0110101001101101 10010110 2/4 Binary Code & CPUs Digital Signals digital versus analog, examples Binary Numbers Transistors: introduction Binary Code bits & bytes types: ASCII, UNICODE, EBCDIC 0110101001101101 10010110

Digital Signals: why they are discussed. Virtually everything in a computer runs in a digital system: data storage, communication, output on the screen, … Everything is in its lowest form either ON or OFF, UP or DOWN, YES or NO. Bits & bytes are combinations of digital signals and codes.

Digital Signals: what are they? Digital signals have two settings: ON or OFF. Examples: smoke signals, Morse code, fluorescent lights, pass or fail Anything that can be compared to ON or OFF can be a digital signal: Magnets: north or south Voltage: high or low Light: light or dark Gates: open or shut

Digital Signals versus Analog Signals Digital signals have two settings: ON or OFF. Analog signals have ranges of settings: dimmer switches, human voices, ocean waves Sound: Digital versus analog. Analog is a wave: continuous, gradual Digital is a step: non-continuous, ON/OFF Analog signal Digital signal

Binary Numbers A digital system Can represent any decimal number with only two characters: 0 & 1 Why not use decimal numbers? Computers use digital systems (on or off) Decimal Binary 0 0 1 1 2 10 3 11 4 100 5 101 6 110 7 111 8 1000 9 1001 10 1010 11 1011 12 1100

Transistors: tiny ON/OFF switches Tiny electrical gates with two paths: 1. Control path (gatekeeper) 2. Signal path (goes through gate) Only two possible states: gate is OPEN or gate is CLOSED. Transistors are what make up computer chips. AMD Athlon chip has 22 million transistors. Image courtesy of AMD

Binary Code: Bits & Bytes Bit: a single element of code. 0 or 1. Contraction of “Binary digit” Byte: a collection of 8 bits. 00000000. Possible number of different bytes: 256 00000000 00000001 00000010 00000011 00000100 00000101 00000110 00000111 00001000 00001001 00001010 00001011 00001100 00001101 00001110 00001111 00010000 00010001 00010010 00010011 00010100 00010101 00010110 00010111 00011000 00011001 00011010 00011011 00011100 00011101 00011110 00011111 etc.

Binary Code: Bits & Bytes Each byte represents 1 character or command. A simple text file ( log.txt ) can be only a few hundred bytes. A spreadsheet ( book1.xls ) can be millions. kilobyte: KB 2 to the 10th (1,024) bytes. megabyte: MB 2 to the 20th (1,048,576) gigabyte: GB 2 to the 30th (1,073,741,824) terabyte: TB 2 to the 40th (1,099,511,627,766)

When is a kilobyte NOT a kilobyte? Common usage (not exactly correct, but close) kilobyte: KB 1,000 bytes megabyte: MB 1,000,000 bytes gigabyte: GB 1,000,000,000 bytes terabyte: TB 1,000,000,000,000 bytes

Why we don’t type in binary digits. Codes (lookup tables) in the computer. Each character corresponds to a byte. As we type, the keystrokes are translated into bytes by the computer. The computer reverse-translates to show the characters on the monitor. Common code sets: ASCII, UNICODE, EBCDIC

Code Types. ASCII “As-key” American Standard Code for Information Interchange. 1st half of the slots in the table are for “standard” ASCII characters. The second half contains the “extended” ASCII character set. UNICODE uses 2 bytes/char rather than 1. Supports many more characters (34,168). Esp. used for non-English languages EBCDIC “eb-see-dik” Extended Binary Coded Decimal Interchange Code. Mainly used on mainframe computers