IB Computer Science Topic A1.3.1: Role of Operating Systems Notes

The Operating System: The Layer That Hides the Hardware

1. The operating system sits between the user, the applications and the hardware, so neither the user nor a program ever speaks to the hardware directly.
2. Every request for CPU time, memory, storage, network access or a peripheral is made to the operating system, which decides whether to grant it and when.
3. Operating systems abstract hardware complexities to manage system resources, and those two jobs, abstraction and resource management, make up the whole of its role.

Abstraction Turns Hardware Detail into a Simple Interface

1. A program that needs 50 megabytes of memory asks the operating system for 50 megabytes.
2. It does not need to know how much RAM is fitted, which addresses are free, or whether part of that memory will actually sit on an SSD.
3. The same request works on a laptop with 4 GB and a server with 512 GB, which is why one compiled program runs on both.

The Resources an Operating System Controls

1. CPU time: the operating system decides which process runs, for how long and in what order.
   1. The four approaches to doing this are compared in A1.3.3.
2. Memory: each process is given its own protected block of memory, which is reclaimed when the process ends.
3. Storage: the operating system presents named files inside folders rather than raw disk block addresses.
4. Input and output: data is routed to and from keyboards, mice, printers, screens and network cards.
5. Access rights: the operating system decides which user or process may read, write or execute a given file or device.
6. Each of these becomes one of the named functions of an operating system in A1.3.2.

What Abstraction Buys: Simpler Code, Enforced Security, Fuller Hardware Use

1. Simpler development: a developer writes one program against the operating system's interface instead of one version per hardware configuration.
2. Enforced security: because every resource request passes through the operating system, there is a single place where access rules can be applied.
   1. A process cannot read another process's memory, because it is never handed a real hardware address to read from.
3. Fuller use of the hardware: the operating system can hand the CPU to a second process while the first one waits for a slow disk read.
4. Abstraction is one of the four concepts of computational thinking in B1.1.2, and the operating system is its clearest working example.

Fairness, Security and Efficiency Are the Three Guarantees

1. Fairness: no single process can starve the others of CPU time or memory, because the operating system decides the share rather than the process.
2. Security: access controls and process isolation keep one user's data out of another user's reach.
3. Efficiency: resources sit idle for less of the time, because the operating system can reassign them as soon as a process stops needing them.
4. Remove any one of the three and the machine becomes unusable, unsafe or slow.