Primary Memory

---

Memory is required in computers to store data and instructions. Memory is physically organized as a large number of cells that are capable of storing one bit each. Logically they are organized as groups of bits called words that are assigned an address. Data and instructions are accessed through these memory address. The speed with which these memory addresses can be accessed determines the cost of the memory. Faster the memory speed, higher the price.

Computer memory can be said to be organized in a hierarchical way where memory with the fastest access speeds and highest costs lies at the top whereas those with lowest speeds and hence lowest costs lie at the bottom. Based on this criteria memory is of two types – primary and secondary. Here we will look at primary memory in detail.

The main features of primary memory, which distinguish it from secondary memory are −

• It is accessed directly by the processor
• It is the fastest memory available
• Each word is stored as well as
• It is volatile, i.e. its contents are lost once power is switched off

As primary memory is expensive, technologies are developed to optimize its use. These are broad types of primary memory available.

RAM

RAM stands for Random Access Memory. The processor accesses all memory addresses directly, irrespective of word length, making storage and retrieval fast. RAM is the fastest memory available and hence most expensive. These two factors imply that RAM is available in very small quantities of up to 1GB. RAM is volatile but my be of any of these two types

DRAM (Dynamic RAM)

Each memory cell in a DRAM is made of one transistor and one capacitor, which store one bit of data. However, this cell starts losing its charge and hence data stored in less than thousandth of a second. So it needs to be refreshed thousand times a second, which takes up processor time. However, due to small size of each cell, one DRAM can have large number of cells. Primary memory of most of the personal computers is made of DRAM.

SRAM (SRAM)

Each cell in SRAM is made of a flip flop that stores one bit. It retains its bit till the power supply is on and doesn’t need to be refreshed like DRAM. It also has shorter read-write cycles as compared to DRAM. SRAM is used in specialized applications.

ROM

ROM stands for Read Only Memory. As the name suggests, ROM can only be read by the processor. New data cannot be written into ROM. Data to be stored into ROM is written during the manufacturing phase itself. They contain data that does not need to be altered, like booting sequence of a computer or algorithmic tables for mathematical applications. ROM is slower and hence cheaper than RAM. It retains its data even when power is switched off, i.e. it is non-volatile. ROM cannot be altered the way RAM can be but technologies are available to program these types of ROMs −

PROM (Programmable ROM)

PROM can be programmed using a special hardware device called PROM programmer or PROM burner.

EPROM (Erasable Programmable ROM)

EPROM can be erased and then programmed using special electrical signals or UV rays. EPROMs that can be erased using UV rays are called UVEPROM and those that can be erased using electrical signals are called EEPROM. However, handling electric signals is easier and safer than UV rays.

Cache Memory

Small piece of high speed volatile memory available to the processor for fast processing is called cache memory. Cache may be a reserved portion of main memory, another chip on CPU or an independent high speed storage device. Cache memory is made of fast speed SRAMs. The process of keeping some data and instructions in cache memory for faster access is called caching. Caching is done when a set of data or instructions is accesses again and again.

Whenever the processor needs any piece of data or instructions, it checks the cache first. If it is unavailable there, then the main memory and finally secondary memory is accessed. As cache has very high speed, time spent in accessing it every time is negligible as compared to time saved if data indeed is in the cache. Finding data or instruction in cache is called cache hit.

Evaluation of Microprocessor

---

The first microprocessor introduced in 1971 was a 4-bit microprocessor with 4m5KB memory and had a set of 45 instructions. In the past 5 decades microprocessor speed has doubled every two years, as predicted by Gordon Moore, Intel co-founder. Current microprocessors can access 64 GB memory. Depending on width of data microprocessors can process, they are of these categories−

• 8-bit
• 16-bit
• 32-bit
• 64-bit

Size of instruction set is another important consideration while categorizing microprocessors. Initially, microprocessors had very small instructions sets because complex hardware was expensive as well as difficult to build.

