In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources (the communication between hardware and software components).Usually as a basic component of an operating system, a kernel can provide the lowest-level abstraction layer for the resources (especially processors and I/O devices) that application software must control to perform its function. It typically makes these facilities available to application processes through inter-process communication mechanisms and system calls.
Operating system software on the computer is the first layer is placed on computer memory, (computer memory in this case there is the hard drive, not the ram memory) when the computer starts up. While other software is run after the computer operating system is running, and the Operating System will perform the core public service for the software.
Common core services such as access to the disk, memory management, task scheduling, and user interface. So that each software no longer have a common core tasks, as can be serviced and performed by the operating system. Section of code that performs the core tasks and the general was named by an Operating System kernel.
Kernel (core operating system)
In a computing device, the kernel is the core component of the System Operations running in the device. Kernel is responsible for managing resource sharing systems; communication between hardware and software components.
Kernel link between software applications and computer hardware. Kernel provides the lowest level abstraction layer for resource-resource such as memory, processor and device I / O in which a software application should control the resource-resource in order to function. The kernel is able to provide facilities this kind of application processes through the mechanism of IPC (Inter Process Communication) and system calls.
Kernel basic facilities
The kernel's primary function is to manage the computer's resources and allow other programs to run and use these resources. Typically, the resources consist of:
- The Central Processing Unit. This is the most central part of a computer system, responsible for running or executing programs. The kernel takes responsibility for deciding at any time which of the many running programs should be allocated to the processor or processors (each of which can usually run only one program at a time)
- The computer's memory. Memory is used to store both program instructions and data. Typically, both need to be present in memory in order for a program to execute. Often multiple programs will want access to memory, frequently demanding more memory than the computer has available. The kernel is responsible for deciding which memory each process can use, and determining what to do when not enough is available.
- Any Input/Output devices present in the computer, such as keyboard, mouse, disk drives, printers, displays, etc. The kernel allocates requests from applications to perform I/O to an appropriate device (or subsection of a device, in the case of files on a disk or windows on a display) and provides convenient methods for using the device (typically abstracted to the point where the application does not need to know implementation details of the device).
Issues of kernel support for protection
An important consideration in the design of a kernel is the support it provides for protection from faults (fault tolerance) and from malicious behaviors (security). These two aspects are usually not clearly distinguished, and the adoption of this distinction in the kernel design leads to the rejection of a hierarchical structure for protection.An important kernel design decision is the choice of the abstraction levels where the security mechanisms and policies should be implemented. Kernel security mechanisms play a critical role in supporting security at higher levels One approach is to use firmware and kernel support for fault tolerance (see above), and build the security policy for malicious behavior on top of that (adding features such as cryptography mechanisms where necessary), delegating some responsibility to the compiler. Approaches that delegate enforcement of security policy to the compiler and/or the application level are often called language-based security.
The lack of many critical security mechanisms in current mainstream operating systems impedes the implementation of adequate security policies at the application abstraction level. In fact, a common misconception in computer security is that any security policy can be implemented in an application regardless of kernel support.
Hardware-based protection or language-based protection
Typical computer systems today use hardware-enforced rules about what programs are allowed to access what data. The processor monitors the execution and stops a program that violates a rule (e.g., a user process that is about to read or write to kernel memory, and so on). In systems that lack support for capabilities, processes are isolated from each other by using separate address spaces. Calls from user processes into the kernel are regulated by requiring them to use one of the above-described system call methods.
An alternative approach is to use language-based protection. In a language-based protection system, the kernel will only allow code to execute that has been produced by a trusted language compiler. The language may then be designed such that it is impossible for the programmer to instruct it to do something that will violate a security requirement.
Advantages of this approach include:
- No need for separate address spaces. Switching between address spaces is a slow operation that causes a great deal of overhead, and a lot of optimization work is currently performed in order to prevent unnecessary switches in current operating systems. Switching is completely unnecessary in a language-based protection system, as all code can safely operate in the same address space.
- Flexibility. Any protection scheme that can be designed to be expressed via a programming language can be implemented using this method. Changes to the protection scheme (e.g. from a hierarchical system to a capability-based one) do not require new hardware.
- Longer application start up time. Applications must be verified when they are started to ensure they have been compiled by the correct compiler, or may need recompiling either from source code or from bytecode.
- Inflexible type systems. On traditional systems, applications frequently perform operations that are not type safe. Such operations cannot be permitted in a language-based protection system, which means that applications may need to be rewritten and may, in some cases, lose performance.
Monolithic kernels
In a monolithic kernel, all OS services run along with the main kernel
thread, thus also residing in the same memory area. This approach
provides rich and powerful hardware access. Some developers, such as UNIX developer Ken Thompson, maintain that it is "easier to implement a monolithic kernel"
than microkernels. The main disadvantages of monolithic kernels are the
dependencies between system components — a bug in a device driver might
crash the entire system — and the fact that large kernels can become
very difficult to maintain.
