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1. Chapter 1 – Introduction to Memory When we speak of a computer system, we tend to think of it as a calculating machine that can calculate extremely fast and hold enormous data. But how is this possible? The key to this is memory. One of the most vital components of a computer system is memory. If a computer were without memory, then it would be just a piece of inert hardware that cannot process or hold anything. Similarly, as the human brain retains, processes, and remembers things, the computer's memory does exactly the same. It keeps data, instructions, programs, and outcomes so that the processor can utilize them whenever needed. Memory is not only about "storing." It also determines how quickly a program will run, how many programs may be run simultaneously, and how well the entire system will operate. For instance, a computer with minimal memory will have slow performance and will not support proper multitasking, while a computer with more memory and high- speed memory will have smooth performance. Memory can be classified into various categories based on: Speed – How quick it is to access. Cost – How costly it is to produce.

2. Capacity – How much information it holds. Volatility – If it holds information when power is turned off. So, learning about memory is essential for any computer science student as it gives us an insight into the actual basis of computer performance Importance of Memory in Computers Memory assists in storing instructions and data temporarily as well as permanently. A CPU cannot run any program without memory. Memory helps the computer to run several activities simultaneously. Better system performance is brought about by faster memory. Chapter 2 – Definition of Memory Memory is defined as:

3. > "Memory is the storage area in a computer system where data and instructions are stored temporarily or permanently, for processing and future use." When we turn on the computer, the operating system is loaded into the memory, programs are run from the memory, and even minute instructions such as arithmetic calculations are retrieved from the memory. Chapter 3 – Characteristics of Memory Each memory contains some features that distinguish it from others. These include: Speed – The higher the speed of memory, the faster the CPU can retrieve and execute instructions. The register and cache are the fastest forms. Volatility – Memories such as RAM lose their data when power is turned off (volatile), but memories such as ROM and HDD retain data even when powered off (non- volatile). Cost per Bit – More expensive are the faster memories. That's why cache is minimal but extremely fast, while hard disk is large but slower. Capacity – Deals with how much information can be held in a specific memory.

4. Secondary memory such as hard disks have the largest capacity. 5. Accessibility – Some of the memories are accessible directly by the CPU (RAM, Cache, Registers) while others have to be accessed through I/O operations (HDD, SSD, CD/DVD). Chapter 4 – Types of Memory Computer memory is classified into different types based on speed, cost, size, and use. The broad categories are: Primary Memory (RAM & ROM) Secondary Memory Cache Memory Virtual Memory Registers 4.1 Primary Memory

5. Primary memory is also referred to as main memory. Primary memory is accessed directly by the CPU. All the data and instructions being processed need to be in the primary memory. Primary memory is of a volatile type (except ROM). That is, when the power supply is turned off, the contents of RAM are lost. A. RAM (Random Access Memory) RAM. The computer's working memory. The operating system, application programs, and data in use are stored there so they can be quickly accessed by the CPU. Volatile – The data gets erased when the computer is shut down. Read/Write Memory – CPU can read from and write to it. Speed – Significantly faster than secondary storage but slower than cache. Types of RAM

6. 1. Static RAM (SRAM) Stores data using flip- flops. Fast, doesn't require frequent refreshing. Costly, therefore used in cache memory. 2. Dynamic RAM (DRAM) Saves data through capacitors, which lose charge, so is refreshed periodically. Slightly slower than SRAM but less costly. Utilized in main system memory. B. ROM (Read Only Memory)

7. ROM is an irreversible memory, meaning its data persists even after power is shut down. Saves firmware (software permanently embedded at the time of manufacturing). Data cannot be altered readily. Stores BIOS, embedded systems, etc. Types of ROM PROM (Programmable ROM) – Can be programmed once by the user. EPROM (Erasable PROM) – Can be erased using ultraviolet light exposure. EEPROM (Electrically Erasable PROM) – Erasable and re- programmable with electrical charge. 4.2 Secondary Memory Secondary memory are storage devices that are not directly accessed by the CPU. Secondary memory data needs to first be loaded into RAM in order to be processed.

8. Examples: Hard Disk Drives (HDD) Solid State Drives (SSD) Optical Discs (CD/DVD) USB Flash Drives Characteristics: Non-volatile – Data is stored permanently. Large capacity – GBs to TBs. Slower than RAM – Access time is greater. Less expensive per bit than main memory.

