BASIC STRUCTURE OF COMPUTERS

1. BASIC STRUCTURE OF COMPUTERS Computer is a fast electronic calculating machine that Accepts digitized input information Processes the information according to a list of internally stored instructions Produces the results as an output information. The instruction is called a computer program and internal storage is called memory.

2. Types of Computers Computers now a days used differ widely in size, speed and cost. The most common type is the Personal computer available with a processing unit , a cathode ray tube display and keyboard all be located on a home or office desk . Portable notebook computers are used mainly for word processing .In this type ,the key board ,flat panel display and processor are all arranged in a single unit.

3. High performance work stations needs computational power higher than that of personal computers. Work stations are often used in Engineering applications. Although still in desktop dimensions they require graphic input/output capability and hence are used to interactive design work.

4. Mainframe & Super Computers Beyond workstations a range of large and very powerful computer system exist . The systems existing in the lower range are called mainframes and those at the high end are called Super computers . In these category, the main functional unit may comprise a number of separate parts. They are used for numerical calculations in weather forecasting and aircraft design and simulation.

5. FUNCTIONAL UNITSThe computer consists of the following five functionally independent units.1.Input unit 2.Memory unit 3.Arithmetic and logic unit 4. Out put unit and 5.Control unit

6. The input unit accepts coded information from operators through electromechanical device such as key-board or from other computers over digital communication lines .The information received is either stored in the memory or used by the ALU to perform the desired operations . The processing steps are determined by the Program stored in the memory. Finally the results are sent back to the outside world through the output unit. All these actions are coordinated by the control unit.

7. The Arithmetic and Logic Circuits in conjunction with the main control circuit is termed as the Central Processing Unit(CPU) or simply the Processor. Modern system often contains many processors but the term CPU is still used widely.The Input and output equipment is usually combined under the term input-output unit(I/O).In almost all standard equipment ,both input and output functions are available. These functions are separated within the terminal.

8. It is convenient to categorize the information as either instruction or data.Instructions(machine instructions) are commands that1. Govern the transfer of information's within a computer as well as b/w computers and its I/O devices.2. Specify the arithmetic and logic operations to be performed.A set of instructions that performs a task is called a program. The program is stored in the memory.

9. Data are numbers and encoded characters that are used as operands by the instructions. The term data means any digital information . An entire program ( set of instructions) may be considered as data if it is processed by another program. An example is the source program which is the input data to the compiler program. The compiler translates the source program in to machine language program.

10. Information handled by a computer must be encoded suitably. Each number, character, or instruction is encoded as a string of binary digits called bits. The binary coded decimal format is employed for the purpose of encoding in which each decimal digit is encoded by 4 bits.

11. Alpha numeric characters are also expressed in terms of binary codes.The widely used such codes are 1.American Standard Code for Information Interchange(ASCII).Here each character is represented as a 7-bit code.2. Extended Binary Coded Decimal Interchange Code(EBCDIC).In this case to denote a character,8-bits are used.

12. Input Unit Computers accept coded information through input units which reads the data. The widely used input device is the key board of a video terminal. The keyboard is wired in such a way that whenever a key is pressed, the corresponding letter or digit is automatically translated in to its corresponding code and sent directly to either the memory or the processor. Other kinds of input devices like joysticks, track balls etc are also available which are used as graphic input devices in conjunction with video displays.

13. Memory Unit The function of the memory unit is to store the data. There are two classes of memory devices. Primary storage or main memory and Secondary storage or auxiliary memory

14. Main memory The main memory is a fast memory that operates at electronic speeds. It contains a large number of semiconductor storage cells capable of storing one bit of information. These cells are processed in group of fixed size called words. This memory is organized in such a way that the contents of one word containing n bits can be stored or retrieved in one basic operation

15. For easy access to any word in the main memory ,an address is associated with each word location.Addresses are numbers that identify successive locations.When main memory is accessed ,one word of data is read from the memory or written in to it.

16. The number of bits in each word is often referred to as word length of the computer. Typical word length ranges from 16 to 64 bits. Small machine may have only a few million words whereas medium and large machines have tens of millions of words. Data in a computer is manipulated in units of words.

17. Program must reside in the main memory during execution. It is essential to be able to access any word location within the main memory as quick as possible. Memories in which any location can be reached in a short time after specifying its address are called Random-Access-Memories (RAM).The time required is called memory access time. For most modern computers, it ranges from 10 to 100 nanoseconds

18. Secondary storage devices Secondary storage devices are essential, because primary storage is expensive. Further in this type of storage, large amounts of data can be stored. A wide selection of secondary storage is available including magnetic disks and tapes.

