1.1 Data Representation in a Digital Computer

A digital computer is an electronic machine. It is built from millions of tiny electronic switches. Because these switches can only be in two states, the computer needs a two-symbol system to represent everything it stores and processes. That system is the binary number system.

Main Idea

A digital computer represents all data using only two binary digits — 0 and 1. These two digits correspond directly to the two possible states of an electronic switch (OFF and ON). Groups of 0s and 1s are used to represent numbers, letters, and special characters.

Core Concept

The Electronic Switch A digital computer is an electronic machine as it consists of millions of electronic switches. An electronic switch is very similar to an electric switch used to turn a light ON/OFF. The main difference is that it works on very low voltage. Closed switch electricity flows switch is ON. Open switch electricity does not flow switch is OFF. Therefore, a switch has only two states. Why Binary? The binary number system used for counting also consists of two digits, 0 and 1. Because the switch has exactly two states, binary digits 0 and 1 are used in computers to represent the two states of a switch: Current flows switch is ON represents binary 1. No current flows switch is OFF represents binary 0. Bit and Binary Code A binary digit, 0 or 1, is referred to as a bit.

Groups of 1s and 0s are used in computers to represent data. This is known as binary code. Example: the binary code 1000100 may represent the letter D.

Detailed Explanation

Alphanumeric Codes A computer must be able to recognize codes that represent numbers, letters, and special characters. These codes are known as alphanumeric codes. Alphanumeric codes include the following characters: Lower-case letters, a to z Upper-case letters, A to Z Numeric digits, 0 to 9 Special characters including punctuation marks and special symbols such as %, $, &, #, +, etc. ASCII The American Standard Code for Information Interchange (ASCII) is the most commonly used alphanumeric code. It uses 7 bits or 8 bits to represent a character . The total number of characters it represents is 128. DC Voltage The circuit of a digital computer works on DC voltage. Such voltage can have only two possible states (which is why binary representation is natural for a digital computer).

Important Points

• A digital computer is an electronic machine made of millions of electronic switches.
• A switch has only two states: ON (closed) and OFF (open).
• ON represents binary 1; OFF represents binary 0.
• A single binary digit (0 or 1) is called a bit.
• Groups of 1s and 0s used to represent data are called binary code.
• Codes that represent numbers, letters, and special characters are called alphanumeric codes.
• ASCII is the most commonly used alphanumeric code. It uses 7 bits or 8 bits to represent a character and represents 128
• characters in total.
• The circuit of a digital computer works on DC voltage, which has only two possible states.

Remember

• Switch closed = current flows = ON = binary 1.
• Switch open = no current = OFF = binary 0.
• A bit is a single 0 or 1.
• ASCII 7 or 8 bits per character 128 characters total.
• Alphanumeric codes cover: lower-case letters, upper-case letters, numeric digits, and special characters.

Common Mistakes

• Confusing the switch state with the binary digit: ON 1 and OFF 0, not the reverse.
• Thinking a bit is a group of digits. A bit is a single binary digit (0 or 1).
• Mixing up binary code (groups of 1s and 0s representing data) with alphanumeric code (codes representing letters, digits,
• and symbols).
• Forgetting that ASCII represents 128 characters, not more.
• Writing that the computer works on AC voltage. The circuit of a digital computer works on DC voltage.

Quick Revision

A digital computer is an electronic machine made of millions of electronic switches. Each switch has only two states — ON (current flows) and OFF (no current). Because the binary number system also has only two digits (0 and 1), these digits are used to represent the two switch states: 1 for ON and 0 for OFF. A single binary digit is called a bit. Groups of 1s and 0s used to represent data are called binary code (e.g., 1000100 may represent the letter D). To represent numbers, letters, and special characters, computers use alphanumeric codes. The most common alphanumeric code is ASCII, which uses 7 bits or 8 bits per character and represents 128 characters in total. The circuit of a digital computer works on DC voltage, which has only two possible states.

60-Second Revision

Digital computer = electronic machine with millions of switches. Switch: ON = 1, OFF = 0 (two states only). Binary digits 0 and 1 match the two switch states. Bit = one binary digit (0 or 1). Binary code = groups of 1s and 0s representing data. Alphanumeric codes represent letters, digits, special characters. ASCII: most common alphanumeric code; 7 or 8 bits; 128 characters. Warning: DC voltage has only two states — do not confuse with AC. analog and idgiatl signals