FUNCTIONAL UNITS: A Computer consists of five functionally independent main parts 1) input 2) memory 3) arithmetic and logic 4) control 5) output. The input unit accepts coded information from human operators, from electromechanical devices such as keyboards, or from other computers over digital communication lines. The information received is either stored in the computer’s memory for later reference or immediately used by the arithmetic and logic circuitry to perform the desired operations. The processing steps are determined by a program stored in the memory. Finally, the results are sent back to the outside world through the output unit. All of these actions are coordinated by the control unit.
Input unit: Computers accept coded information through input units, which read the data. The most well-known input device is the keyboard. Whenever a key is pressed, the corresponding letter or digit is automatically translated into its corresponding binary code and transmitted over a cable to either the memory or the memory or the processor.
Many other kinds of input devices are available, including joysticks, trackballs and mouse. These are often used as graphic input devices in conjunction with displays. Microphones can be used to capture audio input which is then sampled and converted into digital codes for storage and processing.
Memory Unit: The function of the memory unit is to store programs and data. There are two classes of storage, called primary and secondary.
Primary storage is a fast memory that operates at electronic speeds. Programs must be stored in the memory while they are being executed. The memory contains a large number of semi conductor storage cells. Each capable of storing one bit of information. These cells are rarely read or written as individual cells but instead are processed in groups of fixed size called words. The memory is organized so that the contents of one word, containing n bits, can be stored or retrieved in one basic operation.
To provide easy access to any word in the memory, a distinct address is associated with each word location. Addresses are numbers that identify successive locations. A given word is accessed by specifying its address and issuing a control command that starts the storage or retrieval process.
The number of bits in each word is often referred to as the word length of the computer. Typical word lengths range from 16 to 64 bits. The capacity of the memory is one factor that characterized the size of a computer. Small machines typically have only a few tens of millions of words, whereas medium and large machines normally have many tens or hundreds of millions of words. Data are usually processed within a machine in units of words, multiples of words, or parts of words. When the memory is accessed, usually only one word of data is read or written.
Programs must reside in the memory during execution. Instructions and data can be written into the memory or read out under the control of the processor. It is essential to be able to access any word location in the memory as quickly as possible. Memory in which any location can be reached in a short and fixed amount of time after specifying its address is called random access memory (RAM). The time required to access one word is called the memory access time. This time is fixed, independent of the location of the word being accessed. It typically ranges from a few nanoseconds (ns) to about 100 ns for modern RAM units. The memory of a computer is normally implemented as a memory hierarchy of three or four levels of semiconductor RAM units with different speeds and sizes. The small, fast, RAM units are called caches. They are tightly coupled with the processor and are often contained on the same integrated circuit chip to achieve high performance. The largest and slowest unit is referred to as the main memory. Secondary storage is used when large amount of data and many programs have to be stored, particularly for information that is accessed infrequently. A wide selection of secondary storage devices is available, including magnetic disks and tapes and optical disks (CD-ROMS).
ARITHMETIC AND LOGIC UNIT: Arithmetic and logic unit performs arithmetic operations and logical decisions. Suppose two numbers located in the memory are to be added. They are brought into the processor, and the actual addition is carried out by the ALU. The sum may be stored in the memory or retained in the processor for immediate use.
Any other arithmetic or logic operations, for example multiplication, division or comparison of numbers are initiated by brining the required operands into the processor, where the operation is performed by the ALU. When operands are brought into the processor, they are stored in high-speed storage elements called registers. Each register can store one word of data. Access times to registers are somewhat faster than access times to the fastest cache unit in the memory hierarchy.
The control and the arithmetic and logic units are many times faster than other devices connected to a computer system. This enables a single processor to control a number of external devices such as keyboards, displays, magnetic and optical disks, sensors, and mechanical controllers.
Output Unit: The output unit is the counter part of the input unit. It function is to send processed results to the outside world. The most familiar example of such a device is a printer. Any peripheral that receives or displays output from a computer.
