System unit

Ch 4 The System Unit

Competencies

Describe the three basic types of system units.

Discuss how a computer uses binary codes to represent data in electronic form.

Describe each of the major system unit components.

Explain the differences among the three types of memory.

Describe the five principal types of bus lines.

Discuss the most widely used types of ports.

Introduction

• Differences in microcomputer power focus on speed, capacity and flexibility.
• Competent end users need to understand the functionality of the basic components in the system unit, including the system board, microprocessor, memory, system clock, expansion slots and cards, bus lines, ports, cables, and power supply units.
• Some terms:
• Disk Drives: are secondary storage devices for saving data, programs, and information.
• Bus Lines: provide data pathways that connect various system components
• Power Supply Unit: converts Alternating Current (AC) to Direct Current (DC) providing power to the system unit.
• Systems Board: connects all system components to one another.
• System Unit: contains most of the computer’s electronic components.
• Memory: holds data, instructions, and information, memory circuit boards plug into slots on the system board
• Microprocessor: controls operations and performs arithmetic and logical operations, microprocessor cartridges plug into a special slot on the systems board.
• Expansion Cards: allow external devices to connect to and expand a computer’s capabilities, expansion cards plug into slots on the systems board.
• Expansion Slots: provide connections for expansion cards to the system board.

System Unit

• The System unit (aka system cabinet or chassis) is a container that houses most of the electronic components that make up a computer system. 
• Three basic types for microcomputers include:
• Desktop system units: contain the system’s electronic components, and selected secondary storage. Input & Output devices like mouse, keyboard, monitor are located outside the system unit. It can be placed either horizontally or vertically. Vertical units are called “tower models”.
• Notebook system units: are portable and much smaller. They contain the electronic components, selected secondary storage and input devices (keyboard and pointing device). Monitor is located outside the system unit attached to it by hinges.
• Personal digital assistant (PDA) system units: smallest and designed to fit into the palm of one hand. It contains the entire computer system including the electronic components, secondary storage and input and output devices

Electronic Data and Instructions

• Computers only recognize digital electronic signals.
• Much information comes via analog signals: continuous signals, which vary to represent different tones, pitches, and volume such as our voices.
• Computers must be able to convert analog signals to digital for processing.
• Computer data and information are represented electronically with a binary, or two-state system.
• Binary systems use only two digits, 0 & 1.
• Each 0 or 1 is called a bit – short for binary digit.
• Bits are combined into groups of eight bits called bytes to represent numbers, letters and other characters.
• Each byte typically represents one character.

Binary Coding Schemes

Characters are represented as a series of bits through the use of a binary
coding scheme.  Popular schemes include:

• ASCII: American Standard Code for Information Interchange – the most widely used code for microcomputers.  It typically uses an 8 bit encoding scheme, enough to handle 28 (256) different characters.  An example for the letter A is 0100 0001.
• EBCDIC: Extended Binary Coded Decimal Interchange Code – an IBM standard used for large computers.  It also uses 8 bits, but differs slightly from the ASCII coding scheme, e.g. A = 1100 0001
• Unicode: a 16 bit (2 byte) code used to support international languages such as Chinese and Japanese that has too many characters that cannot be supported by 8-bit codes. It can represent 216 characters (65,536 characters)

When you press a key on the keyboard, the keyboard hardware sends an electronic signal to the system unit, converting the key such as the number 3 to the ASCII code of 0011 0011.
All instructions and data have to be converted into binary data before they can be executed. For example, instructions 3 + 5 require 24 bits (8 bits X 3 characters) using the ASCII coding scheme.

System Board

The system board connects all system components and allows input and output devices to communicate with the system unit.

• The system board is also known as main board or motherboard.
• It is the communications medium for the entire computer system. 
• Every component of the system unit connects directly to the system board.
• It acts as the data path allowing the various components to communicate with one and another.

The system board is a large flat circuit board covered with sockets and other electronic parts including variety of chips:

• Sockets: provide a connection point for holding small specialized electronic parts called chips.
• Chips (aka silicon chips, semiconductors, or integrated circuits): consist of tiny circuit-boards etched on to squares of sand-like material called silicon. Chips are mounted on carrier packages that plug into sockets on the system board.
• Slots: provide a connection point for specialized cards or circuit boards.
• Bus lines: provide pathways to support communication among electronic components.

