Introduction
The Core OS Layer is the last layer of the iOS stack and sits directly on top of the device hardware. This layer provides a variety of services including low level networking, access to external accessories and the usual fundamental operating system services such as memory management, file system handling and threads.
Accelerate Framework
In the Xcode Framework library it is represented as "Accelerate.framework" and in the Xcode Objective-C class we import it with #import "<Accelerate/Accelerate.h>". It provides a hardware optimized C-based API for performing complex and large number math, vector, digital signal processing and image processing tasks and calculations. Its most comon use is in gamming apps.
External Accessory Framework
In the Xcode Framework library it is represented as "ExternalAccessory.framework" and in the Xcode Objective-C class we import it with #import "<ExternalAccessory/ExternalAccessory.h>". It provides the ability to interrogate and communicate with external accessories connected physically to the iPhone via the 30-pin dock connector or wirelessly via Bluetooth. After using it we can implement a method related to Accessory.
Security Framework
In the Xcode Framework library it is represented as "Security.framework" and in the Xcode Objective-C class we import it with #import "<Security/Security.h>". The iOS Security framework provides all the security interfaces you would expect to find on a device that can connect to external networks including certificates, public and private keys, trust policies, key chains, encryption, digests and Hash-based Message Authentication Code (HMAC).
Core Foundation Framework
In the Xcode Framework library it is represented as "CoreFoundation.framework" and in the Xcode Objective-C class we import it with #import "<CoreFoundation/CoreFoundation.h>". This framework is a C-based Framework that provides basic functionality such as data types, string manipulation, raw block data management, URL manipulation, threads and run loops, date and times, basic XML manipulation and port and socket communication. Additional XML capabilities beyond those included with this framework are provided via the libXML2 library.
System
iOS is built upon a UNIX-like foundation. System components of the Core OS Layer provides much of the same functionality as any other UNIX like operating system. This layer includes the operating system kernel and the kernel is the foundation on which the entire iOS platform is built and provides the low level interface to the underlying hardware. Amongst other things, the kernel is responsible for memory allocation, process lifecycle management, input/output, inter-process communication, thread management, low level networking, file system access and thread management.