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1. Central Processing Unit (CPU): 2. Memory Unit Department of Electrical Engineering. First Year / 2016-2017 By: Salwa Adel Al-agha Lecture 2 1. Central Processing Unit (CPU): The Central Processing Unit (CPU) is an internal component of the computer, portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU is the brain of the computer. On personal computers and small workstations, the CPU is housed in a single silicon chip called a microprocessor. Modern CPUs are large scale integrated circuits in small, rectangular packages, with multiple connecting pins. Two typical components of a CPU are: - Arithmetic Logic Unit (ALU). - Control Unit (CU). Modern CPUs are small and square and contain multiple metallic connectors or pins on the underside. Pentium chip or processor, made by Intel, is the most common CPU though there are many other companies that produce processors for personal computers. 2. Memory Unit: Memory is a collection of storage registers used to transfer information in and out of the unit. Memory is one of the easiest pieces of hardware to add to computer. The actual work is done in (memory) and the finished result is stored in (disk). The information stored in the memory as binary code in groups of bits called word. The binary is two logic levels: - Logic (1). - Logic (0). Bit: is binary digit (1) or (0), and Byte: is a group of eight bits. 9 Department of Electrical Engineering. First Year / 2016-2017 By: Salwa Adel Al-agha Lecture 2 Memory in general is divided into two general categories: 2.1 Read Only Memory: Read-Only Memory (ROM) is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty. Sometimes called non-volatile memory as it is not erased when the system is switched off. ROM memories have gradually evolved from fixed read-only memories to memories than can be programmed and then re-programmed. - ROM (Read Only Memory). - PROM (Programmable Read Only Memory). - EPROM (Erasable Programmable Read Only Memory). - EEPROM (Electrically Erasable Programmable Read Only Memory). 2.2 Random Access Memory: Random Access Memory (RAM) is the memory that the computer uses to temporarily store the information as it is being processed. Capacity (Memory Size): Capacity (memory size) is an important aspect of system performance; it is a useful and convenient way to describe the size of memory. Most computer memory sizes are described as a specific number of words. Computer 10 Department of Electrical Engineering. First Year / 2016-2017 By: Salwa Adel Al-agha Lecture 2 memory sizes are given in Kilo, Mega, or Gega. The exact size of a 1 Kilo block is (2ˡº), and 1 Mega block is (2²º), and 1 Gega block is (2³º). 3. Input/Output Ports Unit: Input/Output, or I/O, refers to the communication between an information processing system (such as a computer), and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it. keyboard or a mouse may be an input device for a computer, while monitors and printers are considered output devices for a computer. parallel port, all bits of information represented by a byte or word are input or output simultaneously. serial port, all bits of information are input or output one at a time. 4. Buses: Buses are collections of wires or groups of conductors that connect several devices within a computer system. To transfer information internally between units, the computers use buses. Bus is a parallel data communication path over which information is transferred a byte or word at a time, when a word of data is transferred between units; all its bits are transferred in parallel. There are generally two buses within a computer : Internal bus: The internal bus allows the processor to communicate with the system's central memory . Expansion bus: Allows various motherboard components (USB, serial and parallel ports, cards inserted in PCI connectors, hard drives, CD- ROM and CD-RW drives, etc.) to communicate with one another. 11 Department of Electrical Engineering. First Year / 2016-2017 By: Salwa Adel Al-agha Lecture 2 4.1 Characteristics of bus: Bus is characterized by the amount of information that can be transmitted at once. This amount, expressed in bits, corresponds to the number of physical lines over which data is sent simultaneously. A 32-wire cable can transmit 32 bits in parallel. The term width is used to refer to the number of bits that a bus can transmit at once. Additionally, the bus speed is also defined by its frequency (expressed in Hertz), the number of data packets sent or received per second. Each time that data is sent or received is called a cycle. Transfer speed of the bus, the amount of data which it can transport per unit of time, by multiplying its width by its frequency. There are generally two buses within a computer: Internal bus: The internal bus allows the processor to communicate with the system's central memory. Expansion bus: Allows various motherboard components (USB, serial and parallel ports, cards inserted in PCI connectors, hard drives, CD- ROM and CD-RW drives, etc.) to communicate with one another. 4.2 Bus structure: Therefore computer has uses three main types of buses. These are: 1- Data Bus: The bidirectional data bus, handles the transfer of all data and instructions between the units of the computer. Its transfer instructions coming from or going to the processor. 2- Address Bus: It is a unidirectional bus (called the memory bus) transports memory addresses which the processor wants to access in order to read or write data. 12 Department of Electrical Engineering. First Year / 2016-2017 By: Salwa Adel Al-agha Lecture 2 3- Control Bus It is a bidirectional bus (or command bus) transports orders and synchronization signals coming from the control unit and traveling to all other hardware components, as it also transmits response signals from the hardware. Control bus is used by the CPU to direct and monitor the actions of the other functional areas of the computer. 5. BIOS: The BIOS (Basic Input Output System) software is built into the PC, and is the first code run by a PC when powered on. When the PC starts up, the first job for the BIOS is to initialize and identify system devices such as the video display card, keyboard and mouse, hard disk drive, optical disc drive and other hardware. The BIOS locates boot loader software and loads and executes that software, giving it control of the PC. This process is known as booting, or booting up. BIOS software is stored on a non-volatile ROM chip on the motherboard (it is often called a ROM BIOS). This ensures that the BIOS will always be available and will not be damaged by disk failures. Because RAM is faster than ROM, though, many computer manufacturers design systems so that the BIOS is copied from ROM to RAM each time the computer is booted, this is known as shadowing. Many modern PCs have a flash BIOS, which means that the BIOS has been recorded on a flash memory chip, which can be updated if necessary. 13 Department of Electrical Engineering. First Year / 2016-2017 By: Salwa Adel Al-agha Lecture 2 5. Storage Devices: A storage device is a hardware device capable of storing information. There are two storage devices used in computers: - Primary storage device such as computer RAM. - Secondary storage device such as computer Hard Disk. The most common storage devices are: Floppy Disk: They are plastic square disks, usually with a silver or black sliding piece going across the top. These disks come in a variety of colors and they hold about 144 million bytes. CD + R Disc: CD + R Disc (Compact Disc Recordable) is a CD that you can record on. It’s mostly used to record audio and once it’s been written on, you cannot rewrite or erase anything of it. CD + RW Disc: (Compact Disc Rewriteable), this disc looks like a regular CD. The only difference is that you can write on this disc and erase it as many times as you want. A CD + RW disc holds about 650 MB. DVD – R Disc: (Digital Video Recordable) – These discs hold the space of about 4.7 GB and are used to record movies on. The more uncommon storage devices are: Flash Smart Cards Storage Tapes Memory Sticks Smart Media 14 .
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