As technology developed to overcome these issues, more and more complex instructions were added to increase functionality of the microprocessor. However, soon it was realized that having large instruction sets was counterproductive as many instructions that were rarely used sat idle on precious memory space. So the old school of thought that supported smaller instruction sets gained popularity.

Let us learn more about the two types of microprocessors based on their instruction set.

RISC

RISC stands for Reduced Instruction Set Computers. It has a small set of highly optimized instructions. Complex instruction are also implemented using simpler instructions, reducing the size of instruction set. The designing philosophy for RISC incorporates these salient points −

• Number of instructions should be minimum.
• Instructions should be of same length.
• Simple addressing modes should be used
• Reduce memory references to retrieve operands by adding registers

Some of the techniques used by RISC architecture include −

• Pipelining− A sequence of instructions is fetched even if it means overlapping of instructions in fetching and execution.

• Single cycle execution − Most of RISC instructions take one CPU cycle to execute.

Examples of RISC processors are Intel P6, Pentium4, AMD K6 and K7, etc.

CISC

CISC stands for Complex Instruction Set Computers. It supports hundreds of instructions. Computers supporting CISC can accomplish wide variety of tasks, making them ideal for personal computers. These are some characteristics of CISC architecture −

• Larger set of instructions
• Instructions are of variable length
• Complex addressing modes
• Instructions take more than one clock cycle
• Work well with simpler compilers

Examples of CISC processors are Intel 386 & 486, Pentium, Pentium II and III, Motorola 68000, etc.

EPIC

EPIC stands for Explicitly Parallel Instruction Computing. It is a computer architecture that is a cross between RISC and CISC, trying to provide the best of both. Its important features include −

• Parallel instructions rather than fixed width
• Mechanism to communication compiler’s execution plan to hardware
• Programs must have sequential semantics

Some EPIC processors are Intel IA-64, Itanium, etc.

Microprocessor Concepts

---

Microprocessor is the brain of computer, which does all the work. It is a computer processor that incorporates all the functions of CPU (Central Processing Unit) on a single IC (Integrated Circuit) or at the most a few ICs. Microprocessors were first introduced in early 1970s. 4004 was the first general purpose microprocessor used by Intel in building personal computers. Arrival of low cost general purpose microprocessors has been instrumental in development of modern society the way it has.

We will study the characteristics and components of a microprocessor in detail.

Microprocessors Characteristics

Microprocessors are multipurpose devices that can be designed for generic or specialized functions. The microprocessors of laptops and smartphones are general purpose whereas ones designed for graphical processing or machine vision are specialized ones. There are some characteristics that are common to all microprocessors.

These are the most important defining characteristics of a microprocessor −

• Clock speed
• Instruction set
• Word size

Clock Speed

Every microprocessor has an internal clock that regulates the speed at which it executes instructions and also synchronizes it with other components. The speed at which the microprocessor executes instructions is called clock speed. Clock speeds are measured in MHz or GHz where 1 MHz means 1 million cycles per second whereas 1 GHz equals to 1 billion cycles per second. Here cycle refers to single electric signal cycle.

Currently microprocessors have clock speed in the range of 3 GHz, which is maximum that current technology can attain. Speeds more than this generate enough heat to damage the chip itself. To overcome this, manufacturers are using multiple processors working in parallel on a chip.

Word Size

Number of bits that can be processed by a processor in a single instruction is called its word size. Word size determines the amount of RAM that can be accessed at one go and total number of pins on the microprocessor. Total number of input and output pins in turn determines the architecture of the microprocessor.

First commercial microprocessor Intel 4004 was a 4-bit processor. It had 4 input pins and 4 output pins. Number of output pins is always equal to the number of input pins. Currently most microprocessors use 32-bit or 64-bit architecture.

Instruction Set

A command given to a digital machine to perform an operation on a piece of data is called an instruction. Basic set of machine level instructions that a microprocessor is designed to execute is called its instruction set. These instructions do carry out these types of operations −

• Data transfer
• Arithmetic operations
• Logical operations
• Control flow
• Input/output and machine control

Microprocessor Components

Compared to the first microprocessors, today’s processors are very small but still they have these basic parts right from the first model −

• CPU
• Bus
• Memory

CPU

CPU is fabricated as a very large scale integrated circuit (VLSI) and has these parts −

• Instruction register − It holds the instruction to be executed.