19. ARITHMETIC LOGIC UNIT All arithmetical and logical operations such as multiplicatin,division,addition and comparison of numbers are performed by the ALU.This is done by bringing all the required operands in to the ALU. It is to be noted that, not all operands used in the computation reside in the main memory.Temperory storage devices known as registers are used for the purpose. Each register can store on word of data.

20. OUTPUT UNIT The output unit is the counter part of Input unit. Its function is to send processed results to the outside world. The most familiar example is a high speed printer. Some units such as video units and graphic displays provide both an output function and an input function. This dual role is the reason for using the single name I/O unit.

21. CONTROL UNIT The control unit is effectively the nerve center that sends control signals to other units and senses their states. The memory, ALU ,and input and output units store and process information and perform input/output operations. The operations are coordinated by the control unit by software instructions. The proper timing signals(signals that determine when a given action is to take place) that govern the transfers are generated by the control unit.

22. SUMMARY OF OPERATIONS PERFORMED. The operation 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 or in the Registers. The information stored is fetched ,under program control in to ALU where it is processed. Processed information leave the computer through an output unit All activities are directed by the control unit.

23. BASIC OPERATIONAL CONCEPTS The activity of a computer is governed by instructions and a set of instructions are known as a program is stored in the main memory. Data to be used as operands are also stored in memory. As an example , a typical instruction may be of the form

24. Add LOCA, R₀This instruction adds the operand at memory location A to the operand in a register R₀ and places the sum in to the register R₀. In this case the original contents of the location LOCA are preserved ,where as those of R₀ are over written . This instruction requires several steps.

25. The instruction is fetched from the main memory in to the processor.2. The operand at LOCA is fetched and added to the contents of R₀ and3. The resulting sum is stored in the register R₀ In many modern computers, the single operation is performed by following two separate instructions.

26. Load LOCA,R₁ Add R₁,R₀The first instruction transfers the contents of the main memory location LOCA in to the register R₁ and the second instruction adds the contents of registers R₁ and R₀ and places the sum in to R₀. Here the former contents of registers R₁ and R₀ are destroyed ,where as the original contents of the memory location LOCA is preserved.

27. CONNECTIVITY B/W MAIN MEMORY AND PROCESSOR

28. In addition to the ALU and the control circuitry , the processor contains a number of registers used for a temporary storage of data.IR is the instruction register which holds the instruction that is currently being executed. Its output is available to the control circuits. The program counter (PC) register keep tracks of the execution of a program. It contains the memory address of the instruction currently being executed.

29. During the execution of an instruction, the contents of the PC are updated to the address of the next instruction which is to be executed. Actually, PC points to the next instruction that is to be fetched from memory . Besides IR and PC the figure shows n general purpose registers. R₀ through Rn-1.

30. Two more registers are used to communicate with the main memory. They are Memory Address Register(MAR) and the Memory Data Register (MDR) . The MAR holds the address of the location to or from which data are to be transferred. The MDR contains the data to be written in to or readout of the address location.

31. OPERATING STEPS.-1 Program reside in the main memory are get through the input unit PC is set to point to the first instruction of the program The contents of the PC are transferred to the MAR and a read control signal is sent to the memory The first instruction of the program ie the address word is read out of the memory and loaded in to the MDR. The contents of the MDR are transferred to the IR. Here the instruction is ready to be decoded and executed.

32. OPERATING STEPS-2 If the instruction involves an operation to be performed by the ALU it is necessary to obtain the required operands. In such cases the following steps are used to get the operands. The operand residing in the memory is fetched by sending its address to the MAR and initiating a Read cycle. When the operand has been read from the memory in to MDR,it may be transferred from MDR to ALU.

33. OPERATING STEPS-3 The ALU can perform the desired operation if one or more operands are fetched in the same way. If the result of this operation is required to stored in the memory ,it is sent to MDR. The address of the location where the result is to be stored is sent to the MAR and a write cycle is initiated. The contents of the PC are then incremented so that ,it points to the next instruction which is to be executed.

34. BUS STRUCTERS The functional parts of a computer must be connected in some organized way. The simplest and most common way in doing so is discussed below. When a word of data is transferred b/w units ,all its bits are transferred in parallel. This means that the bits are transferred simultaneously over different wires. Many wires are required to establish the necessary connections. A group of wires that connect several devices is called a bus

35. The simplest way to interconnect the functional units is to use a single bus. I t is shown below. Here all units are connected to this bus. The main advantage of the single –bus structure is is its low cost and its flexibility for attaching peripheral devices. Multiple buses gives more parallelism for better performance at an increased cost.