9. 4.3 Cache Memory Cache is a high- speed, small memory between CPU and RAM. It holds frequently used data and instructions so that the CPU doesn't need to retrieve them from slow RAM each time. Speeds up the processing. Extremely expensive, thus tiny in size. Types of cache: L1 Cache – Smallest, located inside CPU. L2 Cache – Larger, located between CPU and RAM. L3 Cache – Shared among cores in multi- core processors. 4.4 Virtual Memory

10. Virtual memory is a technique in which part of the secondary storage (like hard disk) is used as an extension of RAM. Enables program execution beyond the size of physical RAM. Achieved via use of paging and segmentation. Slower than RAM as it employs disk storage. 4.5 Registers Registers are the fastest and smallest memory components within the CPU. They are employed to temporarily hold data, instructions, and addresses in the course of processing. Registertype: Accumulator (ACC) – Used to hold intermediate computations.

11. Program Counter (PC) – Holds the location of the next instruction to be executed. Instruction Register (IR) – Contains the present instruction. Memory Address Register (MAR) – Contains the memory address to be read. Memory Data Register (MDR) – Contains the data being read in/from memory. Chapter 5 – Memory Hierarchy Computer memory is not a single unit but is structured in hierarchical fashion. The principal reason that it is structured hierarchically is to balance three significant factors: Speed – High memory enhances performance but at a cost. Cost – Low-cost memory comes with low speed. Capacity – Large memories can only be afforded when speed is sacrificed. Computer designers provide a balance by structuring memory in a hierarchy, each level with its own attributes.

12. 5.1 Memory Hierarchy Structure The memory hierarchy mostly includes: 1. Registers – Situated within the CPU. Highly fast and expensive. Holds very little data (few bytes). 2. Cache Memory – Little bigger than registers, placed near CPU. Very quick, used to hold frequently used data. Split into L1, L2, and L3 cache.

13. 3. Primary Memory (RAM) – Bigger in capacity than cache. Slower and less expensive than cache. Holds data and programs being used. 4. Secondary Storage (HDD, SSD) – Much bigger in size, less expensive per bit. Slower than RAM. Holds operating system, software, files, etc. 5. Tertiary Storage (CDs, DVDs, Magnetic Tapes, Cloud Storage) –

14. Utilized for backup and archival. Slowest but cheapest. Accessed infrequently in relation to primary or secondary memory. 5.2 Memory Level Characteristics Any level of memory hierarchy can be compared on: Access Time (Speed) Storage Capacity Cost per Bit Proximity to CPU 5.3 Memory Hierarchy Working

15. When CPU is required to access data: It checks registers first. If data is not present there, then it checks cache memory. If not available in cache, then it checks in RAM. If RAM lacks it, data will be retrieved from secondary storage (HDD/SSD). For old or backup files, data could be retrieved from tertiary storage. This procedure provides maximum performance with least cost. 5.4 Memory Hierarchy's advantages Effective use of Memory – Frequently accessed data is stored closer to CPU. Cost Minimization – Merges costly but small memory and inexpensive but large memory. High Performance – Minimizes average access time of the data.

16. Flexibility – Offers balance between storage and speed. 5.5 Example of Memory Hierarchy in Practical Systems A smartphone consists of: Registers within processor (e.g., ARM CPU). L1/L2 cache within CPU chip. RAM (4GB to 12GB). Storage (64GB to 512GB SSD). Cloud backup as third storage.

17. A PC/Laptop contains: Registers within Intel/AMD CPU. Cache memory (L1/L2/L3). RAM (8GB, 16GB, etc.). HDD/SSD (500GB – 2TB). Cloud or external drives for backup.

18. Chapter 6 – Differences Between Different Types of Memory Different types of memory play different roles in computer systems. To understand their significance clearly, we need to learn about the differences between them. 6.1 Difference Between RAM and ROM Though both RAM and ROM are primary memory, they are quite different in function, speed, and volatility. 6.2 Difference Between Primary and Secondary Memory Primary and secondary memory are two broad divisions of storage in computers. 6.3 Difference Between SRAM and DRAM

19. Both are RAM types but of different designs and uses. 6.4 Difference Between Cache and RAM Cache and RAM both hold temporary data but with a different purpose and speed. 6.5 Virtual Memory vs Physical Memory Virtual memory is an extension technique for RAM, while physical memory is real RAM installed. 6.6 Registers vs Cache Registers and cache are both fast memories but of different uses. 6.7 Summary of Key Differences

20. Based on the above tables, we can summarize that: Registers are fastest, then cache, followed by RAM, and secondary storage. Volatile memories lose data upon shutdown, non-volatile do not. Cost is reversely proportional to size – faster memories are smaller and more expensive. Every type of memory plays its own role and can't be substituted by others.