1. www.monash.edu.au FIT1001- Computer Systems Lecture 12 Introduction to System Software

2. Week 12: FIT1001 Computer Systems S12 2006 2 Lecture12 : Learning Objectives Describe the relationship between machine language, assembler code and high level language Understand the use of system software tools including assemblers, linkers, loaders, editors, compilers, interpreters and debuggers describe the concept of relocatability of program code explain some methods used to define the syntax of computer languages including syntax (“railroad”) diagrams and BNF list the stages of high level program compilation differentiate between link/loader systems that perform early, late and dynamic binding

3. www.monash.edu.au FIT1001- Computer Systems Program Translation

4. Week 12: FIT1001 Computer Systems S12 2006 4 Introduction The biggest and fastest computer in the world is of no use if it cannot efficiently provide beneficial services to its users. Users see the computer through their application programs. These programs are ultimately executed by computer hardware. System software can be defined as a collection of programs and associated data that supports the use of a computer system

5. Week 12: FIT1001 Computer Systems S12 2006 5 Programming Tools Overview This study guide will cover program development tools –Editors –Assemblers –Debuggers –Compilers –Linkers –Loaders –Interpreters Integrated Development Environments (IDEs) combine several of the above programming toolsIntegrated Development Environments (IDEs) combine several of the above programming tools

6. Week 12: FIT1001 Computer Systems S12 2006 6 The Program Translation Process Translator Linker Loader Source Object Executable

7. Week 12: FIT1001 Computer Systems S12 2006 7 Program Text Editors Word processors format the appearance of the text Text editors –Format the spacing between words for legibility –Ideal for structured languages –Text is the same font size Examples –DOS – Edit –Windows – Notepad, Wordpad –Unix / Linux – ed, vi, emacs IDEs –MS Visual C++

8. Week 12: FIT1001 Computer Systems S12 2006 8 Programming Language Categories Machine Language –Binary coded instructions Assembly Language –Symbolic coded instructions Procedural Languages –procedural statements or arithmetic notation Fourth-Generation Languages –Natural language and nonprocedural statements Object-oriented Languages –Combination of objects and procedures

9. Week 12: FIT1001 Computer Systems S12 2006 9 Assembly Language When to use –When speed or size of program is critical –Hybrid approach –Hardware Drivers –Can use specialized instructions Disadvantages –Inherently machine specific –Architectures may become obsolete –Lack of programming structure

10. Week 12: FIT1001 Computer Systems S12 2006 10 Assemblers Binary code = machine code Hex code Assembly Language –Mnemonic names  op codes –Labels  memory addresses –Comments –Symbol table –Operations table –Memory Relocation Cross Assembler

11. Week 12: FIT1001 Computer Systems S12 2006 11 Procedural Languages COBOL –Wordy but easier to maintain FORTRAN –Scientists and engineers BASIC Pascal –Highly structured teaching language C –high-level commands and low-level access to hardware

12. Week 12: FIT1001 Computer Systems S12 2006 12 Object-Oriented Languages SmallTalk C++ Java –Based on C++ –Platform independent

13. Week 12: FIT1001 Computer Systems S12 2006 13 Compilers Translates high-level language into low-level instructions High-level language: Source code Machine-level: Object code Changes, including bug fixes, require recompiling

14. www.monash.edu.au FIT1001- Computer Systems Machine Language and Assembler

15. Week 12: FIT1001 Computer Systems S12 2006 15 Machine Language of MARIE(SG4) Consider the simple MARIE program given below –A set of mnemonic instructions stored at addresses 100 - 106 (hex):

16. Week 12: FIT1001 Computer Systems S12 2006 16 Assemblers Mnemonic instructions such as LOAD 104 –Easy for humans to write and understand –Impossible for computers to understand Assemblers are the simplest of all programming tools. They translate mnemonic instructions to machine code. Most assemblers carry out this translation in two passes over the source code. –The first pass partially assembles the code and builds the symbol table –The second pass completes the instructions by supplying values stored in the symbol table. Macro Assemblers can define a recurring code sequence.

17. Week 12: FIT1001 Computer Systems S12 2006 17 Relocatable Code The output of most assemblers is a stream of relocatable binary code. –In relocatable code, operand addresses are relative to where the operating system chooses to load the program. –The origin directive of the assembler implies or specifies the load point. –Absolute (nonrelocatable) code is most suitable for device and operating system control programming. When relocatable code is loaded for execution, special registers provide the base addressing. Addresses specified within the program are interpreted as offsets from the base address.

18. Week 12: FIT1001 Computer Systems S12 2006 18 Binding The process of assigning physical addresses to program variables is called binding. Binding can occur at compile time, load time, or run time. Compile time binding gives us absolute code. Load time binding assigns physical addresses as the program is loaded into memory. –With load time, binding the program cannot be moved! Run time binding requires a base register to carry out the address mapping.