Microkernels
Microkernel (also abbreviated μK or uK) is the term describing an
approach to Operating System design by which the functionality of the
system is moved out of the traditional "kernel", into a set of "servers"
that communicate through a "minimal" kernel, leaving as little as
possible in "system space" and as much as possible in "user space". A
microkernel that is designed for a specific platform or device is only
ever going to have what it needs to operate. The microkernel approach
consists of defining a simple abstraction over the hardware, with a set
of primitives or system calls to implement minimal OS services such as memory management, multitasking, and inter-process communication.The very essence of the microkernel architecture illustrates some of its advantages:
- Maintenance is generally easier.
- Patches can be tested in a separate instance, and then swapped in to take over a production instance.
- Rapid development time and new software can be tested without having to reboot the kernel.
- More persistence in general, if one instance goes hay-wire, it is often possible to substitute it with an operational mirror.
Hybrid (or) Modular kernels
Hybrid kernels are used in most commercial operating systems such as Microsoft Windows NT, 2000, XP, Vista, and 7. Apple Inc's own Mac OS X uses a hybrid kernel called XNU which is based upon code from Carnegie Mellon's Mach kernel and FreeBSD's monolithic kernel. They are similar to micro kernels, except they include some additional code in kernel-space to increase performance. These kernels represent a compromise that was implemented by some developers before it was demonstrated that pure micro kernels can provide high performance.
Unix
In the Unix model, the Operating System consists of two parts; first, the huge collection of utility programs that drive most operations, the other the kernel that runs the programs.Under Unix, from a programming standpoint, the distinction between the two is fairly thin; the kernel is a program, running in supervisor mode, that acts as a program loader and supervisor for the small utility programs making up the rest of the system, and to provide locking and I/O services for these programs; beyond that, the kernel didn't intervene at all in user space.
Modern Unix-derivatives are generally based on module-loading monolithic kernels. Examples of this are the Linux kernel in its many distributions as well as the Berkeley software distribution variant kernels such as FreeBSD, DragonflyBSD, OpenBSD, NetBSD, and Mac OS X. Apart from these alternatives, amateur developers maintain an active operating system development community, populated by self-written hobby kernels which mostly end up sharing many features with Linux, FreeBSD, DragonflyBSD, OpenBSD or NetBSD kernels and/or being compatible with them.
Microsoft Windows
Microsoft Windows was first released in 1985 as an add-on to MS-DOS. Because of its dependence on another operating system, initial releases of Windows, prior to Windows 95, were considered an operating environment (not to be confused with an operating system). This product line continued to evolve through the 1980s and 1990s, culminating with release of the Windows 9x series (upgrading the system's capabilities to 32-bit addressing and pre-emptive multitasking) through the mid 1990s and ending with the release of Windows Me in 2000. Microsoft also developed Windows NT, an operating system intended for high-end and business users. This line started with the release of Windows NT 3.1 in 1993, and has continued through the years of 2000 with Windows 7 and Windows Server 2008.
The release of Windows XP in October 2001 brought these two product lines together, with the intent of combining the stability of the NT kernel with consumer features from the 9x series. The architecture of Windows NT's kernel is considered a hybrid kernel because the kernel itself contains tasks such as the Window Manager and the IPC Manager, but several subsystems run in user mode. The precise breakdown of user mode and kernel mode components has changed from release to release, but the introduction of the User Mode Driver Framework in Windows Vista, and user-mode thread scheduling in Windows 7, have brought more kernel-mode functionality into user-mode processes.
Windows makes use of the FAT, NTFS, exFAT and ReFS file systems (the latter is only supported and usable in Windows Server 8; Windows cannot boot from it).Windows uses a drive letter abstraction at the user level to distinguish one disk or partition from another. For example, the path C:\WINDOWS represents a directory WINDOWS on the partition represented by the letter C. Drive C: is most commonly used for the primary hard disk partition, on which Windows is usually installed and from which it boots. This "tradition" has become so firmly ingrained that bugs came about in older applications which made assumptions that the drive that the operating system was installed on was C. The use of drive letters, and the tradition of using "C" as the drive letter for the primary hard disk partition, can be traced to MS-DOS, where the letters A and B were reserved for up to two floppy disk drives. This in turn derived from CP/M in the 1970s, and ultimately from IBM's CP/CMS of 1967.
