# 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
FeatureCompilerInterpreter
TranslationEntire program before executionLine-by-line
Output.exe fileNo executable
Execution speedFasterSlower
Error reportingAll errors at endStops at first error
DebuggingDifficultEasier
### 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