Operating System Structure
An operating system (OS) is an essential software program that manages a computer’s hardware and software resources. At its core, an OS performs basic tasks like managing memory, prioritizing system requests, controlling input and output devices, and facilitating networking and data storage.
But not all operating systems have the same underlying architecture. Over the decades, computer scientists have developed different internal structures to optimize performance, security, extensibility and other factors.
- Simple structure
- Monolithic structure
- Layered approach
- Microkernel
- Hybrid kernel
- Exokernel
- Virtual machines
Understanding OS design patterns helps illustrate why modern OSes make certain tradeoffs. It also showcases how far we’ve come in balancing competing needs like flexibility, speed and stability.
What is an Operating System?
Before surveying different OS structures, let’s quickly summarize what an operating system actually does. At the highest level, an OS handles resource allocation for hardware and software. The OS makes sure each component — CPU, memory, storage, network card — integrates and runs smoothly.
Key responsibilities include:
- Memory management: Keeping track of memory space allocation and use.
- Processor management: Scheduling tasks and distributing them optimally across CPU cores.
- Device management: Controlling peripherals like printers and coordinating the data flow between hardware and CPU.
- Storage management: Organizing hard disks/file systems to store/retrieve data for users and applications.
- User interface: Displaying information and accepting user inputs through devices like keyboards.
- Networking: Enabling data transfer to/from the computer and other connected devices.
- Security: Authenticating user access and restricting specific user rights.
Accomplishing all this requires an intricate, multi-layer architecture. As we survey the leading options below, keep in mind their impact across these areas.
Layered Operating System Structure
One natural way to handle OS complexity is to separate responsibilities into layers or hierarchical levels:
Like a sandwich stack, the top layers rely on the foundations beneath them to function. Key principles include:
- Layers have defined interfaces enabling communication up and down the hierarchy.
- Each layer hides its inner complexity from adjacent levels.
- The bottommost layer interacts directly with hardware.
The exact layers vary across designs but often include:
- User interface — Primary interaction for end users and external systems.
- Applications — Software services like Microsoft Excel.
- OS core services — Fundamental resource allocation duties.
- Hardware — Physical devices like CPU and memory.
Advantages of Layered Structure
Key perks of the layered operating system approach are:
- Separation of concerns — Breaking down complex tasks makes OS easier to develop, test and manage.
- Portability — Adjusting a single layer has limited impact on others, enabling customization.
- Flexibility — Can swap out implementations of a layer without disrupting rest of system.
Disadvantages of Layered Structure
Downsides to consider with layered operating systems include:
- Performance hits from communication delays between layers.
- Complex coordination logic required between layers.
- Difficult to optimize as a unified whole due to division of labor.
The layered model facilitates OS extensibility over raw speed or simplicity. As such, most modern general purpose OSes leverage layering to some degree even if not formalized into strict boundaries.
Monolithic Operating System Structure
The monolithic architecture takes the opposite approach from layered systems. Rather than decomposing tasks, it combines all core services into a single, unified structure:
This single system kernel handles every basic function required including memory, multitasking, storage and I/O. Direct integration and data sharing aims to achieve maximum performance.
Advantages of Monolithic Structure
Key benefits of the monolithic approach are:
- Faster performance since no communication delays between internal layers.
- Tighter integration enabling holistic optimizations between components.
- Architectural simplicity with clean lines of control/data flow through unified structure.
Disadvantages of Monolithic Structure
Tradeoffs to a monolithic design include:
- Massive codebase that is complex to develop and maintain.
- Not modular thus harder to update or customize a single component.
- Crashes/instability since all code runs in same memory space. A single bug can compromise entire system.
If pure speed on a fixed configuration is the goal, a monolithic kernel has clear advantages. But modern systems prioritize flexibility gained via modularity. As such, strict monolithic designs have fallen out of favor for general computing scenarios.
Microkernel Operating System Structure
Microkernel operating systems take the exact opposite approach from monolithic kernels. They minimize functionality in the core kernel system and delegate other OS tasks out to modular services.
The microkernel itself only handles message passing, memory and processor scheduling. Other managers run separately as their own isolated services – device drivers, network protocols, file system, user authentication and so on.
Advantages of Microkernel Structure
Upsides to the microkernel approach include:
- Improved security and stability since services run isolated in separate spaces. A crash in one module doesn’t take down entire system.
- More customization with ability to swap managers without rebuilding entire OS.
- Easier maintenance with small compact kernel code base.
Disadvantages of Microkernel Structure
Downsides to weigh with microkernels:
- Performance overheads from increased communication between components.
- Programming complexity to coordinate decentralized modules.
- Feature limitations since some functions don’t map cleanly to discrete managers.
Despite some performance tradeoffs, microkernel principles underpin many contemporary OS architectures even among the “monolithic” kernel category. surance services have achieved wide industry adoption due to these advantages.
Modular Operating System Structure
Modularity applies microkernel decentralization principles but in a less extreme fashion. Core system components still exist in kernel space but can be individually loaded/unloaded on demand. This middle ground approach aims to deliver the customizability of microkernels while limiting performance downsides inherent to fragmented architectures.
Advantages of Modular Structure
Benefits of OS modularity include:
- Flexibility to customize environment without rebuilding entire OS.
- Stability since core components still centrally integrated.
- Performance via efficient data exchange within kernel space.
