What is Software? Software, for our purposes, is just a word meaning programs. CS1313: Software Lesson Fall

Transcription

1 1. Software Outline 2. What is Software? 3. What is a Program? 4. What are Instructions? 5. What is a Programming Language? 6. What is Source Code? What is a Source File? 7. What is an Operating System? 8. Operating System Examples 9. A Simple C Program 10. Anatomy of a Simple C Program 11. Block Delimiters 12. What Is a Comment? #1 13. What Is a Comment? #2 14. Are Comments Necessary? 15. hello_world.c with Comments 16. hello_world.c without Comments 17. Flowchart for hello_world.c 18. A Less Simple C Program #1 19. A Less Simple C Program #2 20. A Less Simple C Program #3 21. A Less Simple C Program: Compile & Run 22. Flowchart for my_add.c 23. Languages 24. What Are the Ingredients of a Language? 25. Kinds of Languages 26. Natural Languages #1 27. Natural Languages #2 28. Natural Languages #3 29. Natural Languages #4 Software Outline 30. Programming Languages 31. Natural Languages vs Programming Languages 32. Programming Language Hierarchy 33. High Level Languages 34. Assembly Languages 35. Machine Languages 36. Converting Between Languages 37. Compiler 38. Interpreter 39. Assembler 40. Our Old Friend hello_world.c 41. Compiler Details 42. Compiler Details (cont d) 43. Elements of a Compiler #1 44. Elements of a Compiler #2 45. Phases of Compiling 46. Compiling a C Statement 47. Assembly Code for hello_world.c #1 48. Assembly Code for hello_world.c #2 49. Machine Code for hello_world.c 50. How to Program in Machine Language Directly 51. Why Not Do Everything in Machine Language? 52. Why Not Do Everything in Assembly Language? 53. The Programming Process 54. What is an Algorithm? 55. Algorithms 56. Algorithm Example: Eating a Bowl of Corn Flakes 57. Top-Down Design 58. Eating Cornflakes: Top Level Fall

2 What is Software? Software, for our purposes, is just a word meaning programs. Fall

3 What is a Program? A program is a set of storage locations RAM, disk, etc and a sequence of actions on those storage locations. The actions in a program are known as instructions. Fall

4 What are Instructions? The actions in a program are known as instructions. Examples: Arithmetic/Logical calculation: e.g., add, subtract, multiply, divide, square root, cosine, etc. Memory operations: load from or store into RAM I/O: read from or write to secondary storage Branch: jump to an instruction that is out of sequence Repetition Allocation of resources and many more. Fall

5 What is a Programming Language? A programming language is a well-defined set of rules for specifying a program s sequence of instructions. Examples: C, C++, Fortran 90, Java, Basic, HTML, Perl, Haskell, Prolog, Pascal, Unix shell, SAS, Pentium4 assembly language, etc. Fall

6 What is Source Code? What is a Source File? Source code is a sequence of instructions, written in a human-readable programming language, that constitutes a program, or a piece of a program. An example is shown on page 9. A source file is a file of source code. Fall

7 What is an Operating System? An operating system is a program that manages interactions between: users and hardware; users and software; hardware and software;... and so much more. Fall

8 Operating System Examples MS Windows/MS-DOS MacOS PalmOS Unix Linux (portable) FreeBSD (portable, underlies MacOS X) Solaris (Sun Microsystems) AIX (IBM) IRIX (SGI) Tru64 (Hewlett-Packard) HP-UX (Hewlett-Packard) Unicos (Cray) Fall

9 A Simple C Program % cat hello_world.c /* ************************************************* *** Program: hello_world *** *** Author: Henry Neeman (hneeman@ou.edu) *** *** Course: CS Fall 2005 *** *** Lab: Sec 011 Fridays 9:30am *** *** Description: Prints the sentence *** *** "Hello, world!" to standard output. *** ************************************************* */ #include <stdio.h> int main () { /* main */ /* ******************************** *** Execution Section (body) *** ******************************** * * Print the sentence to standard output * (i.e., to the terminal screen). */ printf("hello, world!\n"); } /* main */ % gcc -o hello_world hello_world.c % hello_world Hello, world! Fall

10 Comment block Preprocessor directive Anatomy of a Simple C Program Unix command to output to the screen % cat hello_world.c /* ************************************************* *** Program: hello_world *** *** Author: Henry Neeman (hneeman@ou.edu) *** *** Course: CS Fall 2005 *** *** Lab: Sec 011 Fridays 9:30am *** *** Description: Prints the sentence *** *** "Hello, world!" to standard output. *** ************************************************* */ #include <stdio.h> Block open Execution section (also known as the program body) Block close Main function header int main () { /* main */ /* ******************************** *** Execution Section (body) *** ******************************** * * Print the sentence to standard output * (i.e., to the terminal screen). */ printf("hello, world!\n"); } /* main */ % gcc -o hello_world hello_world.c % hello_world Hello, world! Unix command to compile Unix command to run Program output Fall

