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Chapter 10: Memory Organization (Memory Cache)
Friday 2 December 2016 • 06:05 • 0 comments



    A memory used to store data and instruction. Computer memory is the storage space in computer where data is to be processed and instructions required for processing are stored.

Memory is primarily of two types
  • Internal Memory − cache memory and primary/main memory
  • External Memory − magnetic disk / optical disk etc.


Characteristics of Memory Hierarchy are following when we go from top to bottom.
  • Capacity in terms of storage increases.
  • Cost per bit of storage decreases.
  • Frequency of access of the memory by the CPU decreases.
  • Access time by the CPU increases.
The Principle of Locality

The Principle of Locality:
• Program access a relatively small portion of the address space at any instant of time. 
• Example: 90% of time in 10% of the code  

Two Different Types of Locality: 
• Temporal Locality (Locality in Time): If an item is referenced, it will tend to be referenced again soon. 
• Spatial Locality (Locality in Space): If an item is referenced, items whose addresses are close by tend to be referenced soon.

Leads to memory hierarchy at two main interface levels: 
Processor - Main memory -> Introduction of caches – Main memory - Secondary memory -> Virtual memory (paging systems)

Cache Memory

Cache memory is a very high speed semiconductor memory which can speed up CPU. It acts as a buffer between the CPU and main memory. It is used to hold those parts of data and program which are most frequently used by CPU. The parts of data and programs, are transferred from disk to cache memory by operating system, from where CPU can access them.

Advantages:

  • Cache memory is faster than main memory.
  • It consumes less access time as compared to main memory.
  • It stores the program that can be executed within a short period of time.
  • It stores data for temporary use.

Disadvantages:


  • Cache memory has limited capacity.
  • It is very expensive.
Oh and last but not least, here's a video for an introduction to cache mapping technique include direct mapping, associative mapping and set-associative mapping. Hope it helps! :-)





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