CHAPTER 3 MEMORY AND STORAGE SYSTEMS Chapter Outline Chapter Objectives 3.1 Introduction In this chapter, we will learn: 3.2 Memory Representation ∑ The concept of memory and its 3.3 Random Access Memory representation. 3.3.1 Static RAM ∑ How data is stored in Random Access 3.3.2 Dynamic RAM Memory (RAM) and the various types of 3.4 Read Only Memory RAM. 3.4.1 Programmable ROM ∑ How data is stored in Read Only Memory 3.4.2 Erasable PROM (ROM) and the various types of ROM. 3.4.3 Electrically Erasable PROM ∑ The concept of storage systems and the 3.4.4 Flash ROM various types of storage systems. 3.5 Storage Systems ∑ The criteria for evaluating storage 3.6 Magnetic Storage Systems systems. 3.6.1 Magnetic Tapes 3.6.2 Magnetic Disks 3.7 Optical Storage Systems 3.1 INTRODUCTION 3.7.1 Read only Optical Disks 3.7.2 Write Once, Read Many Disks Computers are used not only for processing of data 3.8 Magneto Optical Systems for immediate use, but also for storing of large 3.8.1 Principle used in Recording Data volume of data for future use. In order to meet 3.8.2 Architecture of Magneto Optical Disks these two specifi c requirements, computers use two 3.9 Solid-State Storage Devices types of storage locations—one, for storing the data 3.9.1 Structure of SSD that are being currently handled by the CPU and the 3.9.2 Advantages of SSD other, for storing the results and the data for future 3.9.3 Disadvantages of SSD use. The storage location where the data are held 3.10 Storage Evaluation Criteria temporarily is referred to as the primary memory Chapter Summary while the storage location where the programs Key Terms to Remember and data are stored permanently for future use is Review Questions referred to as the secondary memory. The primary Fill in the Blanks memory is generally known as “memory” and the secondary memory as “storage”. Multiple Choice Questions The data and instructions stored in the primary Discussion Questions memory can be directly accessed by the CPU 44 Fundamentals of Computers using the data and address buses. However, the information stored in the secondary memory is not directly accessible to CPU. Firstly, the information has to be transferred to the primary memory using I/O channels and then, to the CPU. Computers also use a third type of storage location known as the internal process memory. This memory is placed either inside the CPU or near the CPU (connected through special fast bus). Figure 3.1 illustrates all the three categories of computer memory and their relative speed, storage capacity and cost. Fig. 3.1 Memory categories Primary memory (also known as main memory) includes two types, namely, Random Access Memory (RAM) and Read Only Memory (ROM). The data stored in RAM are lost when the power is switched off and therefore, it is known as volatile memory. However, the data stored in ROM stay permanently even after the power is switched off and therefore ROM is a non-volatile memory. Secondary memory (also known as auxiliary memory) includes primarily magnetic disks and magnetic tapes. These storage devices have much larger storage capacity than the primary memory. Information stored on such devices remains permanent (until we remove it). Internal process memory usually includes cache memory and registers both of which store data temporarily and are accessible directly by the CPU. This memory is placed inside or near the CPU for the fast access of data. We have discussed briefl y cache memory and various registers used in Chapter 2. In this chapter, we shall discuss the various types of memory units, RAM and ROM and their functions. We shall also discuss the classifi cation of storage devices such as magnetic disks, magnetic tapes and optical disks and their functions. 3.2 MEMORY REPRESENTATION As we discussed earlier, data being worked on is stored in the computer memory. In the memory, values are represented by sequences of binary digits, known as bits. Most computers use a group of eight bits, Memory and Storage Systems 45 known as a byte, to represent a character. How does the computer know what any particular sequence of bits represents? We can think of memory as a “bunch” of bytes or cells into which we can place data. Each cell, known as a data item, is assigned a unique number known as address (which is like the index in an array). The CPU can identify each cell by its address as shown in Fig. 3.2. Fig. 3.2 Memory representation The byte is defi ned as the “smallest addressable unit” of