Memory System Design: Understanding Memory Characteristics and Hierarchy, Slides of Computer Architecture and Organization

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• Memory System Design I Chapter

Basic Concepts Characteristics of Memory inside CPU

1) Memory Location
CPU (Cache, Registers)
Internal (RAM)
External (Hard Disk)

2) Memory Capacity
Word size ■ The natural unit of organisation
Number of words ■ or Bytes

4) Memory Access Methods
Sequential ■ Start at the beginning and read through in order ■ Access time depends on location of data andprevious location ■ e.g. tape
Direct ■ Individual blocks have unique address ■ Access is by jumping to vicinity plus sequentialsearch ■ Access time depends on location and previouslocation ■ e.g. disk

Memory Access Methods…
Random ■ Individual addresses identify locations exactly ■ Access time is independent of location orprevious access ■ e.g. RAM
Associative ■ Data is located by a comparison with contents ofa portion of the store ■ Access time is independent of location orprevious access ■ e.g. cache

©Javaid Khurshid

Memory Hierarchy - Diagram

©Javaid Khurshid

Memory Hierarchy – Diagram…

5) Performance

^ Access time^ ■^ Time between presenting the address andgetting the valid data
^ Memory Cycle time^ ■^ Time may be required for the memory to“recover” before next access^ ■^ Cycle time is access + recovery
^ Transfer Rate^ ■^ Rate at which data can be moved

6) Physical Types

^ Semiconductor^ ■^ RAM
^ Magnetic^ ■^ Disk & Tape
^ Optical^ ■^ CD & DVD
^ Others^ ■^ Bubble (Not used today, Stores in magnetic way)

8) Organisation

^ Physical arrangement of bits into words
^ Not always obvious
^ e.g. interleaved

The Bottom Line

^ How much?^ ■^ Capacity
^ How fast?^ ■^ Time is money
^ How expensive?

So you want fast?

^ It is possible to build a computer which usesonly static RAM
^ This would be very fast
^ This would need no cache^ ■^ How can you cash cache?
^ This would cost a very large amount

Locality of Reference
During the course of the execution of a program,memory references tend to cluster
e.g. loops
The effectiveness of a memory hierarchy dependson the principle of moving information into the fastmemory infrequently and accessing it many timesbefore replacing it with new information.
This principle is possible due to a phenomenoncalled locality of reference

, i.e. within a given period of time, programs tend to reference relatively aconfined area of memory repeatedly.