36. SOFT WARE A system software is a collection of programs. They are executed as needed to perform the following function. Receiving and interpreting user commands Entering and editing application programs and storing them as files in secondary storage. Running standard application programs(spread sheets or games) Controlling I/O units to receive input information and produce output results. Translating source program in to object form consisting of machine instructions. Linking and running user application programs with existing standard Library routine.

37. Application programs are usually written in high-level programming language such as C, Pascal or Fortran. The programmer is not expected to know the details of the machine program instructions. A system software program called a Compiler translates the high level language program in to a suitable machine language program.Another important system program is the Text entry and Editing.

38. Another system software component is the Operating System (OS).This is a large program or a collection of routines that acts as an interface between the user of a computer and the computer hardware. The computer hardware requires a driver in order to make the hardware function correctly. Simply, this driver is called software.The software operates and controls the hardware units .

39. The Memory System A computer’s memory can be viewed as a list of cells of in to which numbers can be placed or read. Each cell has a numbered address and can store a single number. Letters ,numbers or even computer instructions can be placed in to memory with equal ease. A computer can store any kind of information in memory if it can be represented numerically. Modern computers have billions or even trillions of bytes of memory.

40. Basic Concepts.-1 Memory is like the pages of a note book with space for a fixed number of binary numbers on each line. These pages are in general made of semiconductor material. Typically each line is an 8-bit register that can store 8 binary bits and several of these registers are arranged in a sequence .These registers are grouped together in powers of two. A group of 1024 eight bit registers on a semi conductor chip is known as 1K byte of memory.

41. BASIC CONCEPTS-2 The maximum size of the memory is determined by the addressing schemes. A sixteen bit computer that generates 16 bit addresses is capable of addressing 2¹⁶ =64K memory locations .Similarly ,machine whose instructions generate 32-bit addresses can utilize a memory that contains up to 2³²=4 G (giga) memory location ,whereas machine with 40 bit addresses can access up to 2⁴⁰=1T(tera) locations.

42. MEMORY SPEED A usual measure of the speed of memory is the time that elapses between the initiation of an operation and the completion of that operation. It is the time between the read and the MFC signal (Memory Function Completed).This is referred to as the memory access time. Another important measure is the memory cycle time. Memory cycle time is the minimum time delay between the initiation of two successive memory operations. It is observed that the cycle time is slightly longer than the access time.

43. SEMI CONDUCTOR RAM MEMORIES Semiconductor memories are available in a wide range of speeds. Their cycle time changes from a few hundred nano sec to less than 10 nano seconds. The introduction of VLSI(very large scale integration) reduces the cost of semiconductor memories.

44. STATIC MEMORIES memories that consists of circuits that are capable of retaining their state as long as power is applied are called Static memories(SRAM).Here two inverters are cross connected to form a latch. The latch is connected to two-bit lines by two transistors T₁ &T₂.These transistors act as switches that can be opened or closed under control of a word line.

45. Memory cells are organized in the form of an array and each row of cells constitute a memory word. All cells of a row are connected to a common line which is referred to as the word line

46. When word line is at ground level, the transistors are turned off and the latch retains its state. When the logic value at point X=1 and at point Y=0,let the cell is in state 1.This state is maintained as long as the signal on the word line is at ground level.

47. Read Operation In order to read the state of the SRAM, the word line is activated to close switches T₁ &T₂. If the cell is in state 1,the signal on bit line b is high and on bit line b’ low.bis low and b’ is high if the cell is in state 0..Thus b and b’ are compliments of each other.

48. Write Operation The state of the cell is set by placing the appropriate value on bit line b and its complement on b’. Then activate the word line. This forces the cell in to the corresponding state. The required signals on bit lines are generated by the write circuit.

49. DYNAMIC MEMORIES It is found that, Rams are fast and their cells require several transistors and hence they are expensive. Less expensive RAMs can be implemented if simpler cells are used. They do not retain their state indefinitely .hence they are called dynamic RAMS(DRAMS)

50. In dynamic memory, information is stored in the form of a charge on a capacitor. A DRAM is capable of storing information for only a few milliseconds. Since each cell is usually required to store information for a much longer time, its contents must be periodically refreshed by restoring the capacitor charge to its full value. An example of a dynamic memory cell that consists of a capacitor C and a transistor T is shown in the figure. In order to store information in this cell, transistor T is turned on and an appropriate voltage is applied to the bit-line . This causes a known amount of charge to be stored on the capacitor. After the transistor is turned off, the capacitor begins to discharge. The read operation discharges the capacitor in the cell that is being accessed. In order to retain the information stored in the cell DRAM includes special circuitry that writes back the value that has been read. A memory cell is therefore refreshed every time its contents are read.