memory. Most computers use groups of bytes, usually 2 or 4, known as “words” to represent information. Computer memories are often rated in terms of their capacity to store information. Typically, capacities are described using the unit of byte as follows: 1 KB (Kilobyte) = 1,024 bytes 1 MB (Megabyte) = 1,048,576 bytes 1 GB (Gigabyte) = 1,073,741,824 bytes 1 TB (Terabyte) = 1,099,511,627,776 bytes 46 Fundamentals of Computers 3.3 RANDOM ACCESS MEMORY Random Access Memory (RAM) is a volatile memory and loses all its data when the power is switched off. It is the main memory of the computer system that stores the data temporarily and allows the data to be accessed in any order. As compared to the secondary storage, the data can be accessed at a faster speed in RAM because it is the internal memory of the computer. Figure 3.3 shows RAM with ICs. Fig. 3.3 Random access memory with ICs RAM is made up of different ICs, which are mounted on a printed circuit board. RAM stores the application programs and the data on which the user is currently working so that the processor can easily access the required application program and data in a less amount of time. RAM is also known as read/write memory because it can perform both read as well as write operations. The speed of RAM is faster than the other memory devices, such as hard disk, fl oppy disk, etc. The programs, which are being currently executed by the computer system, are stored in RAM. RAM is volatile and, therefore, the programs and the data stored in the RAM get lost when the power supply is switched off. The storage capacity of RAM is usually less than the secondary storage devices. RAM can be categorised into two main types, namely, Static RAM (SRAM) and Dynamic RAM (DRAM), which can be further divided into various types for storing data. Figure 3.4 shows the hierarchy of RAM memory. Fig. 3.4 Types of random access memory Memory and Storage Systems 47 3.3.1 Static RAM Static RAM (SRAM) is a type of RAM in which data is stored till the power of the computer system is switched on. SRAM uses a number of transistors to store a single bit of digital information. Figure 3.5 shows the organisation of data in a cell of SRAM. In the fi gure, b1 and b2 represent the two bit lines and Wx represents the word line. In the memory, the data is stored in the form of a two- dimensional array, containing rows Fig. 3.5 Organisation of data in a cell of static random access memory and columns. The row can be accessed globally by a line (known as word line) and the column can be accessed individually by a line (known as bit line). Depending on the function performed by SRAM, it can be divided into the following three types: ∑ Asynchronous SRAM (ASRAM) ASRAM performs its operations without the use of system clock. It makes use of three signals for working, namely, Chip Select (CS), Write Enable (WE) and Output Enable (OE). The CS signal enables the processor to select the memory for performing read and write operations. If the value of CS signal equals zero, then the memory is enabled to perform the operations. On the other hand, if the value of the CS signal equals one, then the memory is disabled and operations—such as reading and writing in ASRAM—cannot be performed. The signal WE makes the decisions related to data, i.e., whether it should be read from or write to the memory. If the value of WE signal equals zero, then no data can be read from or written to the memory. The signal OE is an active low signal that enables the processor to give the output for the data. If the value of OE signal equals zero, then only it will output the data. ∑ Burst SRAM (BSRAM) BSRAM works in association with the system clock and is also known as synchronous SRAM. BSRAM is most commonly used with high-speed applications because the read and write cycles are synchronised with the clock cycles of the processor. The access-waiting time gets reduced after the read and write cycles are synchronised with the clock cycles. The speed and the cost of BSRAM increases or decreases simultaneously. ∑ Pipeline Burst SRAM (PBSRAM) PBSRAM makes use of the pipeline technology in which a large amount of data is broken up in the form of different packets containing data. These packets are arranged in a sequential manner in the pipeline and are sent to the memory simultaneously. PBSRAM can handle a large amount of data at a very high speed. It is the fastest type of SRAM because it can operate at bus rates as high as 66 MHz.