A1.3 Operating systems and control systems

Why an operating system?

An operating system (OS) sits between hardware and applications so programmers do not have to control the hardware directly. Its main jobs:

• Process management: starting, pausing and ending programs, and deciding which runs next on the CPU.
• Memory management: giving each process its own region of RAM and preventing one from overwriting another.
• File management: organising data into files and folders, and controlling access rights.
• Device management: talking to hardware through drivers and buffering input and output.
• Security: user accounts, authentication and permissions.
• User interface: a GUI or command line.

Scheduling

Only one process can use a core at a time, so the scheduler decides the order. Common approaches:

Worked round-robin trace: P1 needs 4 ms, P2 needs 6 ms, P3 needs 2 ms, slice 3 ms, queue order P1, P2, P3.

A process that needs less than a full slice gives up the CPU early, so the next one starts sooner.

Polling versus interrupts

• Polling: the CPU repeatedly checks a device ("any data yet?"). Simple and predictable, but it wastes CPU time when events are rare.
• Interrupt: the device sends a signal when it needs attention. The CPU finishes its current instruction, saves its state, runs an interrupt service routine (ISR), then restores the state and carries on.

flowchart TD
  A[CPU running a program] --> B{Interrupt pending at end of instruction?}
  B -- No --> A
  B -- Yes --> C[Save registers and PC on the stack]
  C --> D[Run the interrupt service routine]
  D --> E[Restore registers and PC]
  E --> A

Keyboards and network cards normally use interrupts because key presses are unpredictable and rare compared with CPU speed. Polling suits a sensor that must be read at a steady rate anyway, or simple embedded devices.

Control systems

A control system keeps a physical quantity at a target value.

• Sensor: measures the environment (temperature, light, pressure) and produces a signal, often converted by an analogue-to-digital converter.
• Controller (microprocessor): compares the reading with the set point and decides what to do.
• Actuator: acts on the world (motor, heater, valve, pump).
• Feedback: the result of the action changes what the sensor reads next time, closing the loop.

TARGET = 21.0
while True:
    temp = read_sensor()
    if temp < TARGET - 0.5:
        heater_on()
    elif temp > TARGET + 0.5:
        heater_off()
    wait(5)  # seconds

The 0.5 margin stops the heater flicking on and off constantly. An open-loop system (a basic toaster timer) has no feedback; a closed-loop system does.

Multitasking, resources and memory (HL)

• Multitasking: the OS switches between processes so quickly that they appear to run together. Each switch is a context switch: the state of one process is saved and another is loaded.
• Resource allocation: the OS shares CPU time, memory and devices. Deadlock happens when processes each hold a resource the other needs; starvation when a process never gets a turn.
• Paging: memory is split into fixed-size pages and frames, so a process need not sit in one continuous block. Virtual memory uses disk space as an overflow for RAM; if pages are swapped in and out constantly the system thrashes and slows dramatically.

Common mistakes

• Saying an interrupt stops the CPU mid-instruction (it waits for the instruction to finish).
• Describing round robin without stating what happens to unfinished processes (back of the queue).
• Confusing sensors with actuators.
• Leaving out the comparison with a set point when describing feedback.

Exam tips

• For "evaluate a scheduling method", give one benefit and one drawback linked to the scenario.
• Draw a timeline (Gantt) for any scheduling calculation; it avoids arithmetic slips.
• In control-system answers, name the specific sensor and actuator for the context.