Is the iOS operating system the same thing as the phone's firmware? Basically, when reinstalling the firmware (such as with DFU mode), are the system files being reinstalled, or something else?

What is the purpose of the firmware on iDevices (iPhone, iPod Touch, iPad)?

  • Why are you asking?
    – bneely
    Feb 16, 2012 at 9:07
  • Because I'm interested in this stuff, and I wanted to know so I can gain a better understanding of it. Plus I can't find the answer anywhere on the web. Articles seem to be ambiguous when they refer to both. I'm in the computer field, and I thought this was essential to know
    – rubixibuc
    Feb 16, 2012 at 9:20

2 Answers 2


Heres a short overview.

The term "firmware" for iOS devices covers some or all of the following items dependant on the device:

  • the baseband (the phone code implementing the cell tower communications for voice and data)
  • the device firmware (akin to the BIOS) otherwise known as the boot ROM
  • the device software (iOS itself)

Devices like the iPod touch and the non-3G iPads don't have basebands, however the builds for the devices may cover both the WiFi only and 3G enabled devices so will include the baseband update.

Builds that target different generations of devices may contain multiple baseband's for different versions of phones since the baseband chip may be from different manufactuer's as in Qualcomm for the newer devices or Infineon for the older devices.

So when the phone turns on, the code in the Boot ROM runs, it activates the hardware (the phone and the baseband), and then proceeds to start the software (iOS)

  • Let's say on the iPhone, is the device software, considered part of he firmware on that device, because you said it varies?
    – rubixibuc
    Feb 16, 2012 at 9:43
  • In the strictest meaning, the only firmwares in the devices are the baseband code, and the Boot ROM. iOS is not considered part of the firmware, it is software even though the update process for the device updates it as if it was firmware.
    – Stu Wilson
    Feb 16, 2012 at 9:58

Wikipedia describes firmware like this:

In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices.

Wikipedia describes the major layers of iOS like this:

In iOS, there are four abstraction layers: the Core OS layer, the Core Services layer, the Media layer, and the Cocoa Touch layer.

There's a site called theiphonewiki, and its firmware page links to what appear to be complete .ipsw images including all of the software for a given iOS release. So, theiphonewiki considers firmware to be all of the software.

After reading the Wikipedia definition, I consider the firmware of iOS to reside entirely within the Core OS layer, and consisting of the operating system kernel, device drivers, and other hardware-facing software components. Without knowing exactly how iOS is implemented, I can only speculate that these components are clearly defined and distinct (although this is not always the case in practice).

Edit: This TUAW article indicates that all non-bundled applications and user data is erased from the phone during a DFU restore. This suggests to me that the internal storage is completely erased, and all of the device software is reinstalled (low-level operating system, baseband if applicable, and bundled iOS applications).

  • When I enter DFU mode and restore the phone, and it implies reinstalling the firmware, does it reinstall the OS and leave all apps intact, or does it do something else?
    – rubixibuc
    Feb 16, 2012 at 9:38

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