Computer Basics: CPU, Memory and Storage

Computer Basics

• A computer consists of a CPU, memory, a hard disk, a floppy disk, a monitor, printer, and communication devices
• The central processing unit(CPU) is the brain of a computer
• It retrieves instructions from memory and executes them
• CPU speed is measured in megahertz (MHz). 1 MHz equals 1 million pulses per second
• Memory stores data and program instructions for CPU to execute
• A memory unit is an ordered sequence of bytes, each of which holds 8 bits
• Programs and data must be brought to memory before they can be executed
• A memory byte is never empty, but its initial content may be meaningless to a program
• The current content of a memory byte is lost whenever new information is placed in it
• Data, such as numbers, characters, and strings, are encoded as a series of bits, either zeros or ones
• Computers use zeros and ones because digital devices have two stable states

Storage Devices

• The three main types of storage devices are disk drives, CD drives, and tape drives
• Memory is volatile, because information is lost when the power is off
• Therefore, programs and data are permanently stored on storage devices and are moved to memory when the computer actually uses them.

Output Devices

• The monitor displays information(text and graphics)
• The resolution and dot pitch determine the monitor's display quality

Programs

• Computer programs, also known as software, are instructions to the computer
• It is through programs that computers are instructed what to do
• Programs are written using programming languages
• Computers do not understand human language

Programming languages

• Machine language is a set of primitive instructions built into every computer
• Assembly languages were developed to make programming easier
• A program called assembler is used to convert assembly language programs into machine code
• High-level languages are English-like and easy to learn and program
• Programming tools referred to as an interpreter or a compiler translates programs written in a high-level language
• An interpreter reads one statement from the source code at a time, translating it to the machine code or virtual machine code, and then executes it right away
• A compiler translates the entire source code into a machine-code file, which is then executed

Operating Systems

• The operating system (OS) is a program that manages and controls a computer's activities
• Popular OS for general-purpose computers includes Microsoft Windows, Mac OS, and Linux
• Application programs, such as a web browser or a word processor, cannot run unless an operating system is installed and running on the computer

Why Java

• Java enables users to develop and deploy applications on the Internet for servers, desktop computers, and small handheld devices
• Java is an Internet programming language
• It is a general purpose programming language suitable for use on stand-alone applications or apps that run from a browser.
• Furthermore it can be used on applications for handheld devices or web servers

Characteristics of Java

• Java is partially modeled on C++, but greatly simplified and improved
• Java is inherently object-oriented. It was designed from the start to be object-oriented
• Java is designed to make distributed computing easy, with networking capability inherently integrated
• To run Java programs requires an interpreter where programs are compiled into bytecode. The bytecode is machine-independent
• Java compilers can detect many problems that would first show up at execution time in other languages
• Java has eliminated certain types of error-prone programming constructs found in other languages
• Java has a runtime exception-handling feature to provide programming support for robustness
• Java implements several security mechanisms to protect your system against harm caused by stray programs
• With a Java Virtual Machine (JVM), you can write one program that will run on any platform
• Java programs are portable, and can be run on any platform.
• Java programs can be run on any platform without being recompiled
• Multithread programming is smoothly integrated in Java, whereas in other languages, procedures specific to the operating system have to be called to enable multithreading
• New code can be loaded on the fly without recompilation
• There is no need for developers to create, and for users to install, major new software versions
• It allows for new features to be incorporated transparently as needed

Java Develoment Kit (JDK) editions

• Java Standard Edition (J2SE) can be used to develop client-side standalone applications or applets
• Java Enterprise Edition (J2EE) can be used to develop server-side applications
• Java Micro Edition (J2ME) can be used to develop applications for mobile devices like cell phones

Popular Java IDEs

• NetBeans
• Eclipse

A Simple Java Program

// This program prints Welcome to Java!
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
}
}

Basic Program Implementation

• Programs can be created using Notepad or WordPad or other text editors
• The Java code source is compiled by invoking javac Welcome.java from the command line
• The program is then executed, displaying output by invoking: java Welcome from the command line

Anatomy of a Java Program

• Class name
• Main method
• Statements
• Statement terminator
• Reserved words
• Comments
• Blocks

Class Names

• Every Java program must have at least one class
• Class names, by convention start with an uppercase letter

Main Method

• To run a class, the class must contain a method named main
• The program is executed from the main method
• Syntax is

public static void main(String[] args) {
}

Statement

• Represents an action or a sequence of actions
• Every statement in Java ends with a semicolon (;)
• Reserved words or keywords have a specific meaning to the compiler and cannot be used for other purposes in the program
• A pair of braces in a program forms a block that groups components of a program
• Double slashes // precedes a comment line
• Opening and closing quotation marks enclose a string (e.g., sequence of characters)

Programming Style and Documentation

• Include a summary at the beginning of the program to explain what the program does, its key features, its supporting data structures, and any unique techniques it uses
• Include author name, class section, instructor, date, and a brief description at the beginning of the program
• Choose meaningful and descriptive names
• Class names should Capitalize the first letter of each word in the name
• Indent two spaces
• Use a blank line to separate segments of the code
• Follow an end-of-line style for braces

Programming Errors

• Syntax Errors are detected by the compiler
• Runtime Errors causes the program to abort
• Logic Errors produces incorrect results