Microprocessor

• In a microcomputer, a Central Processing Unit (CPU) or processor is contained on a single chip called a Microprocessor.
• The microprocessor is often contained within a cartridge that plugs into the special slot on the system board.  The microprocessor is the computer’s “brain”.

It has two basic components:

• Control unit: tells the rest of the computer system how to carry out a program’s instructions. It directs the electronic signals between memory and the ALU, as well as between CPU and input and output devices.
• Arithmetic-logic unit (aka ALU): performs arithmetic operations – addition, subtraction, multiplication and division, and logical operations – comparisons such as equal to (=), less than (<) or greater than (>).

Microprocessor Chips

Microprocessors capacities differ in terms of:

• Word size – the number of bits that can be accessed at one time by the CPU – typically 16, 32, or 64 bits. The more bits in a word, the more powerful – and the faster – the computer is. A 32-bit word computer can access 4 bytes at a time and a 64-bit word computer can access 8 bytes at a time and hence 64-bit computer is faster.
• Speed – how fast the microprocessor can process data and instructions – older microcomputers processed data in microseconds: millionths of seconds.  Newer systems are faster and process data & instructions of billionths of seconds: nanoseconds.
• Supercomputers can process data in picoseconds: trillionths of seconds, 1,000 times as fast as a microcomputer.

There are two types of microprocessor chips:

• Complex Instruction Set Computing (CISC) chips: It is the most common type of microprocessor popularized by Intel.  These are the most popular microcomputer CPU chips, sold by Intel – Pentium and Itanium, and AMD – Athlon and Hammer.
• Reduced Instruction Set Computing (RISC) chips: use fewer instructions, the design is simpler and less costly than CISC chops.  Examples include the PowerPC chip produced by Motorola, the Alpha chip by DEC, SPARC chip by Sun Microsystems, MIPS chip by Silicon Graph's.

Specialty Processors

• Smart cards: a plastic card with the size of credit card with an embedded chip. They can store 80 times more information than contained in normal credit cards. They can record personal financial and medical information in encrypted or coded form by protecting it with passwords or pin number and hence they offer strong security and privacy.
• Co-processors: specialty chips designed to improve specific computing operations.
• Graphics coprocessors: handle the processing for displaying and manipulating 2-D and 3-D graphics images.
• Parallel processors: using specialized software, these are typically used in network servers and supercomputer systems.
• Visa, MasterCard, and American Express have introduced their smart cards to millions of users.

Memory

• Memory is a holding area for data, instructions, and information.
• Memory is contained on chips connected to the system board.

There are three well-known types of memory chips:

Random Access Memory (RAM)

• RAM chips hold the program instructions and data that the CPU is currently processing.
• RAM is called temporary or volatile storage, because the contents are lost when the microcomputer is turned off.  This differs from secondary storage which is permanent or non-volatile: it retains the contents when the power is switched off as happens to data stored on diskettes. It is a good idea to save your work on secondary storage very often. Data from secondary storage must be loaded into RAM before it can be used.
• Flash RAM (aka Flash memory) is a new type of RAM that is non-volatile and can retain data even if power is switched off.  This type of memory is more expensive are used in digital cell phones, digital cameras, and some portable computers.
• It is important to have enough RAM.
• If your computer does not have enough RAM to hold a program some operating systems allow you to run the program using virtual memory, in which large programs are divided into parts and stored on a secondary storage each part is then “swapped” in and out of secondary storage as needed.
• Cache memory (aka RAM cache) is a special high speed area of memory that holds information that is used most frequently.  Having cache memory can speed up the processing time for your computer.
• Other types of RAM include DRAM, SDRAM.

Read Only Memory (ROM) (aka Firmware)

• Programs are built into these chips in the factory. 
• ROM chips are non-volatile and cannot be changed by the user.
• "Read-only" means that the CPU can read the programs written on the ROM chip but the computer cannot write information or instructions in ROM.
• ROM chips usually contain special instructions for detailed computer operations like instructions for starting the computer, for displaying characters on the screen.
• ROM chips are also called firmware.