Disadvantages of Modular Structure
Considerations when adopting modular architecture:
- Complex coordination logic to integrate modules with central kernel and amongst each other.
- Namespace collisions possible between dynamic components.
- Version compatibility issues as modules update on independent timelines from core code.
Most leading desktop and server OSes utilize modular techniques to balance flexibility and performance. But smartphone platforms tend to favor more centralized designs for battery life and security reasons.
Hybrid Kernel Operating System Structure
Hybrid kernels merge aspects of monolithic and microkernel architectures. The fundamental OS services exist within kernel space like a monolithic system. But key managers run in isolated user spaces akin to a microkernel.
This blend aims to deliver the security and customization of microkernels while retaining the performance advantages of monolithic designs. By also separating drivers and services, crashes are less likely to take down the entire OS.
MacOS and iOS also employ a hybrid model derived from NeXTSTEP and BSD roots. Android blends a Linux kernel with separate user space services also affording enhanced stability and flexibility.
Advantages of Hybrid Kernel
Hybrid operating system benefits include:
- Performance optimization across integrated kernel subsystems.
- Stability from isolating processes in separate memory spaces.
- Customization via ability to swap managers and drivers.
Disadvantages of Hybrid Kernel
Considerations around hybrid models:
- Design complexity to optimize compartmentalization boundaries.
- Synchronization overheads for communications between kernel and user space components.
- Potential bottlenecks based on which services run in kernel space.
For broad computing devices expected to run untrusted code, the compartmentalization and safeguards afforded by hybrid architectures generally outweigh the modest potential performance downsides.
Exokernel Operating System Structure
Exokernels further extend microkernel principles for extreme customization. Rather than provide common high-level resources, they intentionally expose lower-level hardware access by default.
[Diagram showing bare metal hardware resources exposed by exokernel directly to separate application spaces.]
Exokernel systems give apps direct communication channels to disk, network, memory, and processors. Library operating systems then implement traditional OS policies as less privileged application layers atop the raw hardware building blocks.
Advantages of Exokernel Structure
Exokernel benefits include:
- Incredible flexibility with hardware level building blocks exposed by default.
- Tighter application integration for leveraging specific chip capabilities.
- Better performance by bypassing general purpose legacy OS layers.
Disadvantages of Exokernel Structure
Challenges around specialty exokernel hypervisors still being hammered out:
- Lack of standards with each application environment defining its own assumptions for security, networking, storage formats and the like.
- Hardware compatibility hurdles given traditional device drivers run within apps themselves.
- Significant technical expertise required around low level programming quirks and hardware intricacies.
Like microkernels, exokernels have radially shifted operating system design assumptions. Mainstream adoption still requires hardware standardization and performance testing across real world application mixes. But innovative new computing products leverage minimalist hypervisors so apps more directly harness sophisticated modern hardware capabilities.
Virtual Machines
Virtual machines (VMs) play a growing role in operating system structure discussions given prevalence of hypervisor technologies from vendors like VMware, Microsoft and Red Hat.
Rather than yet another kernel design however, virtualization solutions function differently. A hypervisor creates virtual containers that mimic physical computer hardware without directly mirroring lower level component architectures.
Advantages of Virtual Machines
Upsides to hardware virtualization include:
- Server consolidation savings since VMs share physical resources efficiently.
- Cross platform compatibility with uniform virtual hardware assumptions despite heterogeneous physical servers.
- Workload isolation via strong separation between virtual systems running on same hardware.
- Dynamic scalability to provision new VMs based on usage spikes.
Disadvantages of Virtual Machines
Considerations around extensive virtualization:
- Hypervisor licensing costs can add up despite consolidation savings.
- Performance overheads from virtualization abstraction layers hindering native speeds.
- Increased management complexity around hypervisor.
Simple Operating System Structure
Lastly, for the sake of completeness, we should quickly contrast the designs above to simple operating system structures used in much more constrained computing environments.
[Simple boxes diagram showing minimalist scheduler, memory manager and hardware interface components.]
Embedded devices like smart watches and internet-of-things sensors cannot afford the complex, multidimensional architectures found in general purpose computer OSes. Instead, they leverage slimmed down simple kernels containing just the bare essential capabilities required by target functionality.
Advantages of Simple Structure
- Compact code and resource footprint ideal for embedded uses.
- Predictable performance from minimized components.
- High efficiency around narrow intended application range.
Disadvantages of Simple Structure
- Lack of security and isolation best practices baked into sophisticated OS architectures.
- Brittle crash potential given single memory space for entire kernel.
- Portability challenges when targeting different chip/peripheral combinations.
For broadly exposed infrastructure roles, simple kernels pose too many compromises around resilience, standards conformance and future proofing ability.
But when adhering to disciplined embedded design constraints, crafting an OS narrowly fit to purpose still delivers immense value without requiring lavish kernel architecture sophistication.
Conclusion
Operating system design involves complex tradeoffs balancing performance, security, extensibility and hardware resource efficiencies. The patterns covered here expose the spectrum of techniques computing architects employ to best serve target usage contexts.
Strict monolithic kernels optimize throughput by directly exposing hardware to integrated core services. Microkernels and exokernels conversely disaggregate resources to maximize flexibility. Layered hierarchies compartmentalize functionality at the cost of lower agility relative to communication delayed across divisions.
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Operating System Stucture