Learning Objectives
By the end of this topic you will be able to:
List and describe the main functions of an operating system
Explain process management and scheduling
Describe memory management, I/O management and file systems
Explain the role of the OS as an interface between user, applications and hardware
Process Management
Processes and the OS
A process is a program in execution. The OS maintains a Process Control Block (PCB) for each process: stores PID, state, PC value, register values, priority, and memory allocation.
Process states: New → Ready → Running → (Blocked/Waiting) → Terminated
Context switching: OS saves the PCB of the current process and restores the PCB of the next. Enables multitasking on a single CPU core.
Multitasking: interleaving processes rapidly creates the illusion of simultaneous execution. The OS scheduler decides which process runs next.
Memory Management
Memory Management
Allocation: OS allocates blocks of RAM to each process. Ensures each process has the memory it needs without overwriting another process’s data.
Protection: each process is restricted to its own memory region. Attempting to access another process’s memory causes a segmentation fault; OS terminates the offending process.
Relocation: OS can move process data in RAM and update pointers, allowing dynamic memory allocation and compaction to reduce fragmentation.
Sharing: some memory regions (e.g. shared libraries, DLLs) are shared between processes to avoid duplication.
File Systems & I/O
File System Management and I/O
File System
Organises files in a hierarchical directory structure. Manages metadata (name, size, date, permissions). Handles file creation, deletion, reading and writing. Examples: NTFS, FAT32, ext4, APFS.
I/O Management
Manages communication between CPU and I/O devices via device drivers. Buffers data (stores temporarily while speed is matched). Handles interrupts from devices. Provides uniform interface regardless of hardware.
The OS abstracts hardware differences: an application calls the same file-write function whether saving to HDD, SSD, or network drive — the OS and driver handle the details.
Security
OS Security Management
User authentication: username/password, biometrics, multi-factor; verifies identity before granting access
Access control: permissions define what each user/process can read, write or execute (UNIX: rwx for owner/group/others)
Process isolation: OS ensures one process cannot access or corrupt another process’s memory space
Encryption: OS may provide file-system level encryption (e.g. BitLocker on Windows) to protect data at rest
Security is an A Level addition — at GCSE you only need basic OS roles. At A Level, explain WHY each security function is necessary.
Common Mistakes
Don’t Lose Marks
!
Saying the OS “runs programs” without explaining how — always mention process management, scheduling, and PCBs when asked about OS process control.
!
Confusing memory allocation with file system management. Memory management is about RAM (during execution). File systems manage persistent storage (HDD/SSD).
!
Forgetting context switching stores ALL register values — not just the PC. The entire CPU state (all GPRs, flags, SP) must be saved to the PCB for the process to resume correctly.