Operating system layers

An operating system for a distributed system is organised in layers, each using the services of the layer below. Layering hides hardware detail and lets each layer be changed independently.

+----------------------------------------+
|  Applications, services                |
+----------------------------------------+
|  Middleware (RMI, RPC, naming ...)     |
+----------------------------------------+
|  OS: kernel, libraries, servers        |
|  (process, thread, communication,      |
|   memory management)                   |
+----------------------------------------+
|  Computer and network hardware         |
|  (CPU, memory, NIC, disk)              |
+----------------------------------------+

1. Hardware layer (platform): CPU, memory, disks and network interface. It executes instructions and moves data. Importance: provides raw resources on which everything runs.
2. Kernel layer: runs in privileged mode. It handles process and thread creation and scheduling, low level memory management (virtual memory), interrupt handling, device drivers and basic communication (IPC). Importance: protects resources, gives other layers a simple interface through system calls.
3. OS services / library layer: file system, network protocol stack, naming, user-level servers and libraries. Importance: gives programs convenient abstractions (files, sockets) and allows easy replacement of a service in a microkernel design.
4. Middleware layer: RPC, RMI, message queues, object request broker, transactions, security. Importance: hides differences of hardware, OS and network, so distributed applications are portable and see the system as one.
5. Application layer: user programs and distributed applications (email, web, databases). Importance: the purpose of the system; they use only the middleware API.

The lower two layers form the platform; the upper layers give transparency and openness for distributed programming.