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