• Decoder − It decodes (converts to machine level language) the instruction and sends to the ALU (Arithmetic Logic Unit).

• ALU − It has necessary circuits to perform arithmetic, logical, memory, register and program sequencing operations.

• Register − It holds intermediate results obtained during program processing. Registers are used for holding such results rather than RAM because accessing registers is almost 10 times faster than accessing RAM.

Bus

Connection lines used to connect the internal parts of the microprocessor chip is called bus. There are three types of buses in a microprocessor −

• Data Bus − Lines that carry data to and from memory are called data bus. It is a bidirectional bus with width equal to word length of the microprocessor.

• Address Bus − It is a unidirectional responsible for carrying address of a memory location or I/O port from CPU to memory or I/O port.

• Control Bus − Lines that carry control signals like clock signals, interrupt signal or ready signal are called control bus. They are bidirectional. Signal that denotes that a device is ready for processing is called ready signal. Signal that indicates to a device to interrupt its process is called an interrupt signal.

Memory

Microprocessor has two types of memory

• RAM − Random Access Memory is volatile memory that gets erased when power is switched off. All data and instructions are stored in RAM.

• ROM − Read Only Memory is non-volatile memory whose data remains intact even after power is switched off. Microprocessor can read from it any time it wants but cannot write to it. It is preprogrammed with most essential data like booting sequence by the manufacturer.

Number System Conversion

---

As you know decimal, binary, octal and hexadecimal number systems are positional value number systems. To convert binary, octal and hexadecimal to decimal number, we just need to add the product of each digit with its positional value. Here we are going to learn other conversion among these number systems.

Decimal to Binary

Decimal numbers can be converted to binary by repeated division of the number by 2 while recording the remainder. Let’s take an example to see how this happens.

The remainders are to be read from bottom to top to obtain the binary equivalent.

43₁₀ = 101011₂

Decimal to Octal

Decimal numbers can be converted to octal by repeated division of the number by 8 while recording the remainder. Let’s take an example to see how this happens.

Reading the remainders from bottom to top,

473₁₀ = 731₈

Decimal to Hexadecimal

Decimal numbers can be converted to octal by repeated division of the number by 16 while recording the remainder. Let’s take an example to see how this happens.

Reading the remainders from bottom to top we get,

423₁₀ = 1A7₁₆

Binary to Octal and Vice Versa

To convert a binary number to octal number, these steps are followed −

• Starting from the least significant bit, make groups of three bits.

• If there are one or two bits less in making the groups, 0s can be added after the most significant bit

• Convert each group into its equivalent octal number

Let’s take an example to understand this.

1011001010₁₂ = 2625₈

To convert an octal number to binary, each octal digit is converted to its 3-bit binary equivalent according to this table.

Octal Digit	0	1	2	3	4	5	6	7
Binary Equivalent	000	001	010	011	100	101	110	111

 

54673₈ = 101100110111011₂

Binary to Hexadecimal

To convert a binary number to hexadecimal number, these steps are followed −

• Starting from the least significant bit, make groups of four bits.

• If there are one or two bits less in making the groups, 0s can be added after the most significant bit.

• Convert each group into its equivalent octal number.

Let’s take an example to understand this.

10110110101₂ = DB5₁₆

To convert an octal number to binary, each octal digit is converted to its 3-bit binary equivalent.

Number System

---

The technique to represent and work with numbers is called number system. Decimal number system is the most common number system. Other popular number systems include binary number system, octal number system, hexadecimal number system, etc.

Decimal Number System

Decimal number system is a base 10 number system having 10 digits from 0 to 9. This means that any numerical quantity can be represented using these 10 digits. Decimal number system is also a positional value system. This means that the value of digits will depend on its position. Let us take an example to understand this.

