Memory Hierarchy Study Notes for Computer Science Engineering Exams

By Mukesh Kumar|Updated : October 6th, 2020

Memory Hierarchy

  • The memory unit is used for storing programs and data. It fulfills the need for storage of information.
  • The additional storage with main memory capacity enhances the performance of the general-purpose computers and makes them efficient.
  • Only those programs and data, which are currently needed by the processor, reside in the main memory. Information can be transferred from auxiliary memory to main memory when needed.
Table of Content


Cache Memory

  • A small, fast storage memory used to improve average access time Or We can say that cache is a very high-speed memory that is used to increase the speed of processing by making current programs and data available to the CPU at a rapid rate.
  • The cache is used for storing segments of programs currently being executed in the CPU and temporary data frequently needed in the present calculations.


Cache Performance

When the processor needs to read or write to a location in main memory, it first checks whether a copy of that data is in the cache. If so, the processor immediately reads from or writes to the cache.

Cache hit If the processor immediately reads or writes the data in the cache line.

Cache miss If the processor does not found the required word in cache, then cache miss has occurred.

Hit ratio Percentage of memory accesses satisfied by the cache.

Miss ratio = 1– Hit ratio

  • Temporal Locality: The word referenced now is likely to be referenced again soon. Hence it is wise to keep the currently accessed word handy for a while.
  • Spacial Locality: Words near the currently referenced word are likely to be referenced soon. Hence it is wise to prefetch words near the currently referenced word and keep them handy for a while.
  • Write through writes the data to memory as well as to the cache
  • .
  • Write back
  • : Don't write to memory now, do it later when this cache block is evicted.

Main Memory

  • The main memory refers to the physical memory and it is one central storage unit in a computer system.
  • The main memory is relatively large and fast memory used to store programs and data during the computer operation.
  • The main memory in a general purpose computer is made up of RAM integrated circuit.

Latency: The latency is the time taken to transfer a block of data either from main memory or caches.

  • As the CPU executes instructions, both the instructions themselves and the data they operate on must be brought into the registers, until the instruction/data is available, the CPU cannot proceed to execute it and must wait. The latency is thus the time the CPU waits to obtain the data.
  • The latency of the main memory directly influences the efficiency of the CPU.

Auxiliary-Memory: The common auxiliary memory devices used in computer systems are magnetic disks and tapes.

Magnetic Disks

  • A magnetic disk, is a circular plate constructed of metal or plastic coated with magnetised material.
  • Often, both sides of the disk are used and several disks may be stacked on one spindle with read/write heads available on each surface.
  • All disks rotate together at high speed. Bits are stored in the magnetised surface in spots along concentric circles called tracks. The tracks are commonly divided into sections called sectors.

Magnetic Tapes

  • A magnetic tape is a medium of magnetic recording, made of a thin magnetisable coating on a long, narrow strip of plastic film.
  • Bits are recorded as magnetic spots on the tape along several tracks. Magnetic tapes can be stopped, started to move forward or in reverse. Read/write heads are mounted one in each track, so that data can be recorded and read as a sequence of characters.

Memory Hierarchial Design

Level NameRegistersCache MemoryMain MemorySecondary memory
Typical Size<1KB<16MB<16GB>100GB
ImplementationCustomized MultiportsSRAM (Flip-Flops)DRAM (Capacitor)Magnetic devices
Bandwidth (MB/s)(20,000-1,00,000)(5,000-10,000)(1,000-5,000)(20-150)
Access Time (ns)(0.25-0.5)(0.5-2.5)(80-250)5000000
Managed ByCompilerHardwareOSOS
Backed ByCache Memory Main MemorySecondary MemoryCompact Disk

Memory Organisation

Based on the style of accessing the memory, it can be classified into two


  • Simult
  • aneous Memory Access Organi
  • sation.
  • Hiera
  • rchial Memory Access Organis
  • ation.


aneous Access Memory Organisation

  • In this memory Organisation, CPU is directly connected to all the levels of memory but accessing is allowed in a sequence i.e., Whenever there is a miss in level 1 memory, data can be accessed directly from level 2 without copying it into level 1. 
  • Levels are designed in such a way that lower the level, higher the performance, lower the access time and higher the level, lower the performance, and higher the access time.
  • Hit ratio of last level must be 1 since it represent the whole system.

Average Memory Access time= H1T1+ (1-H1)H2T2+ (1-H1)(1-H2)H3T3+.....

Where, H1, H2, H3..are the hit ratio for level 1, level 2, Level3 and

so on.


rchial Memory Access Organisation

  • In this memory organisation, CPU always access the data from the first level of memory. Whenever there is a miss in level 1, data is transferred from higher level of memory to the lower level of memory. After transfer of data, CPU access the data from lower level.
  • It is the better approach if spatial locality is needed in the program.


Average Access time= H1T1+ (1-H1)H2(T2+ T1)+ (1-H1)(1-H2)H3(T3+ T2+T1) +...

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Mukesh KumarMukesh KumarMember since Feb 2020
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Post material for SBI-SO Systems Manager too
Dharmedra kumar Lodhi
@Mallesham Devasane  please post hardware implementation of cache
Pratap Kumar

Pratap KumarJan 11, 2017

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Pages from COA.pdf
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Amruta Korde

Amruta KordeSep 14, 2019

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Cnu Ch

Cnu ChSep 24, 2019

Please send computer organization notes

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