第6章インフラストラクチャーの抽象化層：オペレーティングシステム

The hardware abstraction layer (HAL) is an example of layering in modern OSs, and it allows an OS to interact with a hardware device at a general or abstract level rather than going deep into a detailed hardware level, which improves readability and maintainability. In general, the goal of HAL is to separate hardware-specific routines from the core OS. HAL enables portability of core code across different pieces of hardware.

Another alternative OS design to monolithic OS design is a microkernels architecture. Within a microkernel, the functionality and capabilities are added to a minimal core OS as plug-ins. Microkernel architecture is also called a plug-in architecture because functionalities are added as plug-ins (Figure 6.13).

The goal of a microkernel architecture is to minimize what goes into the kernel and implement everything else that traditionally goes in an OS in terms of user-level processes. This results in improving reliability due to the isolation between components. Also, there is less code running at full privilege, and greater ease of extension and customization. However, performance is generally poor due to user and kernel boundary crossings, which represent a security risk when the kernel code operates on the data.

The first microkernel system was Hydra (CMU, 1970), followed by Mach (CMU), Chorus (French UNIX-like OS), and OS X (Apple). Windows OS used to use a microkernel, but now uses a hybrid kernel architecture that combines the benefits of monolithic, microkernel, and plug-in OS architectures (Figure 6.14).

Read this article about how Apple uses kernel architecture (https://openstax.org/r/76kernelarch) in its macOS.

Two concepts that each represent critical advancements in the field of computing by offering different methodologies for resource abstraction, allocation, and management are cloud infrastructure and the exokernel.

The exokernel architecture simplifies the OS by making its core (kernel) really small and letting apps have more control over the computer’s hardware. Unlike usual systems that hide hardware details, an exokernel shows these details to apps, letting them use the hardware more smartly. This way, it cuts down on unnecessary steps and lets each app manage resources its own way, which can make the computer run better and faster. This design is especially good for special computing tasks where apps can really benefit from managing hardware directly, and this can lead to better performance and more efficient use of the computer’s parts. There are several limitations associated with exokernels. They lack abstractions for operating systems services, which makes it difficult to achieve consistency across applications. They also require an application developer to manage resource allocation and protection and to implement security mechanisms, which raises concerns.

The virtualized and scalable hardware resources that are delivered over the Internet as a service are called cloud infrastructure. This infrastructure encompasses a range of components including servers, storage devices, network equipment, and virtualization software, all hosted within data centers managed by cloud service providers. Unlike traditional physical infrastructure, cloud infrastructure offers flexibility, scalability, and accessibility, allowing users to access and manage computing resources remotely through the Internet. Cloud infrastructure is categorized into three service models:

• Infrastructure as a Service (IaaS) provides virtualized computing resources over the Internet, such as AWS EC2, Google Compute Engine, and Microsoft Azure VMs.

• Platform as a Service (PaaS) offers hardware and software tools over the Internet, typically targeting developers such as Google App Engine and Microsoft Azure.

• Software as a Service (SaaS) delivers software applications over the Internet, accessible from a web browser without installation or running on the user’s personal devices such as Google Workspace and Microsoft Office 365.

The main disadvantages of cloud infrastructure are potential downtime, security and privacy concerns, vulnerability to attacks, limited control and flexibility, vendor lock-in, cost concerns, latency issues, Internet dependency, technical issues, lack of support, bandwidth issues, and varied performance.