1. BIT 1003 - Presentation 6

2. Contents GENERATIONS OF LANGUAGES COMPILERS AND INTERPRETERS VIRTUAL MACHINES OBJECT-ORIENTED PROGRAMMING SCRIPTING LANGUAGES FUNCTIONAL LANGUAGES LANGUAGE DESIGN LANGUAGE SYNTAX AND SEMANTICS 2

3. GENERATIONS OF LANGUAGES Each computer is wired to perform certain operations in response to instructions. An instruction is a pattern of ones and zeros stored in a word of computer memory. By the way, a “word” of memory is the basic unit of storage for a computer. A 16-bit computer has a word size of 16 bits, or two bytes. A 32-bit computer has a word size of 32 bits, or four bytes. A 64-bit computer has a word size of 64 bits, or eight bytes. When a computer accesses memory, it usually stores or retrieves a word of information at a time. 3

4. instruction set - architecture Each computer is wired to interpret a finite set of instructions. Most machines today have 75 to 150 instructions in the machine “instruction set.” Much of the “architecture” of a computer design is reflected in the instruction set, and the instruction sets for different architectures are different. For example, the instruction set for the Intel Pentium computer is different from the instruction set for the Sun SPARC. 4

5. The earliest computers They were programmed directly in the machine instruction set. The programmer worked with ones and zeros to code each instruction. As an example, here is code (and an explanation of each instruction), for a particular 16-bit computer. 0110000001000000 (Load the A-register from 64) 0100000001000001 (Add the contents of 65) 0111000001000010 (Store the A-register in 66) 5

6. Assembly languages An early improvement in programming productivity was the assembler. Assembly languages are called second-generation languages. An assembler can read mnemonics (letters and numbers) for the machine instructions, and for each mnemonic generate the machine language in ones and zeros. 6

7. These are typical of such mnemonics: LDA m ADA m ALS SSA JMP m Load the A-register from memory location m. Add the contents of memory location m to the contents of the A- register, and leave the sum in the A-register. A Left Shift; shift the bits in the A-register left 1 bit, and make the least significant bit zero. Skip on Sign of A; if the most significant bit in the A-register is 1, skip the next instruction, otherwise execute the next instruction. Jump to address m for the next instruction. 7

8. FORTRAN In 1954 the world saw the first third-generation language. The language was FORTRAN, devised by John Backus of IBM. FORTRAN stands for FORmula TRANslation. example: X = Y + Z 8

9. procedural language FORTRAN is a “procedural language”. The computer is a flexible tool, and the programmer’s job is to lay out the sequence of steps necessary to accomplish the task. The program is like a recipe that the computer will follow mechanically. 9

10. LISP (for LISt Processing) In 1958, John McCarthy at MIT developed a very different type of language. It is a particularly good language for working with lists of numbers, words, and objects, and it has been widely used in artificial intelligence (AI) work. In mathematics, a function takes arguments and returns a value. LISP works the same way, and LISP is called a “functional language” example: (+ 2 5) 10

11. Cobol (COmmercial and Business- Oriented Language) In 1959 a consortium of six computer manufacturers and three US government agencies released Cobol as the computing language for business applications. Cobol, like FORTRAN, is an imperative, procedural language. To make the code more self-documenting, Cobol was designed to be a remarkably “wordy” language. The following line adds two numbers and stores the result in a third variable: ADD Y, Z GIVING X. 11

12. PL/1 and BASIC Both PL/1 and BASIC were introduced in 1964. These, too, are procedural, imperative languages. IBM designed PL/1 with the plan of “unifying” scientific and commercial programming. PL/1 was part of the IBM 360 project, and PL/1 was intended to supplant both FORTRAN and Cobol 12

13. BASIC (Beginner’s All-purpose Symbolic Instruction Code) BASIC was designed at Dartmouth by professors Kemeny and Kurtz as a simple language for beginners. Over time, however, an almost countless number of variations of BASIC have been created, and some are very rich in programming power. Microsoft’s Visual Basic, for example, is a powerful language rich in modern features. 13

14. C Dennis Ritchie created the very influential third- generation language C in 1971. C was developed as a language with which to write the operating system Unix, and the popularity of C and Unix rose together. C is also an imperative programming language. An important part of C’s appeal is its ability to perform low-level manipulations, such as manipulations of individual bits, from a high-level language. 14

15. Smalltalk During the 1970s, the language Smalltalk popularized the ideas of object-oriented programming. Object-oriented languages are another subcategory of imperative languages. Both procedural and object-oriented languages are imperative languages. The difference is that object-oriented languages support object-oriented programming practices such as inheritance, encapsulation, and polymorphism. 15

16. C++ In the mid-1980s, Bjarne Stroustrup, at Cambridge University in Britain, invented an object-oriented language called C++. C++ is a superset of C; any C program is also a C++ program. C++ provides a full set of object-oriented features, and at one time was called “C with classes.” 16

17. Java The most popular object-oriented language today is Java, which was created by James Gosling and his colleagues at Sun Microsystems. Java was released by Sun in 1994, and became an immediate hit due to its appropriateness for web applications, its rich language library, and its hardware independence. Java’s growth in use among programmers has been unprecedented for a new language. Today Java and C are the languages most frequently chosen for new work. 17