1. INTRODUCTION TO COMPUTING CHAPTER NO. 05

2. Computer System Organization The Von Neumann Architecture Memory and Cache Input/Output and Storage devices The ALU and Control Unit The Number Systems (Binary, Decimal & Hexadecimal) Boolean Logic and Gates

3. The Von Neumann Architecture There are a huge number of computer systems in the market manufactured by different companies. They not only differ in size and cost but also in speed, memory capacity, input/output capabilities, software etc. In spite of all these differences virtually every computer in use today is based on a single design called Von Neumann Architecture. Princeton Institute for Advanced Studies Named after the brilliant mathematician John Von Neumann, who first proposed in 946 at Princeton Institute for Advanced Studies.

4. The Von Neumann Architecture Characteristics of Von Neumann Architecture A main memory, which stores both data and instructions. A main memory, which stores both data and instructions. An arithmetic and logic unit (ALU) capable of operating on binary data. An arithmetic and logic unit (ALU) capable of operating on binary data. A control unit, which interprets the instructions in memory and causes them to be executed. A control unit, which interprets the instructions in memory and causes them to be executed. Input and output (I/O) equipment operated by the control unit. Input and output (I/O) equipment operated by the control unit.

5. The Von Neumann Architecture The stored program concept, in which the instructions to be executed by the computer are represented as binary values and stored in memory. The stored program concept, in which the instructions to be executed by the computer are represented as binary values and stored in memory. The sequential execution of instructions, one instruction at a time is fetched and executed. The sequential execution of instructions, one instruction at a time is fetched and executed. With rare exceptions all of the today’s computers have this same general structure and functions and are thus referred to von Neumann machines. With rare exceptions all of the today’s computers have this same general structure and functions and are thus referred to von Neumann machines.

6. Main Memory (M) Arithmetic and Logic unit (CA) Program Control Unit (CC) Central Processing Unit (CPU) I/O Equipment (I,O) The Von Neumann Architecture

7. Memory and Cache Memory is the functional unit of a computer that stores and retrieves the instructions and data being executed. Memory is the functional unit of a computer that stores and retrieves the instructions and data being executed. Computer memory uses an access technique called Random Access, so the memory is called Random Access Memory (RAM). Its Characteristics are: Computer memory uses an access technique called Random Access, so the memory is called Random Access Memory (RAM). Its Characteristics are: Memory is divided into fixed size units called cells, each cell has a unique address, these are unsigned integers 0, 1, 2, …….. Memory is divided into fixed size units called cells, each cell has a unique address, these are unsigned integers 0, 1, 2, …….. All accesses to memory are to a specified address, and we always fetch and store a complete cell, all the bits in that cell. All accesses to memory are to a specified address, and we always fetch and store a complete cell, all the bits in that cell.

8. Memory and Cache Earlier there was no standardized cell size, they were 6, 8, 12, etc but today there is a standard cell size of 8 bits which is called a Byte. Earlier there was no standardized cell size, they were 6, 8, 12, etc but today there is a standard cell size of 8 bits which is called a Byte. With a cell size of 8-bits, the largest unsigned integer value that can be stored in a cell is 11111111(255) that is by using formula as 2 N – 1 where N bits represents the address of a cell. With a cell size of 8-bits, the largest unsigned integer value that can be stored in a cell is 11111111(255) that is by using formula as 2 N – 1 where N bits represents the address of a cell. Operations of Memory Fetch (address) - Fetch a copy of the memory cell with the specified address and return those contents as the result of the operation. Fetch (address) - Fetch a copy of the memory cell with the specified address and return those contents as the result of the operation.

9. Memory and Cache The original contents of the memory cell that was accessed are unchanged, it is called as Nondestructive Fetch. The original contents of the memory cell that was accessed are unchanged, it is called as Nondestructive Fetch. Store (address, value) - Store the specified value into the memory cell specified by address. Store (address, value) - Store the specified value into the memory cell specified by address. The previous contents of the cell are lost, called as Destructive Store. The previous contents of the cell are lost, called as Destructive Store. The time required to fetch or a store operation is called the Memory Access time that is same for all 2 N addresses. The time required to fetch or a store operation is called the Memory Access time that is same for all 2 N addresses.

10. Memory and Cache The memory unit contains two special registers whose purpose is to hold the address and value. The memory unit contains two special registers whose purpose is to hold the address and value. Memory Address Register (MAR) - Holds the address of the cell to be fetched or stored. Memory Address Register (MAR) - Holds the address of the cell to be fetched or stored. It must beat least N bits wide, where 2 N is the address space of the computer. It must beat least N bits wide, where 2 N is the address space of the computer. Memory Data Register (MDR) - contains the data value being fetched or stored. Memory Data Register (MDR) - contains the data value being fetched or stored. The MDR should be W bits wide, where W is the cell size, on most computers the cell size is 8 bits, and most data values occupy multiple cells. The MDR should be W bits wide, where W is the cell size, on most computers the cell size is 8 bits, and most data values occupy multiple cells.

