• Middleware runs on a variety of OS – hardware combinations (platforms) at the nodes of a distributed system. 
• OS running at a node provides 
  • abstraction of local hardware resources for processing, storage and communication.
• Middleware utilizes a combination of these local resources to implement its mechanisms for remote invocations between objects or processes at the nodes. 
• Kernel and server processes are the components that manage resources and present clients with an interface to the resources. 
• Encapsulation
  • Provide a useful service interface to their resources – a set of operations that meet their clients’ needs. 
  • Details such as management of memory and devices used to implement the resources should be hidden from clients.
•  Protection
• Resources require protection from illegitimate accesses 
• Concurrent processing
  • Client may share resources and access them concurrently.
  • Resource managers are responsible for achieving concurrency transparency. 
• Client access resources by making remote method invocations to a server object, or system calls to a kernel. 
• A combination of libraries, kernels and servers may be called upon to perform the following invocation-related tasks:
  • Communication
  • Operation parameters and results have to be passes to and from resource managers, over a network or within a computer. 
  • Scheduling
  • When an operation is invoked, its processing must be scheduled within the kernel or server. 

Figure : System layers

Figure : Core OS functionality

Core OS components 

• Process manager
  • Creation of and operation upon processes.
  • Process is a unit of resource management, including an address space and one or more threads. 
• Thread manager
  • Thread creation, synchronization and scheduling.
• Communication manager
  • Communication between threads attached to different processes on the same computer. 
  • Some kernels also support communication between threads in remote processes. 
• Memory manager
  • Management of physical and virtual memory. 
• Supervisor
• Dispatching of interrupts, and other exceptions
• control of memory management unit and hardware caches.

• OS software is designed to be portable between computer architectures where possible.

• Majority of OS is coded in a high-level language such as C, C++ or Modula-3.
• Its facilities are layered so that machine-dependent components are reduced to a minimal bottom layer. 
• Some kernels can executed on shared-memory multiprocessors.
  • Several processors that share one or more modules of memory (RAM). 
  • Processors may also have their own private memory.
  • Simplest and least expensive way to construct is by incorporating a circuit board holding a few (2 – 8) processors in a personal computer.