Operating System Functions

This lesson examines the five core functions of an operating system in depth: process management, memory management, file management, I/O management and the user interface. The OCR H446 specification requires detailed understanding of each function.

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Process Management

A process is a program that is currently being executed, along with its associated data, registers and state information. The OS must manage many processes that may be competing for CPU time.

Process States

State	Description
New	The process has been created but not yet admitted to the ready queue
Ready	The process is in memory and waiting for CPU time. It has everything it needs except the CPU
Running	The process is currently being executed by the CPU
Waiting (Blocked)	The process is waiting for an event (e.g. I/O completion, user input)
Terminated	The process has finished execution

Process State Diagram

        +-----+       +---------+
        | New | ----> | Ready   | <--------+
        +-----+       +----+----+          |
                            |              |
                     Scheduler dispatches   |
                            |              |
                       +----v----+         |
                       | Running | --------+
                       +----+----+    (time slice expired /
                            |         interrupt / preempted)
                            |
                    +-------+-------+
                    |               |
              I/O request    Process completes
                    |               |
              +-----v-----+  +-----v------+
              | Waiting    |  | Terminated |
              | (Blocked)  |  +------------+
              +-----+------+
                    |
               I/O complete
                    |
                    +-------> Ready

Process Control Block (PCB)

Every process has a Process Control Block — a data structure maintained by the OS that stores all the information needed to manage the process.

PCB Field	Content
Process ID (PID)	Unique identifier for the process
Process state	Current state (ready, running, waiting, etc.)
Program counter	Address of the next instruction to execute
CPU registers	Saved register values when the process is not running
Memory allocation	Information about the memory pages/segments assigned
Priority	The process's scheduling priority
I/O status	List of I/O devices allocated to the process
Accounting info	CPU time used, start time, etc.

Context Switching

When the OS switches the CPU from one process to another, it performs a context switch:

1. Save the state (registers, PC) of the currently running process into its PCB.
2. Load the state of the next process from its PCB.
3. Transfer control to the new process.

Context switching is overhead — the CPU does no useful work during the switch. Frequent switching (e.g. with many processes) increases overhead.

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Memory Management

The OS must allocate and manage main memory (RAM) among all running processes.

Goals of Memory Management

Goal	Explanation
Allocation	Assign memory to processes as they need it
Protection	Prevent one process from accessing another process's memory
Efficient use	Minimise wasted (unused) memory
Virtual memory	Allow processes to use more memory than physically available

Paging

• Memory is divided into fixed-size blocks called pages (in virtual memory) and frames (in physical RAM).
• A page table maps virtual page numbers to physical frame numbers.
• Pages can be stored in any free frame — they do not need to be contiguous.
• This eliminates external fragmentation (wasted space between allocated blocks).

Segmentation

• Memory is divided into variable-size segments based on the logical structure of the program (e.g. code segment, data segment, stack segment).
• Each segment has a base address and a length.
• Segments can grow or shrink as needed.
• Can cause external fragmentation because segments are variable-size.

Paging vs Segmentation

Feature	Paging	Segmentation
Block size	Fixed (e.g. 4 KB)	Variable
Based on	Physical memory structure	Logical program structure
External fragmentation	None	Can occur
Internal fragmentation	Can occur (last page may not be full)	None
Address translation	Page table	Segment table

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File Management

The OS manages how files are stored on secondary storage, organised into directories, and accessed by users and programs.

File System Functions

Function	Description
Creating and deleting files	The OS allocates space on disk and creates directory entries
Reading and writing	The OS translates file operations into disk read/write commands
Directory management	Organises files into a hierarchical directory (folder) structure
Access control	Manages permissions (read, write, execute) for different users
File naming	Maintains the mapping between human-readable file names and physical disk locations
Disk space allocation	Decides which blocks on the disk to use for each file

File Allocation Methods