Memory hierarchy is a system that organizes various types of computer memory in a hierarchical order based on their speed, size, and cost.
• The primary goal of memory hierarchy is to provide the CPU with fast access to the most frequently used data while also maintaining a balance between speed and storage capacity.
‣Registers:
A register is a small, high-speed storage location within the CPU (Central Processing Unit) of a computer that are used to store temporary data and instructions that are being processed by the CPU.
- Location: Inside the CPU.
- Characteristics: Fastest and most expensive; limited capacity.
- Usage: Used for storing small amounts of data and instructions that the CPU is actively processing.
‣Cache Memory:
Cache is a smaller, faster type of memory located between the CPU and main memory (RAM) in a computer system. Its primary purpose is to temporarily store copies of frequently accessed data and instructions from main memory to reduce the time taken to access them by the CPU.
- Location: Between RAM and CPU.
- Characteristics: Faster than RAM, more expensive, and smaller in size.
- Usage: Stores frequently accessed data to reduce CPU access time to slower RAM.
‣RAM (Main Memory):
RAM, or Random Access Memory, is a type of volatile memory in a computer system that stores data and program instructions temporarily while the computer is running.
- Location: Close to the CPU.
- Characteristics: Faster than secondary storage, volatile, and larger in size compared to cache.
- Usage: Stores data and instructions currently in use by the CPU.
Types of Main Memory:
- Static RAM: Static RAM stores the binary information in flip flops and information remains valid until power is supplied. It has a faster access time and is used in implementing cache memory.
- Dynamic RAM: It stores the binary information as a charge on the capacitor. It requires refreshing circuitry to maintain the charge on the capacitors after a few milliseconds. It contains more memory cells per unit area as compared to SRAM.
‣Secondary Storage (HDD, SSD):
Secondary memory, often referred to as auxiliary storage or external memory, is a non-volatile storage device that holds data and programs even when the power is turned off.
- Location: External to the CPU.
- Characteristics: Slower access speed compared to RAM, non-volatile, and large storage capacity.
- Usage: Long-term storage of data and programs.
‣Magnetic Disk:
Magnetic Disks are simply circular plates that are fabricated with either a metal or a plastic or a magnetized material. The Magnetic disks work at a high speed inside the computer and these are frequently used.
‣Magnetic Tape:
Magnetic tape is simply a magnetic recording device that is covered with a plastic film. It is generally used for the backup of data. In the case of a magnetic tape, the access time for a computer is a little slower and therefore, it requires some amount of time for accessing the strip.
Characteristics of Memory Hierarchy:
‣Speed:
Memory hierarchy is characterized by varying access speeds. At the top of the hierarchy are registers and cache memory, which offer the fastest access times but have limited capacity. As you move down the hierarchy, accessing data becomes slower but the storage capacity increases.
‣Size:
Memory hierarchy encompasses a range of storage capacities, from small but fast registers and cache memory to larger but slower main memory (RAM), and even larger secondary storage devices like hard disk drives (HDDs) or solid-state drives (SSDs).
‣Cost:
Generally, the cost per unit of storage increases as you move up the memory hierarchy. Registers and cache memory, being the fastest but also the smallest, are the most expensive per unit of storage. Main memory (RAM) is less expensive than cache memory but still more costly than secondary storage devices like HDDs or SSDs.
‣Volatility:
Memory hierarchy includes both volatile and non-volatile storage. Registers, cache memory, and main memory (RAM) are volatile, meaning they lose their contents when power is turned off. Secondary storage devices like HDDs and SSDs are non-volatile, meaning they retain data even when power is removed.
‣Access Mechanism:
Different levels of the memory hierarchy may have different access mechanisms. For example, registers and cache memory typically use hardware-based mechanisms for fast access, while accessing data from main memory or secondary storage involves more complex processes that may require the intervention of the operating system and peripheral devices.
‣Management:
Memory hierarchy requires sophisticated management techniques to ensure efficient utilization of resources. This includes strategies for data placement, data movement, caching algorithms, and prefetching mechanisms to minimize access latency and maximize throughput.
‣Hierarchy Levels:
The memory hierarchy typically consists of several levels, including registers, cache memory (L1, L2, L3), main memory (RAM), and secondary storage devices (HDDs, SSDs). Each level serves a specific purpose in balancing the conflicting requirements of speed, capacity, and cost.
