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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

Feature Compiler Interpreter
Translation Entire program before execution Line-by-line
Output .exe file No executable
Execution speed Faster Slower
Error reporting All errors at end Stops at first error
Debugging Difficult Easier

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:

<TEXT>

<digit> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
<integer> ::= <digit> | <integer> <digit>

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
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