The big idea: The operating system sits between your programs and the hardware.
Programs ask the OS for what they need. The OS deals with the hardware and hands back the result.
Saving a file: A program says "save this as report.txt".
It never says which disk, which blocks, or how that particular drive expects to be spoken to. The OS handles all of that.
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Abstraction: Abstraction means giving programs one simple way to ask for something, whatever hardware is underneath.
Every storage device is "a file". Every printer is "a printer". The program never learns the difference.
Without an operating system
- Every program must know every model of disk, printer and network card
- New hardware means rewriting every program
- Two programs could write to the same memory and corrupt each other
- Nothing stops one program taking the whole CPU
With an operating system
- A program asks once, in one standard way
- New hardware needs a driver, and nothing else changes
- The OS gives each program its own memory
- The OS decides who gets the CPU, and when
Why this is worth the layer: Abstraction is what makes the same program run on thousands of different machines.
That is the answer to "why not let programs talk to the hardware directly".
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Managing resources: One CPU, one block of memory, one disk — and dozens of programs wanting all of them.
The OS decides who gets what and when, so that everything keeps running.
The CPU
- Decides which program runs next, and for how long
- Switches between them fast enough to look simultaneous
- Stops one program monopolising the processor
Memory
- Gives each program its own region
- Stops one program reading or overwriting another's data
- Reclaims the memory when a program closes
Devices and files
- Queues requests so two programs do not print at once
- Keeps track of where every file physically lives
- Checks that the user is allowed the access they ask for
Keeping itself safe: The OS also protects itself. A program cannot reach the OS's own memory or hardware directly — it has to ask.
That boundary is what keeps one crashing program from taking down the machine.
How this is tested — you must name the two jobs — hiding hardware detail and sharing resources — and give an example of each. It comes up two ways:
Paper 1 Section A
- Describe the role of an OS, 2-4 marks
- Explain what abstraction means here
- Say why programs do not use hardware directly
Paper 1 Section B — case study
- Explain what the OS does for the case study system
- Link it to that organisation's mix of hardware
The classic trap: Listing software you can see — a file browser, a settings screen. Those are applications that ship with the OS. The role is managing hardware and resources, which happens where the user never looks.
IB-style questionDescribe[4 marks]
Describe the role of an operating system, using the example of two programs saving files at the same time.
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