CHAPTER 4: SYSTEM SOFTWARE

4.1 PURPOSES OF AN OPERATING SYSTEM

4.1.1 Resource Optimisation

How OS Maximises Resources:

• CPU scheduling
• Memory management
• I/O optimisation
• File system management

4.1.2 User Interface

Purpose: Hides complexities of hardware from user. Allows users to interact with application programs without knowing hardware details.

Types:

• Command-line interface (CLI)
• Graphical User Interface (GUI)
• Touch interface

4.1.3 Process Management

Process: A program being executed which has an associated Process Control Block (PCB) in memory.

Process States:

• Ready: New process arrived, PCB created
• Running: Has CPU access
• Blocked: Cannot progress until event occurs

PCB Contents:

• Process state
• Program counter
• CPU registers
• Memory management information
• I/O status information

4.1.4 Scheduling Routines

First-Come-First-Served (FCFS):

• Non-preemptive
• FIFO queue
• Simple but may cause long waits

Round Robin:

• Preemptive
• Allocates time slice to each process
• No prioritization
• Fair but may waste CPU on context switching

Priority-Based:

• Most complex
• Priorities re-evaluated on queue change
• Can be preemptive or non-preemptive

Shortest Job First:

• Non-preemptive
• Processes with shortest execution time first
• Optimal for minimizing average waiting time

Shortest Remaining Time:

• Preemptive version of Shortest Job First

4.1.5 Memory Management

Paging:

• Process split into pages
• Memory split into frames
• All pages can be loaded into memory

Virtual Memory:

• No need for all pages in memory
• CPU address space larger than physical
• Address resolved by Memory Management Unit (MMU)

Benefits:

• Large programs can run with less physical memory
• More programs can run simultaneously

Disk Thrashing:

• Perpetual loading/unloading of pages
• Occurs when too many processes compete for memory
• Performance severely degraded

4.1.6 OS Structure

User Mode:

• Available for users and applications
• Limited access to system resources

Kernel/Privileged Mode:

• Sole access to parts of memory
• Access to certain system functions
• Only OS components run in this mode

4.2 TRANSLATION SOFTWARE

4.2.1 Compilation Stages

Lexical Analysis:

• Converts sequence of characters to sequence of tokens
• Identifies keywords, identifiers, operators

Syntax Analysis:

• Checks code for grammar mistakes
• Identifies syntax errors

Code Generation:

• Generates intermediate code after syntax analysis

Optimization:

• Improves efficiency of code
• Removes redundant operations

4.2.2 Interpreters vs Compilers

4.2.3 BNF (Backus-Naur Form)

Purpose: Formal mathematical way of defining syntax unambiguously.

Components:

• Set of terminal symbols
• Set of non-terminal symbols
• Set of production rules

Example:

4.2.4 Reverse Polish Notation (RPN)

Definition: A method of representing expressions without brackets or special punctuation. Uses postfix notation where operator is placed after variables.

Example:

• Infix: A + B
• RPN: A B +

Advantages:

• No need for brackets
• No need for operator precedence
• Simpler for machine to evaluate
• No backtracking needed

Evaluation:

• Use stack
• When operator encountered, pop two operands, apply operator, push result