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CHAPTER 3
MEMORY AND STORAGE SYSTEMS
Chapter Outline
3.1 Introduction 3.2 Memory Representation 3.3 Random Access Memory 3.3.1 Static RAM 3.3.2 Dynamic RAM 3.4 Read Only Memory 3.4.1 Programmable ROM 3.4.2 Erasable PROM 3.4.3 Electrically Erasable PROM 3.4.4 Flash ROM 3.5 Storage Systems 3.6 Magnetic Storage Systems 3.6.1 Magnetic Tapes 3.6.2 Magnetic Disks 3.7 Optical Storage Systems 3.7.1 Read only Optical Disks 3.7.2 Write Once, Read Many Disks 3.8 Magneto Optical Systems 3.8.1 Principle used in Recording Data 3.8.2 Architecture of Magneto Optical Disks 3.9 Solid-State Storage Devices 3.9.1 Structure of SSD 3.9.2 Advantages of SSD 3.9.3 Disadvantages of SSD 3.10 Storage Evaluation Criteria Chapter Summary Key Terms to Remember Review Questions Fill in the Blanks Multiple Choice Questions Discussion Questions
Chapter Objectives
In this chapter, we will learn: ∑ The concept of memory and its representation. ∑ How data is stored in Random Access Memory (RAM) and the various types of RAM. ∑ How data is stored in Read Only Memory (ROM) and the various types of ROM. ∑ The concept of storage systems and the various types of storage systems. ∑ The criteria for evaluating storage systems.
3.1 INTRODUCTION
Computers are used not only for processing of data for immediate use, but also for storing of large volume of data for future use. In order to meet these two specific requirements, computers use two types of storage locations—one, for storing the data that are being currently handled by the CPU and the other, for storing the results and the data for future use. The storage location where the data are held temporarily is referred to as the primary memory while the storage location where the programs and data are stored permanently for future use is referred to as the secondary memory. The primary memory is generally known as “memory” and the secondary memory as “storage”. The data and instructions stored in the primary memory can be directly accessed by the CPU
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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 briefly 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 classification 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,
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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, floppy 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
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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 figure, b 1 and b 2 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 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.
3.3.2 Dynamic RAM
DRAM is the RAM in which data is stored in a storage cell, consisting of a transistor and a capacitor. Unlike SRAM, the DRAM needs to be continuously refreshed with power supply because the capacitor has the tendency to get discharged. DRAM retains the data for a very short span of time, even after the power supply is switched off. Figure 3.6 shows the organisation of data in a cell of DRAM.
Fig. 3.5 Organisation of data in a cell of static random access memory
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3.4 READ ONLY MEMORY
ROM is the memory that stores the data permanently, i.e., it can retain the data even when the power of the computer system is switched off. The data can be easily read from this type of memory but cannot be changed. ROM is most commonly used in devices such as calculators, laser printers, etc. ROM does not allow the random access of data rather it allows sequential access of data. It is less expensive as compared to RAM and other storage devices, such as magnetic disk, etc. ROM is divided into four types, which are as follows: ∑ Programmable ROM (PROM) ∑ Erasable PROM (EPROM) ∑ Electrically Erasable PROM (EEPROM) ∑ Flash ROM
3.4.1 Programmable ROM
Programmable ROM (PROM) is a memory chip on which the write operation of data can be performed only once. The data is stored on this chip permanently, i.e., once a program is written on the PROM, it cannot be erased or destroyed. To write the data on the PROM chip, a device known as PROM programmer or PROM burner is required. The method of writing data on the chip is known as burning the PROM. PROM is reliable and stores the data permanently without making any change in it. It is mostly used in video games and electronic dictionaries.
3.4.2 Erasable PROM
Erasable PROM (EPROM) is a type of ROM in which data can be erased or destroyed using Ultraviolet Light (UL). Erasable ROM provides the facility of changing the contents of the data, i.e., it can be reprogrammed. It contains the floating gate transistors, which have the capability to hold an electric charge, even when the power of the computer system is switched off. It also facilitates the storage of data for a longer period of time.
