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Computer Architecture - Memory: Memory Hierarchy

memory hierarchy
- a design of layered stack of storage to minimize the access time.
- based on the locality of references
  - same data or nearby data is likely to be accessed again and again
Benefits:
- optimizing the memory available in the computer
  - divide into multiple levels that have different size, cost, etc
  - divide into vary types of memory that have different performance and storage value
Types of Memory Hierarchy
- Primary Memory/Internal Memory:
  - directly accessible by the processor
  - e.g., Main Memory, Cache Memory & CPU registers
- Secondary Memory/External Memory:
  - accessible by the processor via an I/O Module
  - e.g., Magnetic Disk, Optical Disk, and Magnetic Tape

Capacity:
- It is the global volume of information the memory can store.
- As we move from top to bottom in the Hierarchy, the capacity increases.
Access Time:
- It is the time interval between the read/write request and the availability of the data.
- As we move from top to bottom in the Hierarchy, the access time increases.
Performance:
- The Memory Hierarch design ensures that frequently accessed data is stored in faster memory to improve system performance.
Cost Per Bit:
- As we move from bottom to top in the Hierarchy, the cost per bit increases i.e. Internal Memory is costlier than External Memory.

register (寄存器)
- small, high-speed memory units located in the CPU.
- used to store the most frequently used data and instructions.
- have the fastest access time and the smallest storage capacity, typically ranging from 16 to 64 bits.

Cache memory
- a small, fast memory unit located close to the CPU.
- used to store frequently used data and instructions that have been recently accessed from the main memory.
- designed to minimize the time it takes to access data by providing the CPU with quick access to frequently used data.
hierarchy
- L1: keep immediately hot code/data
  - used to Feed a core every cycle with instructions (L1I) and data (L1D).
  - One L1I and one L1D per core
  - size: 16–64 KiB
  - very fast, lowest latency.
- L2: hape the algorithm for medium working sets.
  - used to catch L1 misses; hold medium-sized working sets.
  - One unified L2 per core.
  - size: 256 KiB–2 MiB
  - Lower than L1
- L3/Last-Level Cache (LLC): keep big read-mostly data here and avoid cross-core invalidation.
  - Last on-package stop before DRAM; shared across cores; absorbs cross-core traffic.
  - Size: Many MiB
  - higher latency.

Main memory/RAM (Random Access Memory)
- the primary memory of a computer system.
- used to store data and instructions that are currently in use by the CPU.
- has a larger storage capacity than cache memory, but it is slower.
Types of Main Memory
- Static RAM:
  - used to store the binary information in flip flops(电路中的触发器) and information remains valid until power is supplied.
  - has a faster access time and is used in implementing cache memory.
- Dynamic RAM:
  - used to store the binary information as a charge on the capacitor(电容中的电荷).
  - 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
- used to store data and instructions that are not currently in use by the CPU.
- a non-volatile memory unit that has a larger storage capacity than main memory.
- has the slowest access time and is typically the least expensive type of memory in the memory hierarchy.
- e.g., hard disk drives (HDD) and solid-state drives (SSD)

Magnetic Disk
- circular plates that are fabricated with either a metal or a plastic or a magnetized material.

Magnetic Tape
- a magnetic recording device that is covered with a plastic film.
- used for the backup of data.
- the access time for a computer is a little slower and therefore, it requires some amount of time for accessing the strip.