Control Unit: The memory, arithmetic and logic, and input and output units store and process information and perform input and output operations. The operation of these units must be coordinated in some way. This is the task of the control unit. The control unit is effectively the nerve center that sends control signals to other units and senses their states.
I/O transfers, consisting of input and output operations, are controlled by the instructions of I/O programs that identify and devices involved and the information to be transferred. However, the actual timing signals that govern the transfers are generated by the control circuits. Timing signals are signals that determine when a given action is to take place. Data transfers between the processor and the memory are also controlled by the control unit through timing signals. It is reasonable to think of a control unit as a well-defined, physically separate unit that interacts with other parts of the machine.
The operations of a computer can be summarized as follows:
•The computer accepts information in the form of programs and data through an input unit and stores it in the memory.
•Information stored in the memory is fetched, under program control, into an arithmetic and logic unit. Where it is processed.
•Processed information leaves the computer through an output unit.
•All activities inside the machine are directed by the control unit.
Wednesday, January 5, 2011
Introduction to Digital Gates
The AND gate is so named because, if 0 is called "false" and 1 is called "true," the gate acts in the same way as the logical "and" operator. The following illustration and table show the circuit symbol and logic combinations for an AND gate. (In the symbol, the input terminals are at left and the output terminal is at right.) The output is "true" when both inputs are "true." Otherwise, the output is "false."
AND gate
The OR gate gets its name from the fact that it behaves after the fashion of the logical inclusive "or." The output is "true" if either or both of the inputs are "true." If both inputs are "false," then the output is "false."
OR gate
The XOR ( exclusive-OR ) gate acts in the same way as the logical "either/or." The output is "true" if either, but not both, of the inputs are "true." The output is "false" if both inputs are "false" or if both inputs are "true." Another way of looking at this circuit is to observe that the output is 1 if the inputs are different, but 0 if the inputs are the same.
A logical inverter , sometimes called a NOT gate to differentiate it from other types of electronic inverter devices, has only one input. It reverses the logic state.
Inverter or NOT gate
The NAND gate operates as an AND gate followed by a NOT gate. It acts in the manner of the logical operation "and" followed by negation. The output is "false" if both inputs are "true." Otherwise, the output is "true."
The NOR gate is a combination OR gate followed by an inverter. Its output is "true" if both inputs are "false." Otherwise, the output is "false."
NOR gate
The XNOR (exclusive-NOR) gate is a combination XOR gate followed by an inverter. Its output is "true" if the inputs are the same, and"false" if the inputs are different.
Using combinations of logic gates, complex operations can be performed. In theory, there is no limit to the number of gates that can be arrayed together in a single device. But in practice, there is a limit to the number of gates that can be packed into a given physical space. Arrays of logic gates are found in digital integrated circuits (ICs). As IC technology advances, the required physical volume for each individual logic gate decreases and digital devices of the same or smaller size become capable of performing ever-more-complicated operations at ever-increasing speeds.
AND gate
Input1 Input2 Output
0 0 0
0 1 0
1 0 0
1 1 1
The OR gate gets its name from the fact that it behaves after the fashion of the logical inclusive "or." The output is "true" if either or both of the inputs are "true." If both inputs are "false," then the output is "false."
OR gate
Input1 Input2 Output
0 0 0
0 1 1
1 0 1
1 1 1
The XOR ( exclusive-OR ) gate acts in the same way as the logical "either/or." The output is "true" if either, but not both, of the inputs are "true." The output is "false" if both inputs are "false" or if both inputs are "true." Another way of looking at this circuit is to observe that the output is 1 if the inputs are different, but 0 if the inputs are the same.
XOR gate
Input1 Input2 Output
0 0 0
0 1 1
1 0 1
1 1 0
A logical inverter , sometimes called a NOT gate to differentiate it from other types of electronic inverter devices, has only one input. It reverses the logic state.
Inverter or NOT gate
Input Output
1 0
0 1
The NAND gate operates as an AND gate followed by a NOT gate. It acts in the manner of the logical operation "and" followed by negation. The output is "false" if both inputs are "true." Otherwise, the output is "true."
