Chapter 3: Primary & Secondary Storage Devices PDF

**Chapter 3: Primary Storage and Secondary Storage Devices** **3.0 Objectives** This chapter, *Primary Storage and Secondary Storage Devices*, will enable you to: - Explain what a storage unit is and the criteria according to which the different types of storage are ranked. - Describe va...

**Chapter 3: Primary Storage and Secondary Storage Devices** **3.0 Objectives** This chapter, *Primary Storage and Secondary Storage Devices*, will enable you to: - Explain what a storage unit is and the criteria according to which the different types of storage are ranked. - Describe various types of storage devices and their storage capacities. - Discuss various concepts related to computer memory. **3.1 Introduction** **Storage Unit:** A storage unit is a component of the computer system used to store data and instructions for processing. There are two main types of storage: 1. **Primary Storage** 2. **Secondary Storage** - **Primary Storage**: Also known as internal or main memory, it holds program instructions, input data, and intermediate results. - **Secondary Storage**: External memory used for long-term storage of programs and data. **Characteristics of Storage Units:** The efficiency and performance of a computer system depend on the following characteristics of storage units: - **Access Time**: Time required to locate and retrieve data. - **Storage Capacity**: The amount of data a storage unit can hold, measured in bytes (e.g., KB, MB, GB, TB). - **Cost**: Lower cost is desirable. Storage units with faster access time, higher storage capacity, and lower costs are considered superior. **3.2 Primary Storage** Primary storage is an integral part of all computer systems. It is characterized by: - Faster access time - Smaller storage capacity - Higher costs compared to secondary storage **Key Features:** - **Volatility**: Data is lost when power is turned off. - **Random Access Memory (RAM)**: Allows random access to storage locations and is used during program execution. **Types of Primary Storage:** 1. **RAM (Random Access Memory)**: - Volatile and used for temporary data storage. - Known as read/write memory. 2. **ROM (Read-Only Memory)**: - Non-volatile and used to permanently store data. - Cannot be rewritten or altered once programmed. 3. **PROM (Programmable ROM)**: - Programmable using special tools. - Data cannot be altered after programming. 4. **EPROM (Erasable PROM)**: - Data can be erased and reprogrammed using ultraviolet light. 5. **Cache Memory**: - High-speed memory that temporarily stores frequently accessed data for the CPU. 6. **Registers**: - Special temporary memory units not part of main memory. - Common types include: - Memory Address Register (MAR) - Memory Buffer Register (MBR) - Program Control Register - Instruction Register - Accumulator Register - Input/Output Register **3.3 Secondary Storage Devices** Secondary storage provides non-volatile, long-term data storage at a lower cost but with slower access time than primary storage. Data can be accessed in two ways: 1. **Sequential Access** 2. **Direct Access** **3.3.1 Sequential Access Devices:** Data is accessed in the order it is stored. - **Punch Paper Tape**: Early low-cost storage medium, now rarely used. - **Magnetic Tape**: - Stores data as magnetized and non-magnetized spots. - High storage capacity but supports only sequential access. **3.3.2 Direct Access Storage Devices:** Allows random access to data at any location. **Examples:** - **Magnetic Disk**: - Thin metallic/Mylar platters coated with magnetic material. - Data is recorded as magnetic spots. - Divided into concentric circles (tracks) and cylinders. - **Floppy Disk**: - Flexible Mylar disk coated with iron oxide. - Sizes include 8-inch, 5¼-inch, and 3½-inch disks. - Portable and low-cost. - **Winchester Disk**: - Permanently enclosed in a sealed container for high reliability and storage density. - **Magnetic Drum**: - A rotating cylinder coated with magnetic material. - Faster than disks but with limited storage capacity. **3.3.3 Optical Devices:** Uses laser techniques to write and read data. - **Optical Disk**: - Coated with a thin reflective metal. - Read-only and offers high storage capacity (e.g., CD-ROM). - **Optical Card**: - Laser-encoded strip storing up to 2 MB of data. - **Optical Tape**: - Similar to magnetic tape but uses laser techniques. **3.4 Summary** - Storage units are used to store data and instructions. - Types of storage include primary (volatile, faster, and costly) and secondary (non-volatile, slower, and cheaper). - **Primary storage** includes RAM, ROM, PROM, EPROM, cache memory, and registers. - **Secondary storage** includes sequential access devices (e.g., magnetic tape) and direct access devices (e.g., magnetic disk, floppy disk, optical devices). **3.6 Questions for Self-Study** 1. What are storage units? What are their types? Describe their characteristics. 2. Explain the difference between volatile and non-volatile memory. 3. Define registers. Describe commonly used registers in computer systems. 4. How can data on secondary storage devices be accessed? 5. Describe any two sequential access devices. 6. Describe any two direct access storage devices. 7. Explain moving head and fixed head systems. 8. Discuss two types of optical storage devices.