3.4.3 Electrically Erasable PROM
Electrically Erasable PROM (EEPROM) is a type of ROM in which data can be erased or destroyed by exposing it to an electric charge. It has the ability to retain the data stored in it, even if the power of the computer system is switched off. It stores the data permanently but allows us to make changes in the data by erasing it with the help of electric charges. In this type of memory, the data can be written or erased only one byte at a time because of which it works very slowly.
3.4.4 Flash ROM
Flash ROM is a type of EEPROM that stores the information using floating-gate transistors, which can store electric charge for a longer period of time as compared to the normal transistors. This memory is mainly used in the memory cards of mobile phones, digital cameras and ipods for storing data. The data stored in flash ROM memory can be easily transferred using transmission mediums such as data cable, bluetooth and infrared technology. For example, we can transfer the data stored in flash ROM memory of mobile phone to the memory of a computer using data cable. We can easily erase the data stored in flash ROM memory and reprogram this type of memory. Flash ROM has faster speed of reading data, as compared to any other type of ROM. It uses continuous memory cells for storing data.
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The memory cells of flash ROM are made up of floating-gate transistors. A Single-level Cell (SLC) can store only one bit of data, whereas Multi-level Cell (MLC) provides the facility of storing more than one byte. The two types of flash ROM memory are as follows:
∑ NAND flash In NAND flash or NAND gate flash memory, each cell behaves like a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), which has two gates for amplifying an electric signal.
∑ NOR flash NOR gate flash or NOR flash memory provides the facility to write data or information, with the help of tunnel injection. It can be defined as the process of inserting electrons to an electric conductor using a layer of electric insulator. It also provides the facility to erase the instructions using tunnel release.
3.5 STORAGE SYSTEMS
Storage systems are the devices—such as hard disks, optical disks and magnetic disks—used for data storage. The main objective of the storage system is to permanently store data—which can be of any type, such as images, videos and text—for future use. The storage systems provide the facility to use the data at any time. Different storage systems have varying storage capacities. For example, the normal storage capacity of Compact Disk (CD) is 700 Megabytes (MB). The storage systems can be classified as shown in Fig. 3.7.
Fig. 3.7 Classification of storage systems
3.6 MAGNETIC STORAGE SYSTEMS
Magnetic storage systems can be defined as the storage systems that store the data on a magnetised medium, with the help of magnetised particles. Magnetic tapes, magnetic disks, hard disks, floppy disks are examples of magnetic storage systems. The magnetic storage systems are non-volatile and provide the facility to store any type of data, such as text, audio, video, image, etc. In case of magnetic storage systems, data can be accessed randomly as well as sequentially. For example, we can access data sequentially from a magnetic
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predetermined for reading and writing data. When the magnetic tape moves at high speed, slows down or stops, the data cannot be read or written to the tape. Major advantages of magnetic tapes are: ∑ Low cost ∑ Large storage capacity ∑ Easily transportable ∑ Easy to handle and store Major disadvantages of using magnetic tapes are: ∑ Low data transmission speed due to sequential access. ∑ Not suitable for random access. ∑ Require protected environment for storage. ∑ Updating, such as insertion or deletion is difficult. ∑ Fast wear-out.
3.6.2 Magnetic Disks
Magnetic disk is a flat disk that is covered with magnetic coating for holding information. It is a type of secondary memory device that is used to store different programs and files. It is used to store digital information in the form of small and magnetised needles. These needles help in encoding a single bit of information by getting polarized in one direction represented by 1, and opposite direction represented by 0. Magnetic disk can store a large amount of data and is less expensive as compared to RAM. As it takes more time to read the information from a specified location, its data access rate is slow compared to the main memory. It allows the random access of data and provides the facility of erasing and re-recording the data as many times as required. Figure 3.9 shows a magnetic disk with sectors and tracks.
Fig. 3.9 Magnetic disk surface
Tracks are the concentric circles on the magnetic disk, having a common centre and containing a block of recorded data. The thickness between two tracks affects the storage capacity of a magnetic disk. If the thickness between the two tracks is less, then the magnetic disk can store a large amount of data. On the other hand, if the thickness between the two tracks is more then less amount of data can be stored in the