11 Block Delimiters The open curly brace, also known as the left brace, { acts as the start of a block and is known as the block open. The close curly brace, also known as the right brace, } acts as the end of a block and is known as the block close. The block open and block close are said to delimit the block: they indicate where the block begins and where the block ends. Delimit: indicate where something begins and ends. Fall

12 What Is a Comment? #1 A comment is a piece of text in a source file that: tells human beings (e.g., programmers) something useful about the program, but is ignored by the compiler, so it has absolutely no affect on how the program runs. In C, the start of a comment is indicated by /* and the end of a comment is indicated by */ All text appearing between these comment delimiters are part of the comment, and therefore are ignored by the compiler. Fall

13 What Is a Comment? #2 A comment is a piece of text in a source file that: tells human beings (e.g., programmers) something useful about the program, but is ignored by the compiler, so it has absolutely no affect on how the program runs. In C, the start of a comment is indicated by /* and the end of a comment is indicated by */ A comment can use multiple lines of text. The delimiters do not have to be on the same line. Fall

14 Are Comments Necessary? Comments are ignored by the compiler, so strictly speaking they aren t needed to compile and run. But, if you don t put them into one of your CS1313 programming projects, YOU MAY LOSE A FULL LETTER GRADE OR MORE on that project. Why? Comments tell human beings useful things about your program. They help programmers including you, a month later when you ve forgotten everything about your program to understand your program. They also tell graders that you know what the heck you re doing. Fall

15 hello_world.c with Comments % cat hello_world.c /* ************************************************* *** Program: hello_world *** *** Author: Henry Neeman (hneeman@ou.edu) *** *** Course: CS Fall 2005 *** *** Lab: Sec 011 Fridays 9:30am *** *** Description: Prints the sentence *** *** "Hello, world!" to standard output. *** ************************************************* */ #include <stdio.h> int main () { /* main */ /* ******************************** *** Execution Section (body) *** ******************************** * * Print the sentence to standard output * (i.e., to the terminal screen). */ printf("hello, world!\n"); } /* main */ % gcc -o hello_world hello_world.c % hello_world Hello, world! Fall

16 hello_world.c without Comments % cat hello_world.c #include <stdio.h> int main () { printf("hello, world!\n"); } % gcc -o hello_world hello_world.c % hello_world Hello, world! Fall

17 Flowchart for hello_world.c int main () { printf("hello, world!\n"); } Start An oval denotes either the start or the end of the program, or a halt operation within the program (which we ll learn about later). References: Output Hello, world! End A parallelogram denotes either an input operation or an output operation. An arrow denotes the flow of the program. home/k5htm/f2.htm#flowchart symbol Fall

18 A Less Simple C Program #1 /* ************************************************ *** Program: my_add *** *** Author: Henry Neeman (hneeman@ou.edu) *** *** Course: CS Fall 2005 *** *** Lab: Sec 011 Fridays 9:30am *** *** Description: Input two integers, compute *** *** their sum and output the result. *** ************************************************ */ #include <stdio.h> int main () { /* main */ /* *************************** *** Declaration Section *** *************************** * ***************************** * Named Constant Subsection * ***************************** */ const int program_success_code = 0; /* ***************************** * Local Variable Subsection * ***************************** * * addend: the addend value that the user inputs. * augend: the augend value that the user inputs. * sum: the sum of the addend and the augend, * which is output. */ int addend, augend, sum; Continued on the next slide. Fall

19 A Less Simple C Program #2 /* ************************* *** Execution Section *** ************************* * *********************** * Greeting Subsection * *********************** * * Tell the user what the program does. */ printf("i ll add a pair of integers.\n"); /* ******************** * Input subsection * ******************** * * Prompt the user to input the addend & augend. */ printf("what two integers do you want to add?\n"); /* * Input the integers to be added. */ scanf("%d %d", &addend, &augend); Continued on the next slide. Fall

20 A Less Simple C Program #3 /* ************************** * Calculation Subsection * ************************** * * Calculate the sum. */ sum = addend + augend; /* ********************* * Output Subsection * ********************* * * Output the sum. */ printf("the sum of %d and %d is %d.\n", addend, augend, sum); return program_success_code; } /* main */ Fall

21 A Less Simple C Program: Compile & Run % gcc -o my_add my_add.c % my_add I ll add a pair of integers. What two integers do you want to add? 5 7 The sum of 5 and 7 is 12. % my_add I ll add a pair of integers. What two integers do you want to add? The sum of 1593 and 9832 is Fall

