on the iPhone, it later was utilized by the iPod touch, iPad, and Apple TV (second generation and newer). As such, iPhone OS was renamed "iOS" with the fourth major version. This name was utilized for all iDevices for many years.

With the release of the original Apple Watch on , Apple released "watchOS", a trimmed down version of iOS that's designed for the small screen of the Watch. Unlike the (then unreleased) tvOS and iPadOS which kept version number parity with iOS, the original Apple Watch was released with watchOS 1.0, based on iOS 8.2.

Later that year, with the release of the fourth generation Apple TV in , Apple released "tvOS" 9 alongside iOS 9. tvOS, like watchOS, is (internally) still iOS, but designed for a 10-foot user interface. The second and third generation Apple TVs however, continued to use the "iOS" branding.

Four years after the release of iOS 9, with iOS 13, Apple again rebranded iOS, but this time for their iPad line of products. The resulting operating system was named "iPadOS." According to Apple, the change was made to differentiate the iPad from the iPhone and iPod touch lines as it (iPadOS) would contain a greater emphasis on multitasking.[AppleUnveilsIPadOS]


iOS (and watchOS, tvOS, and iPadOS) feature(s) a hybrid kernel known as XNU that runs Darwin. It is mostly programmed in ARM assembly, C, C++, Objective-C, and Swift. As XNU is Unix-like, it uses "union mounting" (compared to the Windows way of using drive letters). The flash is partitioned into two separate blocks: one for the operating system (mounted at /) and one for the user data (mounted at /private/var).

Installation Size

Over the years, iOS has required more and more space as the operating system has become more complex. Initially, iPhone OS required less than half a gigabyte to function, but with almost each major release, the root filesystem partition grew bigger. Currently, iOS 13 will claim over six gigabytes for itself.

Versions of iOS

Apple has maintained a steady release schedule with iOS' (and related) major releases; A new major version is released with every new flagship iPhone. Currently, iOS is on it's 16th major release.