11. Memory and Cache Cache Memory In Von Neumann computer there was only a single type of memory that was RAM. In Von Neumann computer there was only a single type of memory that was RAM. With the passage of time, computer designers felt the requirement of another high-speed memory unit to store some of the data from “regular RAM memory”. With the passage of time, computer designers felt the requirement of another high-speed memory unit to store some of the data from “regular RAM memory”. That special memory unit is called as “Cache Memory”, which is typically 5 - 10 times faster than RAM but smaller on the order of KBs of storage rather than MBs. That special memory unit is called as “Cache Memory”, which is typically 5 - 10 times faster than RAM but smaller on the order of KBs of storage rather than MBs. It is used to store that data which is most recently and most frequently used by the processor. It is used to store that data which is most recently and most frequently used by the processor.

12. Memory and Cache When the computer needs a piece of information it carries out the following three steps: When the computer needs a piece of information it carries out the following three steps: Look first in the cache memory to see whether the information needed is there, if it is, then the computer will be able to access it at a higher speed. Look first in the cache memory to see whether the information needed is there, if it is, then the computer will be able to access it at a higher speed. If it is not there, then access the desired information from RAM at the slower speed, using the fetch operation. If it is not there, then access the desired information from RAM at the slower speed, using the fetch operation. Copy the data just fetched into the cache, if cache is currently full, then discard one of the older data that has not recently been accessed. Copy the data just fetched into the cache, if cache is currently full, then discard one of the older data that has not recently been accessed.

13. This computer had a total of 21 instructions, that are grouped as: This computer had a total of 21 instructions, that are grouped as: Data Transfer(07): Move data between memory and ALU registers or between two ALU registers. Data Transfer(07): Move data between memory and ALU registers or between two ALU registers. Unconditional Branch(02): Normally the control unit executes instructions in sequence queue from memory. This sequence can be changed by a branch instruction. Unconditional Branch(02): Normally the control unit executes instructions in sequence queue from memory. This sequence can be changed by a branch instruction. Conditional Branch(02): The branch can be made dependent on a condition, thus allowing decision points. Conditional Branch(02): The branch can be made dependent on a condition, thus allowing decision points. Memory and Cache

14. Arithmetic(08): Operations performed by the ALU. Arithmetic(08): Operations performed by the ALU. Address modify(02): Permits addresses to be computed in ALU and then inserted into the instructions stored in memory. Address modify(02): Permits addresses to be computed in ALU and then inserted into the instructions stored in memory. Memory and Cache

15. The I/O units are the devices that allow a computer system to communicate and interact with the outside world as well as store information. The I/O units are the devices that allow a computer system to communicate and interact with the outside world as well as store information. There are dozens of different I/O and storage devices manufactured by different companies. There are dozens of different I/O and storage devices manufactured by different companies. Input/Output devices come in two types: Input/Output devices come in two types: Represent Human-readable information for human consumption (keyboards, screens, printers etc). Represent Human-readable information for human consumption (keyboards, screens, printers etc). Represent machine-readable information for access by the computer system (floppy disks, hard disks, CD ROMs, tapes etc). Represent machine-readable information for access by the computer system (floppy disks, hard disks, CD ROMs, tapes etc). Input / Output and Storage Devices

16. Storage devices come in two forms: Direct access storage devices (DASDs) and Sequential access storage devices (SASDs). Storage devices come in two forms: Direct access storage devices (DASDs) and Sequential access storage devices (SASDs). A DASD stores information in units called sectors, which contains address and data block. A DASD stores information in units called sectors, which contains address and data block. A fixed number of sectors are placed on a concentric circle on the surface of the disk, called a track. A fixed number of sectors are placed on a concentric circle on the surface of the disk, called a track. The surface of the disk contains many tracks, and there is a single read/write head moved in or out to position itself over any track. The surface of the disk contains many tracks, and there is a single read/write head moved in or out to position itself over any track. Entire disk is rotating at high speed under the read/write head. Entire disk is rotating at high speed under the read/write head. Input / Output and Storage Devices

17. The access time to any individual sector is made up of three components: Seek time, Latency and Transfer time. The access time to any individual sector is made up of three components: Seek time, Latency and Transfer time. Seek Time – the time needed to position the read/write head over the correct track. Seek Time – the time needed to position the read/write head over the correct track. Latency – time for the beginning of the desired sector to rotate under the read/write head. Latency – time for the beginning of the desired sector to rotate under the read/write head. Transfer time – the for the entire sector to pass under the read/write head and have its contents read into or written from memory. Transfer time – the for the entire sector to pass under the read/write head and have its contents read into or written from memory. These values depend on the specific sector being accessed and the current position of the read/write head. These values depend on the specific sector being accessed and the current position of the read/write head. Input / Output and Storage Devices

18. In SASDs, sequential access technique is used in which all the data is searched sequentially. In SASDs, sequential access technique is used in which all the data is searched sequentially. A SASD is just like an audio cassette tape, to locate a specific song, runt he tape for a while and then stop and listen. A SASD is just like an audio cassette tape, to locate a specific song, runt he tape for a while and then stop and listen. This process is repeated until the desired song or come to the end. This process is repeated until the desired song or come to the end. These devices can be useful in specific situations such as sequentially copying the entire contents of memory or of a disk drive. These devices can be useful in specific situations such as sequentially copying the entire contents of memory or of a disk drive. Input / Output and Storage Devices