Say we have three numbers – 734, 971 and 207. The value of 7 in all three numbers is different−

• In 734, value of 7 is 7 hundreds or 700 or 7 × 100 or 7 × 10²
• In 971, value of 7 is 7 tens or 70 or 7 × 10 or 7 × 10¹
• In 207, value 0f 7 is 7 units or 7 or 7 × 1 or 7 × 10⁰

The weightage of each position can be represented as follows −

In digital systems, instructions are given through electric signals; variation is done by varying the voltage of the signal. Having 10 different voltages to implement decimal number system in digital equipment is difficult. So, many number systems that are easier to implement digitally have been developed. Let’s look at them in detail.

Binary Number System

The easiest way to vary instructions through electric signals is two-state system – on and off. On is represented as 1 and off as 0, though 0 is not actually no signal but signal at a lower voltage. The number system having just these two digits – 0 and 1 – is called binary number system.

Each binary digit is also called a bit. Binary number system is also positional value system, where each digit has a value expressed in powers of 2, as displayed here.

In any binary number, the rightmost digit is called least significant bit (LSB) and leftmost digit is called most significant bit (MSB).

And decimal equivalent of this number is sum of product of each digit with its positional value.

11010₂ = 1×2⁴ + 1×2³ + 0×2² + 1×2¹ + 0×2⁰

= 16 + 8 + 0 + 2 + 0

= 26₁₀

Computer memory is measured in terms of how many bits it can store. Here is a chart for memory capacity conversion.

• 1 byte (B) = 8 bits
• 1 Kilobytes (KB) = 1024 bytes
• 1 Megabyte (MB) = 1024 KB
• 1 Gigabyte (GB) = 1024 MB
• 1 Terabyte (TB) = 1024 GB
• 1 Exabyte (EB) = 1024 PB
• 1 Zettabyte = 1024 EB
• 1 Yottabyte (YB) = 1024 ZB

Octal Number System

Octal number system has eight digits – 0, 1, 2, 3, 4, 5, 6 and 7. Octal number system is also a positional value system with where each digit has its value expressed in powers of 8, as shown here −

Decimal equivalent of any octal number is sum of product of each digit with its positional value.

726₈ = 7×8² + 2×8¹ + 6×8⁰

= 448 + 16 + 6

= 470₁₀

Hexadecimal Number System

Octal number system has 16 symbols – 0 to 9 and A to F where A is equal to 10, B is equal to 11 and so on till F. Hexadecimal number system is also a positional value system with where each digit has its value expressed in powers of 16, as shown here −

Decimal equivalent of any hexadecimal number is sum of product of each digit with its positional value.

27FB₁₆ = 2×16³ + 7×16² + 15×16¹ + 10×16⁰

= 8192 + 1792 + 240 +10

= 10234₁₀

Number System Relationship

The following table depicts the relationship between decimal, binary, octal and hexadecimal number systems.

HEXADECIMAL	DECIMAL	OCTAL	BINARY
0	0	0	0000
1	1	1	0001
2	2	2	0010
3	3	3	0011
4	4	4	0100
5	5	5	0101
6	6	6	0110
7	7	7	0111
8	8	10	1000
9	9	11	1001
A	10	12	1010
B	11	13	1011
C	12	14	1100
D	13	15	1101
E	14	16	1110
F	15	17	1111

ASCII

Besides numerical data, computer must be able to handle alphabets, punctuation marks, mathematical operators, special symbols, etc. that form the complete character set of English language. The complete set of characters or symbols are called alphanumeric codes. The complete alphanumeric code typically includes −

• 26 upper case letters
• 26 lower case letters
• 10 digits
• 7 punctuation marks
• 20 to 40 special characters

Now a computer understands only numeric values, whatever the number system used. So all characters must have a numeric equivalent called the alphanumeric code. The most widely used alphanumeric code is American Standard Code for Information Interchange (ASCII). ASCII is a 7-bit code that has 128 (27) possible codes.