19. www.monash.edu.au FIT1001- Computer Systems Programming Language Definition

20. Week 12: FIT1001 Computer Systems S12 2006 20 Language Components Language can be defined as three interrelated components: Lexicon –All legal words in the language –Meaning and type Syntax –Form and structure –grammar rules Semantics –meaning of command

21. Week 12: FIT1001 Computer Systems S12 2006 21 Computer Language Descriptions Expressing the grammar of high-level computer language requires formal and precise syntax specifications Narrative descriptions –the method of description useful to the programmer, reference source –Difficult to implement in a compiler Syntax (Railroad) Diagrams –Formal definition of grammar for a programming language BNF –Backus-Naur Form >Most precise method of describing the grammar of a computer language >Also known as Context-Free Grammar

22. Week 12: FIT1001 Computer Systems S12 2006 22 Railroad Diagram Examples

23. Week 12: FIT1001 Computer Systems S12 2006 23 Typical BNF Rules for Java

24. Week 12: FIT1001 Computer Systems S12 2006 24 Parsed English Sentence

25. www.monash.edu.au FIT1001- Computer Systems Compilers and Interpreters

26. Week 12: FIT1001 Computer Systems S12 2006 26 Compilers Compilers bridge the semantic gap between the higher level language and the machine’s binary instructions. Most compilers effect this translation in a six-phase process. The first three are analysis phases: 1.Lexical analysis extracts tokens, eg. reserved words and variables. 2.Syntax analysis (parsing) checks statement construction. 3.Semantic analysis checks data types and the validity of operators.

27. Week 12: FIT1001 Computer Systems S12 2006 27 Compilers The last three compiler phases are synthesis phases: 4.Intermediate code generation creates three address code to facilitate optimization and translation. 5.Optimization creates assembly code while taking into account architectural features that can make the code efficient. 6.Code generation creates binary code from the optimized assembly code. Through this modularity, compilers can be written for various platforms by rewriting only the last two phases.

28. Week 12: FIT1001 Computer Systems S12 2006 28 The Stages of program Compilation

29. Week 12: FIT1001 Computer Systems S12 2006 29 Process of Parsing Lexical analysis –Also known as scanning –Divides the string of input characters into single elements, tokens, based on strict computer punctuation Syntactic analysis –Checks for errors in grammar rules Semantic parsing –Determines the meaning of the string

30. Week 12: FIT1001 Computer Systems S12 2006 30 Source Code Instructions Data declarations: –Data type such as floating point, integer Data operations –Instructions that update or compute data value (lots of moving around!) Control Structures –Branches, Goto (yetch!), If-then-else, loops such as While-do and Repeat-until Function, procedure, or subroutine calls –Receives control via a call instruction, receives and possibly modifies parameters, and returns control to the instruction after the call

31. Week 12: FIT1001 Computer Systems S12 2006 31 Optimization Compiler analyzes code in order to –Reduce amount of code –Eliminate repeated operations –Reorganize parts of of the program to execute faster and more efficiently –Use computer resources more effectively Example –Move a calculation repeated within the body of a loop that does not use any value modified by the loop Different compilers can produce different results!

32. Week 12: FIT1001 Computer Systems S12 2006 32 Interpreters Interpreters produce executable code from source code in real time, one line at a time. Consequently, this not only makes interpreted languages slower than compiled languages but it also affords less opportunity for error checking. Interpreted languages are, however, very useful for teaching programming concepts, because feedback is nearly instantaneous, and performance is rarely a concern. Examples of interpreted languages –Java, BASIC, LISP

33. Week 12: FIT1001 Computer Systems S12 2006 33 Interpreter vs. Compiler Resources during executionInterpreterCompiler Contents in memory Interpreter/compilerYesNo Source codePartialNo Executable codeYes CPU cycles Translation operationsYesNo Library linkingYesNo Application programYes

34. www.monash.edu.au FIT1001- Computer Systems Linking and Loading

35. Week 12: FIT1001 Computer Systems S12 2006 35 Linking Object file Object file or module Linker Executable file C library

36. Week 12: FIT1001 Computer Systems S12 2006 36 Linkers Searches program libraries to find library routines used by the program –Library: collection of pre-written functions and subroutines made available to perform commonly required activities Determines the memory locations that code from each module will occupy and relocates instructions by adjusting absolute references Resolves references among files Like assemblers, link editors perform two passes: The first pass creates a symbol table and the second resolves references to the values in the symbol table.

37. Week 12: FIT1001 Computer Systems S12 2006 37 Why Link? Construct single executable program from multiple object code files compiled at different times Program can be subdivided into components and parceled out to different developers Example –Main program and multiple subroutines written and compiled by different programmers at different times

38. Week 12: FIT1001 Computer Systems S12 2006 38 Loader Loads binary files that have been linked into main memory Program is ready for execution There are four link/loading scenarios –Absolute code generated at the end of a development cycle, not requiring link editing, eg. Single-user embedded control environment –Using a loader several times without repeat linking, early binding –Using memory efficiently and linker/loader functions are combined, late binding –Using dynamic link library modules, dynamic binding

39. Week 12: FIT1001 Computer Systems S12 2006 39 Link Loading System With Early Binding

40. Week 12: FIT1001 Computer Systems S12 2006 40 Dynamic Linking Dynamic linking is when the link editing is delayed until load time or at run time. External modules are loaded from from dynamic link libraries (DLLs). Load time dynamic linking slows down program loading, but calls to the DLLs are faster. Run time dynamic linking occurs when an external module is first called, causing slower execution time. –Dynamic linking makes program modules smaller, but carries the risk that the programmer may not have control over the DLL.

41. www.monash.edu.au FIT1001- Computer Systems Debugging and Testing

42. Week 12: FIT1001 Computer Systems S12 2006 42 Debuggers Assembly language debuggers Source code debuggers Step through programs Check variable values

43. Week 12: FIT1001 Computer Systems S12 2006 43 Next… Revision of FIT1001 follows this lecture