22 Flowchart for my_add.c A rectangle denotes an operation other than I/O or branching (e.g., calculation). Fall

23 What is a language? Kinds of languages Natural languages Languages Programming languages (also known as Formal languages) Converting between programming languages Compilers Interpreters Assemblers Fall

24 What Are the Ingredients of a Language? Symbols: a set of words and punctuation (in computing, words and punctuation are together known as tokens) Grammar (also known as syntax): a set of rules for putting tokens together to get valid statements Semantics: a set of rules for interpreting the meaning of a valid statement Fall

25 Kinds of Languages Natural languages: used in human communication Programming languages (also known as formal languages): used by computers (among others) Fall

26 Natural Languages #1 Examples: English, Chinese, Swahili, Navajo, Quechua, Maori Typically can be described by formal rules (grammar), but often are not rigidly governed by these rules in everyday use: Any noun can be verbed. I might could get me one o them there computers. Fall

27 Natural Languages #2 Can mix words from different languages and even syntax (elements of grammar) from different languages in a single sentence: Hey, amigo, is it all right by you if I kibbitz your parcheesi game while we watch your anime? Fall

28 Natural Languages #3 Can be ambiguous: When did he say she was going? could be interpreted as: State the time at which he said, She was going. According to him, at what time was she going? Fall

29 Natural Languages #4 Plenty of flexibility regarding correctness; e.g., ain t, split infinitives, ending a sentence with a preposition That is something up with which I will not put. Fall

30 Programming Languages Examples: C, Java, HTML, Haskell, Prolog, SAS Also known as formal languages Completely described and rigidly governed by formal rules Cannot mix the words of multiple languages, or the syntax of multiple languages, in the same program Cannot be ambiguous Words and syntax must be EXACTLY correct in every way Fall

31 Natural Languages vs Programming Languages PROPERTY NAT L PROG Completely described and rigidly governed by formal rules CAN mix the words of multiple languages, or Y N the syntax of multiple languages, in the same program CAN be ambiguous Y N N Y Words and syntax must be EXACTLY correct in every way N Y Fall

32 Programming Language Hierarchy High Level Languages Assembly Languages Machine Languages Fall

33 High Level Languages Human-readable Most are standardized, so they can be used on just about any kind of computer. Examples: C, Fortran 90, Java, HTML, Haskell, SAS Typically they are designed for a particular kind of application; for example: C for operating system design Fortran 90 for scientific & engineering applications Java for web applets and embedded systems HTML for hypertext (webpages) SAS for statistics But often, their uses in real life are broader their original purpose. Fall

34 Assembly Languages Human-readable Specific to a particular CPU family; for example: Intel Pentium4/AMD Athlon XP (PC) Motorola PowerPC G5 (Macintosh) Intel Itanium2 (servers) So, for example, a program in Pentium4 assembly language cannot be directly run on a G5 machine. Set of simple commands; for example: Load a value from a location in main memory Add two numbers Branch to an instruction out of sequence Fall

35 Machine Languages Not human-readable, except with immense effort Binary code that the CPU family understands directly Binary representation of the CPU family s assembly language Fall

36 Converting Between Languages Compilers, interpreters and assemblers are programs that convert programs that people can read into actions that computers can perform. Fall

37 Compiler Converts a human-readable high level language source code of a program into a machine language executable program Converts an entire source code all at once Must be completed before executing the program Examples: Fortran 90, C, C++, Pascal Fall

38 Interpreter Converts a human-readable high level language source code into actions that are immediately performed Converts and executes one statement at a time Conversion and execution alternate Examples: Perl, HTML, SAS, Mathematica, Unix shell (interactive system within Unix) Fall

39 Assembler Converts a human-readable CPU-specific assembly code into CPU-specific machine language Like a compiler, but for a low level assembly language instead of for a high level language Fall

40 Our Old Friend hello_world.c % cat hello_world.c /* ************************************************* *** Program: hello_world *** *** Author: Henry Neeman (hneeman@ou.edu) *** *** Course: CS Fall 2005 *** *** Lab: Sec 011 Fridays 9:30am *** *** Description: Prints the sentence *** *** "Hello, world!" to standard output. *** ************************************************* */ #include <stdio.h> int main () { /* main */ /* ******************************** *** Execution Section (body) *** ******************************** * * Print the sentence to standard output * (i.e., to the terminal screen). */ printf("hello, world!\n"); } /* main */ % gcc -o hello_world hello_world.c % hello_world Hello, world! Fall

41 Compiler Details Fall

42 Compiler Details (cont d) Fall

43 Elements of a Compiler #1 Lexical Analyzer: identifies program s word elements Comments (ignored by compiler) Keywords (e.g., int, while) Constants (e.g., 5, 0.725, "Hello, world!") User-defined Identifiers (e.g., addend) Operators; for example: Arithmetic: + - * / % Relational: ==!= < <= > >= Logical: &&! Fall