19. ALU is the subsystem that perform such mathematical and logical operations such as addition, subtraction, comparison etc. ALU is the subsystem that perform such mathematical and logical operations such as addition, subtraction, comparison etc. The ALU and Control Unit have become fully integrated into a single component called the processor. The ALU and Control Unit have become fully integrated into a single component called the processor. The ALU is made up of three parts: registers, interconnections between components, and the ALU circuitry. The ALU is made up of three parts: registers, interconnections between components, and the ALU circuitry. Register – is a storage cell that holds the operands and the result of an arithmetic operation. Register – is a storage cell that holds the operands and the result of an arithmetic operation. The Arithmetic / Logic Unit

20. Registers do not have a numeric memory address but are accessed by a special register designator such as A, X, or R0. Registers do not have a numeric memory address but are accessed by a special register designator such as A, X, or R0. As there are few registers (one or two dozen), they can be accessed more quickly, therefore they are 2 to 10 times faster than memory cells. As there are few registers (one or two dozen), they can be accessed more quickly, therefore they are 2 to 10 times faster than memory cells. They are not used for general-purpose storage but for specific purposes such as holding the operands for an upcoming arithmetic computation. They are not used for general-purpose storage but for specific purposes such as holding the operands for an upcoming arithmetic computation. The interconnection between components provides a data path for electrical signals termed as a Bus. The interconnection between components provides a data path for electrical signals termed as a Bus. The Arithmetic / Logic Unit

21. The task of Control Unit is to: The task of Control Unit is to: Fetch – from memory the next instruction to be executed. Fetch – from memory the next instruction to be executed. Decode (translate) – determine what is to be done. Decode (translate) – determine what is to be done. Execute – by issuing the appropriate command to the ALU, memory, and I/O controllers. Execute – by issuing the appropriate command to the ALU, memory, and I/O controllers. These three steps are repeated over and over until the last instruction in the program reaches that is HALT, STOP, QUIT. These three steps are repeated over and over until the last instruction in the program reaches that is HALT, STOP, QUIT. Control Unit performs these tasks with the help of two special registers: Program Counter and Instruction Register. Control Unit performs these tasks with the help of two special registers: Program Counter and Instruction Register. The Control Unit

22. Program Counter (PC) – it holds the address of the next instruction to be executed. Program Counter (PC) – it holds the address of the next instruction to be executed. PC gets incremented by 1 after each fetch, because the normal mode of execution is sequential. PC gets incremented by 1 after each fetch, because the normal mode of execution is sequential. Instruction Register (IR) – holds a copy of the instruction fetched from memory. Instruction Register (IR) – holds a copy of the instruction fetched from memory. IR holds both the Op code portion of the instruction and the address. IR holds both the Op code portion of the instruction and the address. The Control Unit

23. The construction of computer circuits is based on the branch of mathematics and symbolic logic called Boolean Logic which deals with two logical values True (1) and False (0). The construction of computer circuits is based on the branch of mathematics and symbolic logic called Boolean Logic which deals with two logical values True (1) and False (0). Digital systems are constructed by using logic gates, these gates are the AND, OR, NOT, NAND, NOR, EXOR and EXNOR gates. Digital systems are constructed by using logic gates, these gates are the AND, OR, NOT, NAND, NOR, EXOR and EXNOR gates. Boolean Logic and Gates

24. AND gate - an electronic circuit that gives a high output (1) only if all its inputs are high. A dot (.) is used to show the AND operation i.e. A.B, bear in mind that this dot is sometimes omitted i.e. AB Boolean Logic and Gates

25. OR gate - an electronic circuit that gives a high output (1) if one or more of its inputs are high. A plus (+) is used to show the OR operation. Boolean Logic and Gates

26. NOT gate - an electronic circuit that produces an inverted version of the input at its output, it is also known as an inverter. If the input variable is A, the inverted output is known as NOT A, also shown as A', or A with a bar over the top. Boolean Logic and Gates

27. NAND gate - This is a NOT-AND gate which is equal to an AND gate followed by a NOT gate. The outputs of all NAND gates are high if any of the inputs are low. The symbol is an AND gate with a small circle on the output. The small circle represents inversion.. Boolean Logic and Gates

28. NOR gate - This is a NOT-OR gate which is equal to an OR gate followed by a NOT gate. The outputs of all NOR gates are low if any of the inputs are high. The symbol is an OR gate with a small circle on the output. The small circle represents inversion. Boolean Logic and Gates

29. Exclusive-OR gate (EXOR)- a circuit which will give a high output if either, but not both, of its two inputs are high. An encircled plus sign is used to show the EOR operation. Boolean Logic and Gates

30. Exclusive-NOR gate (EXNOR)- a circuit that does the opposite to the EOR gate, it will give a low output if either, but not both, of its two inputs are high. The symbol is an EXOR gate with a small circle on the output. The small circle represents inversion. Boolean Logic and Gates