1.6 Types and Hierarchy of Computer Memory
Memory in computing refers to the physical devices used to store data and programs, both temporarily and permanently, for use by a computer. It is essential for the operation of the computer system and the execution of programs. The types and hierarchy of memory can be broadly classified into different categories.
Memory Terminology
- Bit: The smallest unit of memory, representing a binary digit, either 0 or 1. Each bit corresponds to an electronic cell in memory which is either ON (1) or OFF (0).
- Byte: A group of 8 bits constitutes one byte. It is the basic unit of memory used to store one character of data, such as 'A', 'b', or '*'. Memory size is generally measured in bytes, with higher units including:
- Kilobyte (KB): 1 KB = 1024 Bytes
- Megabyte (MB): 1 MB = 1024 KB
- Gigabyte (GB): 1 GB = 1024 MB
- Terabyte (TB): 1 TB = 1024 GB
- Petabyte (PB): 1 PB = 1024 TB
- Exabyte (EB): 1 EB = 1024 PB
- Memory Word: The smallest amount of data that a computer can process as a unit. The size of a memory word, known as word size, typically ranges from 16 to 64 bits. A larger word size allows the CPU to process more data per operation, enhancing performance.
- Word Size: Refers to the number of bits that a CPU can process or manipulate in a single operation. For example, a 32-bit CPU processes data in 32-bit chunks, whereas a 64-bit CPU handles 64-bit chunks. Larger word sizes enable handling of larger integers, more complex arithmetic operations, and greater memory addressing.
Memory Capacity Conversion Chart
| Term (Abbreviation) | Approximate Size |
|---|
| Byte (B) | 8 bits |
| Kilobyte (KB) | 1024 bytes / 10³ bytes |
| Megabyte (MB) | 1024 KB / 10⁶ bytes |
| Zettabyte (ZB) | 1024 EB / 10²¹ bytes |
| Yottabyte (YB) | 1024 ZB / 10²⁴ bytes |
Memory Built-up and Retention Power
- Chip Memory: Refers to memory contained within integrated circuits (ICs) on a semiconductor chip. These chips are fast due to their lack of mechanical parts and reliance on electrical processes. Examples include:
- RAM (Random Access Memory)
- ROM (Read-Only Memory)
- Cache Memory
- Flash Memory Drives
- Memory Cards
- Solid State Drives (SSDs)
- Magnetic Memory: Uses magnetized media to store data. Magnetic memory includes:
- Magnetic Tapes: Data is stored as magnetized spots on a tape.
- Magnetic Disks: Data is stored on disks coated with a magnetic material. Examples include hard disk drives (HDDs), which use a read/write head to detect and modify magnetized spots on the disk.
Magnetic memory is cost-effective and offers large storage capacities but has slower access times compared to other types of memory. - Optical Memory: Stores data using laser technology to encode data onto disks. Optical memory includes:
- CDs (Compact Discs)
- DVDs (Digital Versatile Discs)
- Blu-ray Discs (BDs)
Data is encoded in the form of pits and lands on the disk's surface, which are read by a laser beam. Optical storage is relatively inexpensive and offers high capacity but is generally slower than chip memory.
Main Memory
- RAM (Random Access Memory): Volatile memory that provides temporary storage for data and instructions in use.
- Cache Memory: A smaller, faster type of volatile memory located close to the CPU, used to speed up access to frequently used data.
- ROM (Read-Only Memory): A type of non-volatile memory that is used to store firmware and system software. Once written, it cannot be modified or can only be modified with difficulty.
- Registers
Volatile and Non-Volatile Memory
- Volatile Memory: Memory that loses its data when power is lost. Examples include:
- RAM (Random Access Memory)
- Cache Memory
- Non-Volatile Memory: Memory that retains its data even when the power is turned off. Examples include:
- ROM (Read-Only Memory)
- Flash Memory
- Magnetic Storage (e.g., HDDs)
- Optical Storage (e.g., CDs, DVDs)