NAND gate
Input 1 Input 2 Output
0 0 1
0 1 1
1 0 1
1 1 0
The NOR gate is a combination OR gate followed by an inverter. Its output is "true" if both inputs are "false." Otherwise, the output is "false."
NOR gate
Input 1 Input 2 Output
0 0 1
0 1 0
1 0 0
1 1 0
The XNOR (exclusive-NOR) gate is a combination XOR gate followed by an inverter. Its output is "true" if the inputs are the same, and"false" if the inputs are different.
XNOR gate
Input1 Input2 Output
0 0 1
0 1 0
1 0 0
1 1 1
Using combinations of logic gates, complex operations can be performed. In theory, there is no limit to the number of gates that can be arrayed together in a single device. But in practice, there is a limit to the number of gates that can be packed into a given physical space. Arrays of logic gates are found in digital integrated circuits (ICs). As IC technology advances, the required physical volume for each individual logic gate decreases and digital devices of the same or smaller size become capable of performing ever-more-complicated operations at ever-increasing speeds.
Introduction to Digital Computers
Digital Computers: The digital computer is a digital system that performs various computational tasks. The word digital implies that the information in the computer is represented by variables that take a limited number of discrete values. These values are processed internally by components that can maintain a limited number of discrete states. The decimal digits 0, 1, 2…9 for example, provide 10 discrete values. The first electronic digital computers, developed in the late 1940s, were used primarily for numerical computations. In this case the discrete elements are the digits. From this application the term digital computer has emerged. In practice, digital computers function more reliably if only two states are used. Because of the physical restriction of components, and because human logic tends to be binary (that is true or false, yes or no statements), digital components that are constrained to take discrete values are further constrained to take only two values and are said to be binary. Digital computers use the binary number system, which has two digits: 0 and 1. A Single binary digit is called a bit. A nibble is half a byte, or four bits. A word is the default data size for a processor. The default size does not apply in all cases. The word size is chosen by the processor’s designer(s) and reflects some basic hardware issues (such as internal or external buses). The most common word sizes are 16 and 32, but words have ranged from 16 to 60 bits. Typically there will be additional data sizes that are defined relative to the size of a word: halfword, half the size of a word; longword, usually double the size of a word; doubleword, usually double the size of a word (sometimes double the size of a longword); and quadword, four times the size of a word. Whether or not there is a space between the size designation and “word” is designated by the manufacturer, and varies by processor.
A sequence of instructions for the computer is called a Program. The data that are manipulated by the program constitute the data base.
A computer system is composed of its hardware and the system software available for its use. The system software of a computer consists of a collection of programs whose purpose is to make more effective use of the computer. The programs included in a systems software package are referred to as the operating system.
Computer Organization is concerned with the way the hardware components operate and the way they are connected together to form the computer system. The various components are assumed to be in place and the task is to investigate the organizational structure to verify that the computer parts operate as intended.
Computer Design is concerned with the hardware design of the computer. Once the computer specifications are formulated, it is the task of the designer to develop hardware for the system. Computer design is concerned with the determination of what hardware should be used and how the parts should be connected. This aspect of computer hardware is sometimes referred to as computer implementation.
Computer Architecture is concerned with the structure and behavior of the computer as seen by the user. It includes the information, formats, the instruction set, and techniques for addressing memory. The architectural design of a computer system is concerned with the specifications of the various functional modules, such as processors and memories, and structuring them together into a computer system.
Logic Gates: Binary logic deals with binary variables and with operations that assume a logical meaning. It is used to describe, in algebraic or tabular form, the manipulation and processing of binary information. The manipulation of binary information is done by logic circuits called gates. Gates are blocks of hardware that produce signals of binary 1 or 0 when input logic requirements are satisfied. A variety of logic gates are commonly used in digital computer systems. Each gate has a distinct graphic symbol and its operations can be described by means of an algebraic expression. The input-output relationship of the binary variables for each gate can be represented in tabular form by a truth table.