UNIT-2 : COMPUTER MEMORY UNIT STRUCTURE 2.1 Learning Objectives 2.2 Introduction 2.3 Introduction to Computer Memories 2.4 Primary memory 2.4.1 Random Access Memory(RAM) 2.4.2 Read Only Memory(ROM) 2.5 Secondary Memory 2.6 Let Us Sum Up 2.7 Further Readings 2.8 Answers to Check Your Progress 2.9 Possible Questions 2.1 LEARNING OBJECTIVES After going through this unit, you will be able to- • Describe about computer memories • Identify the types of memory • Narrate about the difference in type of memories • Identify commercial memories 2.2 INTRODUCTION In the previous unit you have come across the fundamental concepts of a computer organisation, software and hardware, operating system etc. In this unit you are going to be introduced with the computer memories. This unit mainly deals with the different types of memories used in a computer and their functions. This unit also highlights on some commercial memories. So let us now first start with the section introduction to'computer memory. 26 Fundamentals of Computer and Management Computer Memory Unit-2 2.3 INTRODUCTION TO COMPUTER MEMORIES As mentioned in the previous unit a complete computer system consists of both software and hardware. Now software is the set of instructions according to which the computer system works. These instructions are required to be stored in a device from where we can recall later whenever we need to run any software. Also it is important to store the outcome of any task performed on a computer system or the result of any execution. Also it becomes sometimes most important to create a data base and store it properly for any further use. All these data are stored in a particular device of a computer system which is known as Memory. Allihe data are stored in digital format in the storing device or element of the computer system i.e. in the memory. These memory devices may be either meant for temporary storage or permanent storage. Both these type are important to run the computer system smoothly and efficiently. The temporary memories are also known as the Volatile memory, as it is capable of storing till when the power is ON, after that it loses all its data. Permanent memories are called the Non volatile memory as it can retain data even after the power is switched OFF. The term memory usually refers to a form of semiconductor storage and sometimes other forms of fast but temporary storage. Similarly, storage today more commonly refers to mass storage such as optical discs, forms of magnetic storage like hard disks and other types. All these are slower than memory, but of a more permanent nature. Generally, the term Primary Memory is used to represent the memory and the term Secondary Memory is used to represent the storage. * CHECK YOUR PROGRESS-1 Fill in the blanks: (i) Software is a set of , according to which a computer works. (ii) The storing device of a computer system is called the ill) Computer memory can be classified as & Fundamentals of Computer and Management 27 Unit-2 Computer Memory 2.4 PRIMARY MEMORY Primary memory also known as memory,is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them.Any data actively operated on is also stored there in uniform manner. It is a form of semiconductor storage known as random access memory(RAM) and sometimes other forms offaM but temporary storage. It is small-sized, light, but quite expensive. This type of memory is divided into the following two types: • RAM (Random Access Memory) • ROM (Read Only Memory) ^ 2.4.1 Random Access Memory(RAM) RAM stands for Random Access Memory. This means Information can be retrieve and store by the computer at any order. RAM gives your computer a temporary place to process electronic data. This means that, RAM chips continue to store information only as long as computer has electrical power. In other words, when you shut off your computer, all the data stored in RAM are lost. All actual computing starts with the CPU (Central Processing Unit). There are generally two broad categories of random access memory: • DRAM memories {Dynamic Random Access Memory) • SRAM memories {Static Random Access Memory) /C; It is a miniaturized Similar to a microprocessor, a memory chip is an integrated circuit electronic circuit that (/C) made of millions of transistors and capacitors. In the most has been manufactured in the common form of computer memory,dynamic random access memory surface of a substrate (DRAM), a transistor and a capacitor are paired to create a memory of a semiconductor cell, which represents a single bit of data. The capacitor holds the bit material. of information — a 0 or a 1. The transistor acts as a switch that lets i8 Fundamentals of Computer and Management Computer Memory Unit-2 the control circuitry on the memory chip read the capacitor or change its state. Given that the capacitors become discharged they must be constantly recharged (the exact term is refresh) at regular intervals, known as the refresh cycle. DRAM memories for example require refresh cycles of around 15 nanoseconds (ns). DRAM--Dynamic RAM Dynamic random access memory(DRAM) is the most common kind of random access memory (RAM) for personal computers and workstations. Memory is the network of electrically-charged points in which a computer stores quickly accessible data in the form of Os and '1's. Random access means that the PC processor can access any part of the memory or data storage space directly rather than having to proceed sequentially from some starting place. DRAM is dynamic in that, unlike static RAM (SRAM), it needs to have its storage cells refreshed or given a new electronic charge every few milliseconds. Static RAM does not need refreshing because it operates on the principle, of moving current that is switched in one of two directions rather than a storage cell that holds a charge in place. Static RAM is generally used for cache memory, which can be accessed more quickly than DRAM. DRAM stores each bit in a storage cell consisting of a capacitor and a transistor. Capacitors tend to lose their charge rather quickly; thds, the need for recharging. A variety of other RAM interfaces to the computer exist, such as EDO RAM and SDRAM. SRAM - Static RAM » . ■ Static RAM is a type of RAM that holds its data without external refresh, for as long as power is supplied to the circuit. This is contrasted to dynamic RAM(DRAM), which rpust berrefreshed many times per second in order to hold its data contents. SRAMs are used for specific applications within the PC,where their strengths outweigh • their weaknesses compared to DRAM: . Fundamentals of Computer and Management 29 Unit-2 Computer Memory • Simplicity: SRAMs don't require external refresh circuitry or other work in order for them to keep their data intact. • Speed: SRAM is faster than DRAM. In contrast, SRAMs have the following weaknesses, compared to DRAMs: • Cost: SRAM is, byte for byte, several times more expensive than DRAM. • Size: SRAMs take up much more space than DRAMs (which is part of why the cost is higher). These advantages and disadvantages taken together obviously show that performance-wise, SRAM is superior to DRAM, and we would use it exclusively if only we could do so economically. Unfortunately, 32 MB of SRAM would be prohibitively large and costly, which is why DRAM is used for system memory. SRAMs are used instead for level 1 cache and level 2 cache memory, for which it is perfectly suited; cache memory needs to be very fast, and not very large. SRAM is manufactured in a way rather similar to how processors are: highly-integrated transistor patterns photo-etched into silicon. Each SRAM bit is comprised of between four and six transistors, which is why SRAM takes up much more space compared to DRAM, which uses only one (plus a capacitor). Because an SRAM chip is comprised of thousand^ or millions of identical cells, it is much easier to make than a CPU, which is a large die with a non-repetitive structure. This is one reason why RAM chips cost much less than processors do. 2.4.2 Read Only Memory(ROM) One major type of memory that is used in PCs Is called read - only memory or ROM for short. ROM is a type of memory that normally can only be read, as opposed to RAM which can be both read and :0 Fundamentals of Computer and Management Computer Memory Unit-2 written. There are two main reasons that read-only memory is used for certain functions within the PC: • Permanence: The values stored in ROM are always there, whether the power is on or not. A ROM can be removed from the PC, stored for an indefinite period of time, and then replaced, and the data it contains will still be there. For this reason, it is called non-volatile storage. A hard disk is also non-volatile, for the same reason, but regular RAM is not. • Security: The fact that ROM cannot easily be modified provides a measure of security against accidental (or malicious) changes to its contents. You are not going to find viruses infecting true ROMs,for example; it's just not possible. (It's technically possible with erasable EPROMs, though in practice never seen) Read-only memory is most commonly used to store system-level programs that we want to have available to the PC at all times. The most common example is the system BIOS program, which is stored in a ROM called (amazingly enough)the system BIOS ROM.