ISCII

ISCII stands for Indian Script Code for Information Interchange. IISCII was developed to support Indian languages on computer. Language supported by IISCI include Devanagari, Tamil, Bangla, Gujarati, Gurmukhi, Tamil, Telugu, etc. IISCI is mostly used by government departments and before it could catch on, a new universal encoding standard called Unicode was introduced.

Unicode

Unicode is an international coding system designed to be used with different language scripts. Each character or symbol is assigned a unique numeric value, largely within the framework of ASCII. Earlier, each script had its own encoding system, which could conflict with each other.

In contrast, this is what Unicode officially aims to do − Unicode provides a unique number for every character, no matter what the platform, no matter what the program, no matter what the language.

Domain Specific Tools

---

Depending on its usage, software may be generic or specific. Generic software is a software that can perform multiple tasks in different scenarios without being modified. For example, a word processor software can be used by anyone to create different types of documents like report, whitepaper, training material, etc. Specific software is a software for a particular application, like railway reservation system, weather forecasting, etc. Let us look at some examples of domain specific tools.

School Management System

School management system handles the diverse activities of a school like examination, attendance, admission, students’ fees, time table, teachers’ training, etc.

Inventory Management

Managing multiple activities like purchase, sales, order, delivery, stock maintenance, etc. associated with raw or processed goods in any business is called inventory management. The inventory management software ensures that stocks are never below specified limits and purchase/deliveries are done in time.

Payroll Software

Payroll software handles complete salary calculations of employees, taking care of leave, bonus, loans, etc. Payroll software is usually a component of HR (human resource) management software in mid-sized to big organizations.

Financial Accounting

Financial management software keeps an electronic record of all financial transactions of the organization. It has many functional heads like account receivables, accounts payable, loans, payroll, etc.

Restaurant Management

Restaurant management software helps restaurant managers in keeping track of inventory levels, daily orders, customer management, employee scheduling, table bookings, etc.

Railway Reservation System

Railway reservation system is a software that handles multiple modules like train routes, train management, seat booking, food booking, train maintenance, train status, travel packages, etc.

Weather Forecasting System

Weather forecasting system is a real-time software that predicts weather of a place by collecting copious amount of live data about atmospheric temperature, humidity, wind level, etc. It is used to predict major disasters like earthquakes, hurricanes, tsunamis, etc.

Office Tools

---

Application software that assist users in regular office jobs like creating, updating and maintaining documents, handling large amounts of data, creating presentations, scheduling, etc. are called office tools. Using office tools saves time and effort and lots of repetitive tasks can be done easily. Some of the software that do this are −

• Word processors
• Spreadsheets
• Database systems
• Presentation software
• E-mail tools

Let us look at some of these in detail.

Word Processor

A software for creating, storing and manipulating text documents is called word processor. Some common word processors are MS-Word, WordPad, WordPerfect, Google docs, etc.

A word processor allows you to −

• Create, save and edit documents
• Format text properties like font, alignment, font color, background color, etc.
• Check spelling and grammar
• Add images
• Add header and footer, set page margins and insert watermarks

Spreadsheet

Spreadsheet is a software that assists users in processing and analyzing tabular data. It is a computerized accounting tool. Data is always entered in a cell (intersection of a row and a column) and formulas and functions to process a group of cells is easily available. Some of the popular spreadsheet software include MS-Excel, Gnumeric, Google Sheets, etc. Here is a list of activities that can be done within a spreadsheet software −

• Simple calculations like addition, average, counting, etc.
• Preparing charts and graphs on a group of related data
• Data entry
• Data formatting
• Cell formatting
• Calculations based on logical comparisons

Presentation Tool

Presentation tool enables user to demonstrate information broken down into small chunks and arranged on pages called slides. A series of slides that present a coherent idea to an audience is called a presentation. The slides can have text, images, tables, audio, video or other multimedia information arranged on them. MS-PowerPoint, OpenOffice Impress, Lotus Freelance, etc. are some popular presentation tools.

Database Management System

Software that manages storage, updating and retrieval of data by creating databases is called database management system. Some popular database management tools are MS-Access, MySQL, Oracle, FoxPro, etc.