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Rhapsody operating system. Archived from the original on Retrieved Conversely, the reason NT is not a microkernel system is because most of the system components run in the same address space as the kernel, as would be the case with a monolithic design in a traditional monolithic design, there would not be a microkernel per se, but the kernel would implement broadly similar functionality to NT's microkernel and kernel-mode subsystems. The Windows NT design includes many of the same objectives as Mach , the archetypal microkernel system, one of the most important being its structure as a collection of modules that communicate via well-known interfaces, with a small microkernel limited to core functions such as first-level interrupt handling, thread scheduling and synchronization primitives.
This allows for the possibility of using either direct procedure calls or interprocess communication IPC to communicate between modules, and hence for the potential location of modules in different address spaces for example in either kernel space or server processes.
Other design goals shared with Mach included support for diverse architectures, a kernel with abstractions general enough to allow multiple operating system personalities to be implemented on top of it and an object-oriented organisation. The reason NT is not a microkernel system is that nearly all of the subsystems providing system services, including the entire Executive, run in kernel mode, in the same address space as the microkernel itself, rather than in user-mode server processes, as would be the case with a microkernel design.
This is an attribute NT shares with early versions of Mach, as well as all commercial systems based on Mach, and stems from the superior performance offered by using direct procedure calls in a single memory space, rather than IPC, for communication amongst subsystems. The user-mode subsystems on NT include one or more emulation subsystems, each of which provides an operating system personality to applications, the Session Manager Subsystem smss. On versions of NT prior to 4. For performance reasons, however, in version 4.
As of , one other operating system personality, UNIX , is offered as an optionally installed system component on certain versions of Windows Vista and Windows Server R2. It was marketed as "Bash on Windows",  because it ran bash, a popular command line interface used on many Linux distributions and macOS , and allows binaries compiled for amd64 to run unmodified within the subsystem. This was intended so that developers could run their tools on Windows without having to emulate them, and thus requires developer mode to be enabled in Windows Settings. Applications that run on NT are written to one of the OS personalities usually the Windows API , and not to the native NT API for which documentation is not publicly available with the exception of routines used in device driver development.
An OS personality is implemented via a set of user-mode DLLs see Dynamic-link library , which are mapped into application processes' address spaces as required, together with an emulation subsystem server process as described previously.
Operating System Design/Kernel Architecture
Applications access system services by calling into the OS personality DLLs mapped into their address spaces, which in turn call into the NT run-time library ntdll. The NT run-time library services these requests by trapping into kernel mode to either call kernel-mode Executive routines or make Local Procedure Calls LPCs to the appropriate user-mode subsystem server processes, which in turn use the NT API to communicate with application processes, the kernel-mode subsystems and each other.
XNU is the kernel that Apple Inc. Like some other modern kernels , XNU is a hybrid, containing features of both monolithic and microkernels , attempting to make the best use of both technologies, such as the message passing capability of microkernels enabling greater modularity [ citation needed ] and larger portions of the OS to benefit from protected memory , [ citation needed ] as well as retaining the speed of monolithic kernels for certain critical tasks.
On the other hand, real-time applications such as multimedia need to be assured of predictable, time-critical, behavior. In contrast, OS X is a preemptive multitasking environment. In OS X, the kernel provides enforcement of cooperation, scheduling processes to share time preemption. This supports real-time behavior in applications that require it. In OS X, processes do not normally share memory.
Instead, the kernel assigns each process its own address space , controlling access to these address spaces. Size is not an issue; with the virtual memory system included in OS X, each application has access to its own 4 GB address space.
Viewed together, all applications are said to run in user space, but this does not imply that they share memory. User space is simply a term for the combined address spaces of all user-level applications. The kernel itself has its own address space, called kernel space.
In OS X, no application can directly modify the memory of the system software the kernel. Although user processes do not share memory by default as in Mac OS 9, communication and even memory sharing between applications is still possible. For example, the kernel offers a rich set of primitives to permit some sharing of information among processes. Mach messaging provides another approach, handing memory from one process to another. Unlike Mac OS 9, however, memory sharing cannot occur without explicit action by the programmer.
The kernel, along with other core parts of OS X are collectively referred to as Darwin. Darwin is a complete operating system based on many of the same technologies that underlie OS X. Figure shows the relationship between Darwin and OS X. Both Darwin and OS X include the BSD command-line application environment; however, in OS X, use of environment is not required, and thus it is hidden from the user unless they choose to access it.
Hybrid kernel - Wikipedia
Darwin technology is based on BSD , Mach 3. Best of all, Darwin technology is Open Source technology, which means that developers have full access to the source code. In effect, OS X third-party developers can be part of the Darwin core system software development team. Developers can also see how Apple is doing things in the core operating system and adopt or adapt code to use within their own products.
Because the same software forms the core of both OS X and Darwin, developers can create low-level software that runs on both OS X and Darwin with few, if any, changes. The only difference is likely to be in the way the software interacts with the application environment. Darwin is based on proven technology from many sources.