The Binary Machine Modern highlevel programming languages are

The Binary Machine • Modern high-level programming languages are designed to make programming easier. • On the other end, the low level, all modern digital computers understand only machine language. • This means that a program written in a high -level language must be translated into machine language. • Translation is performed by an assembler, an interpreter, or a compiler. • A computer has a hard-wired instruction set unique to that computer.

The Binary Machine • The instruction set is etched into the computer’s microprocessor chip. • High-level -----> Translator -----> Low-level Source code --------------> Object code • Source code is what goes into a program translator. Object code is what comes out. • At the low-level, all modern digital computers store programs and information in binary form.

Binary Representations • Modern digital computer hardware circuitry is based on on-off switching. Thus, a computer can distinguish between only the two values “on” (1) and “off” (0). • This means information must be represented in binary form. • Some terminology: bit - binary digit. byte - 8 bits. word - 2 bytes or 16 bits.

Binary Representations • Example: Computer memory is measured in bytes. 64 MB means 64 megabytes or 64 million bytes or 8 x 64 = 512 million bits. • Numbers are represented in binary (base 2) notation. • Example: 235 in decimal notation is 11101011 in binary notation. 235 = 2 x 102 + 3 x 101 + 5 x 100 11101011 = 1 x 27 + 1 x 26 + 1 x 25 + 0 x 24 + 1 x 23 + 0 x 22 + 1 x 21 + 1 x 20

Binary Representations • There’s a decimal-to-binary and binary-todecimal calculator in Module 6 of The Analytical Engine Website. • Characters are represented by coding them as integers. • A commonly used character code is ASCII (American Standard Code for Information Interchange). • Example: The letter A is represented by 65 or 01000001. A right parenthesis ) is represented by 41 or 00101001.

Encoding Instructions • Instructions are also encoded in binary. • accumulator - a location where intermediate results are stored. • Sample coded instructions: Code Info Action 00000100 X Load accumulator with contents of memory location X 00000101 X Store accumulator value in memory location X 00001111 X Halt execution 0000 X Add contents of memory location X to accumulator

Encoding Instructions • With this coding, a simple machinelanguage program might look like this: 00000100 10000000 1000000101 100000001111 0000 • Here’s what it says: Load accumulator with contents of 128. Add contents of 129 to accumulator. Store accumulator contents in 128. Halt.

Encoding Instructions • As you might guess, programming in machine language is hard. • Early programmers developed mnemonics for remembering instructions. • The above machine-language program might be remembered as follows: (x = cell 128, y = cell 129) Load x Add y Store x Halt

Encoding Instructions • The next step was to write a program which would translate these mnemonics to machine language, thereby sparing a programmer the grief of programming directly in machine language. • This led to the first program translator. • assembler - a program that takes an assembly language as its input source code and produces machine language as its output object code. • The PIPPIN assembler.

Beyond the Assembler • Assembly-language code is fast and efficient, but hard to write. • Using a higher-level language closer to our native language would make programming easier. • The idea is to move away from dealing with the computer at its level and work more at our level. • When programming at a high level, we don’t want to think about the details of the computer our program will run on.

Types of Languages • Modern high-level programming languages come in two flavors depending on the type of translator used to take source code and produce object code. • interpreter - a translator which translates one line of source code and instructs the computer to perform the resulting object code before translating another line of source code. • compiler - a translator which translates all the source code and produces a complete machine-language program.

Interpreted Languages • Examples include BASIC, LISP, Java. Script. • Advantages: - Interpreters are easy to write. - Errors are more easily localized. • Disadvantages: - Interpreters are slow. - A source-code line will be translated as often as it is encountered. For example, a line of code in a loop that is repeated 1000 times will be translated 1000 times.

Compiled Languages • Examples include FORTRAN, C, Java. • Advantage: - After compilation, execution is fast. • Disadvantage: - Localizing errors is more difficult.

Language Groups 1. Imperative - the fundamental unit is a procedure called by the main program. Examples are FORTRAN, Pascal, Ada. 2. Functional - processes are defined as functions with no main program. An example is LISP. 3. Declarative - the emphasis is on I/O as opposed to processing algorithms. Examples are COBOL, Prolog. 4. Object-oriented - hybrids organized around programming “objects”. Examples are C++, Java.

Language Translation • Translation of a high-level language into machine language involves three steps: 1. Scanning - breaking the source code into its smallest meaningful pieces or tokens. This is what we do when we read a sentence and note the words it contains. 2. Parsing - arranging the tokens into a sensible logical structure. This is what we do when we recognize nouns, verbs, adjectives, etc. in a sentence. 3. Code generation - generating the machine language object code. This is what we do when we paraphrase a sentence for a kid.

Language Generations • Languages are characterized by how far they are from what computers do and how close they are to what people do. 1. First generation - machine languages. 2. Second generation - assembly languages and assemblers. 3. Third generation - interpreted and compiled languages. 4. Fourth generation - languages that give “computational power to nonprogrammers. ”