Chapter 1.pptx

Chapter 1 Introduction Topics What is a Computer/Computer System? The von Neumann Architecture. Representation of Data and Instructions. Instruction Execution. The Control Unit. The Computer System. Main and Secondary Memory. Input - Output Devices.  A computer is...

Chapter 1 Introduction Topics What is a Computer/Computer System? The von Neumann Architecture. Representation of Data and Instructions. Instruction Execution. The Control Unit. The Computer System. Main and Secondary Memory. Input - Output Devices.  A computer is a data processing What is a machine which is operated automatically under the control computer? of a list of instructions (called a program) stored in its main memory What is a A computer system consists usually of a computer and its peripherals. computer Computer peripherals include input devices, output devices, and system? secondary memories. The Von Neumann Architecture von Neumann computers are general purpose computers. They can solve very different problems depending on the program they got to execute! The Von Neumann Architecture The principles: Data and instructions are both stored in the main memory. The content of the memory is addressable by location (without regard to what is stored in that location). Instructions are executed sequentially (from one instruction to the next, in order of their location in memory) unless the order is explicitly modified. The organization (architecture) of the computer a central processing unit (CPU); it contains the control unit (CU), that coordinates the execution of instructions and the arithmetic/logic unit (ALU) which performs arithmetic and logic operations; (main) memory. General-purpose (von Neumann) Architectures In the von-Neumann architecture, a small set of circuits can be driven to perform very different tasks, depending on the software program which is executed.  General-purpose (von Neumann) Architectures The primary function of a CPU is to execute the instructions fetched from the main memory. An instruction tells the CPU to perform one of its basic operations (an arithmetic or logic operation, to transfer a data from/to main memory, etc.). The Control Unit is the one which interprets (decodes) the instruction to be executed and which "tells" the different other components what to do. The CPU includes a set of registers which are temporary storage devices typically used to hold intensively used data and intermediate results. Representation of Data Inside a computer, data and control information (instructions) are all represented in binary format which uses only two basic symbols: "0" and "1". The two basic symbols are represented by electronics signals. Numeric data are represented using the binary system, in which the positional values are powers of 2: 100101 = 1*20 + 0*21 + 1*22 + 0*23 + 0*24 + 1*25 10110 = 0*20 + 1*21 + 1*22 + 0*23 + 1*24 Binary numbers are added, subtracted, multiplied and divided (by the ALU) directly; it is not needed to convert them to decimal numbers first. 100101 + 10110 = 111011 Machine Instructions A CPU can only execute machine instructions; Each computer has a set of specific machine instructions which its CPU is able to recognize and execute. A machine instruction is represented as a sequence of bits (binary digits). These bits have to define: What has to be done (the operation code) To whom the operation applies (source operands) Where does the result go (destination operand) How to continue after the operation is finished Type of Machine Instructions Machine instructions are of four types: Data transfer between memory and CPU registers Arithmetic and logic operations Program control (test and branch) I/O transfer The Instruction Cycle Each instruction is performed as a sequence of steps; the steps corresponding to one instruction are referred together as an instruction cycle. A simple and refined view of the instruction cycle The Control Unit The task of the Control Unit is to perform the control by question how are the elements inside the CPU and the interface to the external data path controlled (synchronized) in order to work properly?  CPU + main memory constitute the "core" of the computer The system. Computer Secondary memory + I/O devices are the so called peripherals. System Communication between different components of the system is usually performed using one or several buses. Memories The main memory is used to store the program and data which are currently manipulated by the CPU. The secondary memory provides the long-term storage of large amounts of data and program. Before the data and program in the secondary memory can be manipulated by the CPU, they must first be loaded into the main memory. The most important characteristics of a memory is its speed, size, and cost, which are mainly constrained by the technology used for its implementation. Typically: The main memory is fast and of limited size; Secondary memory is relatively slow and of very large size. The Main Memory The main memory can be viewed as a set of storage cells, each of which can be used to store a word. Each cell is assigned a unique address and the addresses are numbered sequentially: 0,1,2,.... Besides the storage cells, there are a memory address buffer (storing the address of the word to be read/written) and a data buffer (storing the data read/to be written), the address decoder and a memory control unit. The most widely used technology to implement main memories is semiconductor memories. The most common semiconductor memory type is random access memory (RAM). The information stored in a RAM semiconductor memory will be lost when electrical power is removed. Main Memory Secondary Memory Hard Disk: - Data are recorded on the surface of a hard disk made of metal coated with magnetic material. The disks and the drive are usually built together and encased in an air tight container to protect the disks from pollutants such as smoke particle and dust. Several disks are usually stacked on a common drive shaft with each disk having its own read/write head. Main features: Direct access Fast access: Seek time » 10 ms Data transfer rate » 5 MB/s Large storage capacity (8MB - several GB) Hard Disk Hard Disk Diskette Data are recorded on the surface of a floppy disk made of polyester coated with magnetic material. A special diskette drive must be used to access data stored in the floppy disk. It works much like a record turntable of gramophone. Main features: Direct access Cheap Portable, convenient to use Main standards: 5 1/4-inch. Capacity » 360 KB/disk 3 1/2-inch. Capacity » 1.44 MB/disk (about 700 pages of A4 text) Magnetic Tape Magnetic tape is made up from a layer of plastic which is coated with iron oxide. The oxide can be magnetized in different directions to represent data. Its operation uses a similar principle as in the case of a tape recorder. Main features: Sequential access (access time about 1-5 s). High value of storage (50 MB/tape). Inexpensive. It is often used for backup or archive purpose. Magnetic Tape Optical Memory CD-ROM (Compact Disk ROM): The disk surface is imprinted with microscopic holes which record digital information. When a low-powered laser beam shines on the surface, the intensity of the reflected light changes as it encounters a hole. The change is detected by a photo sensor and converted into a digital signal. with properties : Huge capacity: 775 MB/disk (»550 diskettes). Inexpensive replication, cheap production. Removable. Read-only. Long access time (could be half a second). Optical Memory WORM (Write-Once Read-Many) CD: A laser beam of modest intensity equipped in the disk drive is used to imprint the whole pattern. Good for archival storage by providing a permanent record of large volumes of data. Erasable Optical Disk: combination of laser technology and magnetic surface technique. Can be repeatedly written and overwritten High reliability and longer life than magnetic disks. CD-ROM Input-Output Devices Input and output devices provide a means for people to make use of a computer. Some I/O devices function also as an interface between a computer system and other physical systems. Such interface usually consists of A/D and D/A converters. Typical Input Devices Typical Output Devices