44 Elements of a Compiler #2 Parser: determines the program s grammar Semantic Analyzer: determines what the program does Intermediate Code Generator: expresses, as an assembly-like program, what the program does Optimizer: makes code more efficient (faster) Assembly Code Generator: produces the final assembly code that represents what the program does Fall

45 Compiler Phases of Compiling Assembler: turns assembly code into machine code Linker/loader: turns machine code into an executable file Both the assembler and the linker/loader are invoked automatically by the compiler, so you don t have to worry about them. Fall

46 Compiling a C Statement Fall

47 Assembly Code for hello_world.c #1 On Pentium4 Using gcc On IBM POWER4 Using gcc pushl %ebp movl %esp, %ebp subl $8, %esp subl $12, %esp pushl $.LC0 call printf addl $16, %esp leave ret Different opcodes! mflr 0 stw 31,-4(1) stw 0,8(1) stwu 1,-64(1) mr 31,1 lwz 3,LC..1(2) bl.printf nop lwz 1,0(1) lwz 0,8(1) mtlr 0 lwz 31,-4(1) blr Fall

48 Assembly Code for hello_world.c #2 On Pentium4 Using gcc (GNU compiler) pushl %ebp movl %esp, %ebp subl $8, %esp subl $12, %esp pushl $.LC0 call printf addl $16, %esp leave ret Different sequences of instructions! On Pentium4 Using icc (Intel compiler) pushl %ebp movl %esp, %ebp subl $3, %esp andl $-8, %esp addl $4, %esp push $ STRING.0 call printf xorl %eax, %eax popl %ecx movl %ebp, %esp popl %ebp ret Fall

49 Machine Code for hello_world.c Fall

50 How to Program in Machine Language Directly 1. Write the assembly code for the program directly by hand; i.e., not in a high level language. 2. For each assembly language instruction, look up the bit pattern of the associated machine code. 3. On the computer console, flip switches to match the bit pattern of the machine code. 4. Press the Run button. Actually, on modern computers, programming directly in machine language is just about impossible. Fall

51 Why Not Do Everything in Machine Language? Incredibly tedious and ridiculously error prone! Fun and easy! Not nearly as tedious or error-prone! Fall

52 Why Not Do Everything in Assembly Language? Can t be run on any other kind of computer. May be completely obsolete in a few years. Portable to many kinds of computers. Will still be useable in 20 years ( legacy codes). Fall

53 The Programming Process Formulate Problem Compile Construct Algorithm Bugs? Yes Debug No Choose Programming Language Run Write Program Bugs? Yes No Get an A/Impress Your Boss/Sell for Zillions! Fall

54 What is an Algorithm? An algorithm is: a step-by-step method that is written in a natural language (e.g., English) or in pseudocode (something that sort of looks like a programming language but isn t as precise), rather than in a programming language, that solves a well-defined (but not necessarily useful) problem on a well-defined set of inputs (which may be empty), using finite resources (e.g., computing time and memory) and that produces a well-defined set of outputs (which may be empty). Fall

55 Algorithms An algorithm is a language-independent way of expressing the method of solving a problem; that is, an algorithm could be expressed in two different languages (e.g., English and Japanese) and still be the same algorithm. A program, by contrast, is a language-dependent implementation of the method of solving a problem; that is, the same set of steps expressed in two different programming languages would be two different programs, even if the two programs accomplished exactly the same result. Many programs, but not all programs, implement algorithms. Fall

56 Algorithm Example: Eating a Bowl of Corn Flakes Get bowl from cupboard Get spoon from drawer Get box of corn flakes from pantry Get jug of milk from refrigerator Place bowl, spoon, corn flakes and milk on table Open box of corn flakes Pour corn flakes from box into bowl Open jug of milk Pour milk from jug into bowl Close jug of milk Go to table Pick up spoon Repeat until bowl is empty of corn flakes Using spoon, pick up corn flakes and milk from bowl Put spoon with corn flakes and milk into mouth Pull spoon from mouth, leaving corn flakes and milk Repeat... Chew... until mouthful is mush Swallow Leave mess for housemates to clean up Fall

57 Top-Down Design Algorithms for most non-trivial problems tend to be fairly complicated. As a result, it may be difficult to march from an algorithm s beginning to its end in a straight line, because there may be too many details to keep in your head all at one time. Instead, you can use a technique called top-down design: start with the whole problem, then break it into a few pieces, then break each of those pieces into a few pieces, then break each of those pieces into a few pieces, and so on, until each piece is pretty small. Fall

58 Eating Cornflakes: Top Level Get stuff Transport stuff Set up stuff Eat Finish Fall