Note for Computer Organisation and Architecture - COA By Amity Kumar

Topics ( 6 )

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Einstein College of Engineering UNIT 1 INTRODUCTION FUNCTIONAL UNITS OF A COMPUTER SYSTEM Digital computer systems consist of three distinct units. These units are as follows: Input unit Central Processing unit Output unit these units are interconnected by electrical cables to permit communication between them. This allows the computer to function as a system. Input Unit A computer must receive both data and program statements to function properly and be able to solve problems. The method of feeding data and programs to a computer is accomplished by an input device. Computer input devices read data from a source, such as magnetic disks, and translate that data into electronic impulses for transfer into the CPU. Some typical input devices are a keyboard, a mouse, or a scanner. Central Processing Unit The brain of a computer system is the central processing unit (CPU). The CPU processes data transferred to it from one of the various input devices. It then transfers either an intermediate or final result of the CPU to one or more output devices. A central control section and work areas are required to perform calculations or manipulate data. The CPU is the computing center of the system. It consists of a control section, an arithmetic-logic section, and an internal storage section (main memory). Each section within the CPU serves a specific function and has a particular relationship with the other sections within the CPU. CONTROL SECTION The control section directs the flow of traffic (operations) and data. It also maintains order within the computer. The control section selects one program statement at a time from the program storage area, interprets the statement, and sends the appropriate electronic impulses to the arithmetic-logic and storage sections so they can carry out the instructions. The control section does not perform actual processing operations on the data. The control section instructs the input device on when to start and stop transferring data to the input storage area. It also tells the output device when to start and stop receiving data from the output storage area. ARITHMETIC-LOGIC SECTION.— The arithmetic-logic section performs arithmetic operations, such as addition, subtraction, multiplication, and division. Through internal logic capability, it tests various conditions encountered during processing and takes action based on the result. At no time does processing take place in the storage section. Data maybe transferred back and forth between these two sections several times before processing is completed. Computer architecture topics Sub-definitions Some practitioners of computer architecture at companies such as Intel and AMD use more fine distinctions:  Macroarchitecture - architectural layers that are more abstract than microarchitecture, e.g. ISA

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Einstein College of Engineering    ISA (Instruction Set Architecture) - as defined above Assembly ISA - a smart assembler may convert an abstract assembly language common to a group of machines into slightly different machine language for different implementations Programmer Visible Macroarchitecture - higher level language tools such as compilers may define a consistent interface or contract to programmers using them, abstracting differences between underlying ISA, UISA, and micro architectures. E.g. the C, C++, or Java standards define different Programmer Visible Macro architecture - although in practice the C micro architecture for a particular computer includes  UISA (Microcode Instruction Set Architecture) - a family of machines with different hardware level micro architectures may share a common microcode architecture, and hence a UISA.  Pin Architecture - the set of functions that a microprocessor is expected to provide, from the point of view of a hardware platform. E.g. the x86 A20M, FERR/IGNNE or FLUSH pins, and the messages that the processor is expected to emit after completing a cache invalidation so that external caches can be invalidated. Pin architecture functions are more flexible than ISA functions - external hardware can adapt to changing encodings, or changing from a pin to a message - but the functions are expected to be provided in successive implementations even if the manner of encoding them changes. Design goals The exact form of a computer system depends on the constraints and goals for which it was optimized. Computer architectures usually trade off standards, cost, memory capacity, latency and throughput. Sometimes other considerations, such as features, size, weight, reliability, expandability and power consumption are factors as well. The most common scheme carefully chooses the bottleneck that most reduces the computer's speed. Ideally, the cost is allocated proportionally to assure that the data rate is nearly the same for all parts of the computer, with the most costly part being the slowest. This is how skillful commercial integrators optimize personal computers. Performance Computer performance is often described in terms of clock speed (usually in MHz or GHz). This refers to the cycles per second of the main clock of the CPU. However, this metric is somewhat misleading, as a machine with a higher clock rate may not necessarily have higher performance. As a result manufacturers have moved away from clock speed as a measure of performance. Computer performance can also be measured with the amount of cache a processor has. If the speed, MHz or GHz, were to be a car then the cache is like the gas tank. No matter how fast the car goes, it will still need to get gas. The higher the speed, and the greater the cache, the faster a processor runs. Modern CPUs can execute multiple instructions per clock cycle, which dramatically speeds up a program. Other factors influence speed, such as the mix of functional units, bus speeds, available memory, and the type and order of instructions in the programs being run. There are two main types of speed, latency and throughput. Latency is the time between the start of a process and its completion. Throughput is the amount of work done per unit time. Interrupt latency is the guaranteed maximum response time of the system to an electronic event (e.g. when the disk drive finishes moving some data). Performance is affected by a very wide range of design choices — for example, pipelining a processor