Windows Registry
The Windows Registry is a hierarchical database that stores
configuration settings and options on Microsoft Windows operating
systems. It contains settings for low-level operating system components
as well as the applications running on the platform: the kernel, device
drivers, services, SAM, user interface and third party applications all
make use of the registry. The registry also provides a means to access
counters for profiling system performance. When first introduced with
Windows 3.1, the Windows registry's primary purpose was to store
configuration information for COM-based components. With the
introduction of Windows 95 and Windows NT, its use was extended to tidy
up the profusion of per-program INI files that had previously been used
to store configuration settings for Windows programs.
Structure
Keys and values
The registry contains two basic elements: keys and
values. Registry keys are similar to folders in addition to values,
each key can contain subkeys, which may contain further subkeys, and so
on. Keys are referenced with a syntax similar to Windows' path names,
using backslashes to indicate levels of hierarchy. Each subkey has a
mandatory name, is a non-empty string that cannot contain any backslash,
and whose letter case is insignificant. The
hierarchy of registry keys can only be accessed from a known root key
handle (which is anonymous but whose effective value is a constant
numeric handle) that is mapped to the content of a registry key
preloaded by the kernel from a stored "hive", or to the content of a
subkey within another root key, or mapped to a registered service or DLL
that provide access to its contained subkeys and values.
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows refers to the subkey "Windows" of the subkey "Microsoft" of the subkey "Software" of the HKEY_LOCAL_MACHINE root key. There are seven predefined root keys, traditionally named according to their constant handles defined in the Win32 API, or by synonymous abbreviations (depending on applications):
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows refers to the subkey "Windows" of the subkey "Microsoft" of the subkey "Software" of the HKEY_LOCAL_MACHINE root key. There are seven predefined root keys, traditionally named according to their constant handles defined in the Win32 API, or by synonymous abbreviations (depending on applications):
- HKEY_LOCAL_MACHINE or HKLM
- HKEY_CURRENT_CONFIG or HKCC (only in Windows 9x/Me and NT-based versions of Windows)
- HKEY_CLASSES_ROOT or HKCR
- HKEY_CURRENT_USER or HKCU
- HKEY_USERS or HKU
- HKEY_PERFORMANCE_DATA (only in NT-based versions of Windows, but invisible in the Windows Registry Editor)
- HKEY_DYN_DATA (only in Windows 9x/Me, and visible in the Windows Registry Editor
Mac OS
Apple Computer first launched Mac OS in 1984, bundled with its Apple Macintosh personal computer. Apple moved to a nanokernel design in Mac OS 8.6. Against this, Mac OS X is based on Darwin, which uses a hybrid kernel called XNU, which was created combining the 4.3BSD kernel and the Mach kernel.Mac OS X
Mac OS X uses a file system that it inherited from classic Mac OS called HFS Plus, sometimes called Mac OS Extended. HFS Plus is a metadata-rich and case preserving file system. Due to the Unix roots of Mac OS X, Unix permissions were added to HFS Plus. Later versions of HFS Plus added journaling to prevent corruption of the file system structure and introduced a number of optimizations to the allocation algorithms in an attempt to defragment files automatically without requiring an external defragmenter.Filenames can be up to 255 characters. HFS Plus uses Unicode to store filenames. On Mac OS X, the filetype can come from the type code, stored in file's metadata, or the filename extension.
HFS Plus has three kinds of links: Unix-style hard links, Unix-style symbolic links and aliases. Aliases are designed to maintain a link to their original file even if they are moved or renamed; they are not interpreted by the file system itself, but by the File Manager code in userland.
Mac OS X also supports the UFS file system, derived from the BSD Unix Fast File System via NeXTSTEP. However, as of Mac OS X 10.5 (Leopard), Mac OS X can no longer be installed on a UFS volume, nor can a pre-Leopard system installed on a UFS volume be upgraded to Leopard.
Newer versions Mac OS X are capable of reading and writing to the legacy FAT file systems(16 & 32). They are capable of reading NTFS Filesystems. Writing is only supported on Mac OS X 10.6 (Snow Leopard) and later but only after a non-trivial system setting change. Third party software exists that automates this. Third party software is still necessary to write to the NTFS file system on Mac OS X versions prior to 10.6 (Snow Leopard).
Mac OS X 10.3 and later include read-only support for NTFS-formatted partitions. The GPL-licensed NTFS-3G also works on Mac OS X through FUSE and allows reading and writing to NTFS partitions. A performance enhanced commercial version, called Tuxera NTFS for Mac, is also available from the NTFS-3G developers. Paragon Software Group sells a read-write driver named NTFS for Mac OS X, which is also included on some models of Seagate hard drives. Native NTFS write support has been discovered in Mac OS X 10.6 and later, but is not activated by default, although hacks do exist to enable the functionality. However, user reports indicate the functionality is unstable and tends to cause kernel panics, probably the reason why write support has not been enabled or advertised.
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