Complementary Metal Oxide Semiconductor (CMOS)

• CMOS chips provide flexibility and expandability for a computer system.  It contains essential information that is required every time the computer is turned on.
• CMOS chips typically hold 64 bytes of data including the current data & time, amount of RAM, type of keyboard, etc.
• CMOS chips are powered by a battery, and don’t lose their contents when the AC power is turned off unlike RAM.
• Unlike ROM, CMOS chips can have their contents changed to update changes in computer system like increased RAM and new hardware devices.

System Clock

• The system clock is located on a small, specialized chip that produces precisely timed electrical beats or pulses.
• The microprocessor uses the clock beats to coordinate and synchronize all computer operations.
• The clock’s speed (aka clock rate) is typically measured in gigahertz, or billions of beats per second.

Expansion Slots and Cards

• Computers are designed based on different “architectures”
• A closed architecture means that you generally can not add new devices.
• An open architecture allows you to expand the system by providing slots on the system board in which one can insert optional devices known as expansion cards.
• Expansion slots provide an open architecture.
• Expansion cards are also called plug-in boards, controller cards, adapter cards, and interface cards.  Some of the most common cards are:
• Video cards (aka graphics cards) connect the system board with the computer’s monitor to convert the internal electronic signals to video signals so they can be displayed on the monitor.
• Sound cards accept audio input from a microphone and convert it to a form that can be processed by the computer and vice-versa.
• Modem cards (aka internal modems) allow computers to communicate with each other by converting electrical signals from within the system unit into electrical signals that can travel over telephone lines and other types of connections.
• Network Interface Cards (NICs aka Network Adapter Cards) connect a computer to one or more other computers forming a network where by users can share data, programs, and hardware, e.g. an Ethernet network
• TV tuner cards (aka television boards, video recorder cards, or video capture cards) allow you to watch TV on your computer as well as capture video input.
• Plug and Play is a set of hardware and software standards developed in part by Intel and Microsoft to create operating systems, processing units, expansion boards, as well as other devices, to automatically configure themselves so they work with your computer.
• PC cards (aka PCMCIA Personal Computer Memory Card International Association cards) are credit card sized adaptors that connect portable computers to various external devices, e.g. a PCMCIA Ethernet card.

Bus Lines (aka Data Bus or Bus)

• The bus connects the parts of the CPU to each other and also links the CPU to various other components on the system board.
• The number of bits that can pass down the bus is called the bus width.
• The more the bus width the faster the processing speed.
• Typical bus widths are 32 or 64 bits.
• A system unit has more than one type of bus line. The principal bus lines are:
• Industry Standard Architecture (ISA) bus: originally an 8 bit bus designed for the first IBM PCs.  Now it is a 16 bit bus and still found on some microcomputers though it is slow.
• Peripheral Component Interconnect (PCI) bus: originally developed for video demands of graphical user interfaces, a high-speed 32-bit or 64-bit bus that is over 20 times faster than the older ISA buses.
• Accelerated Graphics Port (AGP) bus: over twice as fast as PCI buses.  Widely used for acceleration of graphics performance, 3-D animation, and the transfer of video data.
• Universal Serial Bus (USB) combines with a PCI bus on the system board to support several external devices without using expansion cards or slots.  USB buses are widely used to support high-speed scanners, printers and video-capturing devices.
• High Performance Serial Bus (HPSB) (aka Fire Wire bus) operate much like USB buses but at higher speeds and are typically used to connect digital camcorders.

Ports

• A Port is a socket for external devices to connect to the system unit.

Standard Ports

• Serial ports: used to connect a mouse, keyboard, modem, etc.  Send data one bit at a time.  Good for sending data over a longer distance.
• Parallel ports: used to connect external devices that need to send or receive a lot of data over a short distance, such as printers.  Sends eight bits of data at a time across eight parallel wires.
• USB Ports: gradually replacing serial and parallel ports.  They are faster and one USB port can be used to connect several devices into the system unit.
• FireWire Ports (aka HPSB): are faster than USB ports and typically used to connect video devices to a computer.

Cables

• Cables are the “wires” to connect external devices to the system unit via the ports.