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Einstein College of Engineering usually makes latency worse (slower) but makes throughput better. Computers that control machinery usually need low interrupt latencies. These computers operate in a real-time environment and fail if an operation is not completed in a specified amount of time. For example, computer-controlled anti-lock brakes must begin braking almost immediately after they have been instructed to brake. The performance of a computer can be measured using other metrics, depending upon its application domain. A system may be CPU bound (as in numerical calculation), I/O bound (as in a webserving application) or memory bound (as in video editing). Power consumption has become important in servers and portable devices like laptops. Benchmarking tries to take all these factors into account by measuring the time a computer takes to run through a series of test programs. Although benchmarking shows strengths, it may not help one to choose a computer. Often the measured machines split on different measures. For example, one system might handle scientific applications quickly, while another might play popular video games more smoothly. Furthermore, designers have been known to add special features to their products, whether in hardware or software, which permit a specific benchmark to execute quickly but which do not offer similar advantages to other, more general tasks.

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Einstein College of Engineering A Functional Unit is defined as a collection of computer systems and network infrastructure components which, when abstracted, can be more easily and obviously linked to the goals and objectives of the enterprise, ultimately supporting the success of the enterprise‘s mission. From a technological perspective, a Functional Unit is an entity that consists of computer systems and network infrastructure components that deliver critical information assets,1 through network-based services, to constituencies that are authenticated to that Functional Unit. Central processing unit (CPU) — The part of the computer that executes program instructions is known as the processor or central processing unit (CPU). In a microcomputer, the CPU is on a single electronic component, the microprocessor chip, within the system unit or system cabinet. The system unit also includes circuit boards, memory chips, ports and other components. A microcomputer system cabinet will also house disk drives, hard disks, etc., but these are considered separate from the CPU. This is principal part of any digital computer system, generally composed of control unit, and arithmetic-logic unit the ‗heart‖ of the computer. It constitutes the physical heart of the entire computer system; to it is linked various peripheral equipment, including input/output devices and auxiliary storage units o Control Unit is the part of a CPU or other device that directs its operation. The control unit tells the rest of the computer system how to carry out a program‘s instructions. It directs the movement of electronic signals between memory—which temporarily holds data, instructions and processed information—and the ALU. It also directs these control signals between the CPU and input/output devices. The control unit is the circuitry that controls the flow of information through the processor, and coordinates the activities of the other units within it. In a way, it is the "brain", as it controls what happens inside the processor, which in turn controls the rest of the PC. o Arithmetic-Logic Unit usually called the ALU is a digital circuit that performs two types of operations— arithmetic and logical. Arithmetic operations are the fundamental mathematical operations consisting of addition, subtraction, multiplication and division. Logical operations consist of comparisons. That is, two pieces of data are compared to see whether one is equal to, less than, or greater than the other. The ALU is a fundamental building block of the central processing unit of a computer. Memory — Memory enables a computer to store, at least temporarily, data and programs. Memory—also known as the primary storage or main memory—is a part of the microcomputer that holds data for processing, instructions for processing the data (the program) and information (processed data). Part of the contents of the memory is held only temporarily, that is, it is stored only as long as the microcomputer is turned on. When you turn the machine off, the contents are lost. The capacity of the memory to hold data and program instructions varies in different computers. The original IBM PC could hold approximately 6,40,000 characters of data or instructions only. But modern microcomputers can hold millions, even billions of characters in their memory. Input device : An input device is usually a keyboard or mouse, the input device is the conduit through which data and instructions enter a computer. A personal computer would be useless if you could not interact with it because the machine could not receive instructions or deliver the results of its work. Input devices accept data and instructions from the user or from another computer system (such as a computer on the Internet). Output devices return processed data to the user or to another computer system.