Difference between Simultaneous and Hierarchical Access Memory Organisations
Last Updated : 23 Sep, 2024
In the Computer System Design, memory organization is important in order to enhance system performance and efficiency . So, two ways have been devised on the basis of which CPU tries to access different levels of Memory. These two types include Simultaneous Access Memory Organization and Hierarchical Access Memory Organization . Let's explore these memory organizations through this article.
What is Simultaneous Access Memory Organization?
In this memory organization, CPU can access the data simultaneously from all levels of memory i.e. if it was trying to access data from level 1 but it didn't get. So, it can check in other levels at the same time as shown in the figure below.
Figure -Simultaneous Access Memory Organization
Average Memory Access Time for Simultaneous Access Memory Organization
Let there be total three levels in a memory i.e. L1, L2, L3 and H1, H2,H3 are the hit ratios for data to be found in respective levels. T1, T2, T3 be the access time for these levels. Then Average Memory Access time for Simultaneous Access Memory Organization is
Average Memory Access Time = H1*T1 + (1-H1)*H2*T2 + (1-H1)*(1-H2)*H3*T3 = H1*T1 + (1-H1)*H2*T2 + (1-H1)*(1-H2)*1*T3 = H1*T1 + (1-H1)*H2*T2 + (1-H1)*(1-H2)*T3
Because the chances to get data in L3 will be 100% if it is not found in L2 and L3 , so H3=1 is used here.
What is Hierarchical Access Memory Organization?
In this memory organization, CPU can't access data simultaneously from all levels of memory as it is only connected to level 1 of memory here. So, the data will come from other levels to the level 1 then CPU will get its access as shown in the figure below.
Figure - Hierarchical Access Memory Organization
Average Memory Access Time for Hierarchical Access Memory Organization
Let there be total three levels in a memory i.e. L1, L2, L3 and H1, H2,H3 are the hit ratios for data to be found in respective levels. T1, T2, T3 be the access time for these levels. Then Average Memory Access time for Hierarchical Access Memory Organization is
Average Memory Access Time = H1*T1 + (1-H1)*H2*(T1+T2) + (1-H1)*(1-H2)*H3*(T1+T2+T3) = H1*T1 + (1-H1)*H2*(T1+T2) + (1-H1)*(1-H2)*1*(T1+T2+T3) = H1*T1 + (1-H1)*H2*(T1+T2) + (1-H1)*(1-H2)*(T1+T2+T3)
Again, H3=1 will be used here as it was used above for average memory access time in case of Simultaneous Access Memory Organization.
Also, if data is not found in L1 then time to access data will be T1+T2 while checking for it in L2 and so on for further levels.
Difference between Simultaneous and Hierarchical Access Memory Organizations
| Simultaneous Access Memory Organization | Hierarchical Access Memory Organization |
|---|
| In this organization, CPU is directly connected to all the levels of Memory. | In this organization, CPU is always directly connected to L1 i.e. Level-1 Memory only. |
| CPU accesses the data from all levels of Memory simultaneously. | CPU always accesses the data from Level-1 Memory. |
| For any "miss" encountered in L1 memory, CPU can directly access data from higher memory levels (i.e. L2, L3, .....Ln). | For any "miss" encountered in L1 memory, CPU cannot directly access data from higher memory levels(i.e. L2, L3, .....Ln). First the desired data will be transferred from higher memory levels to L1 memory. Only then it can be accessed by the CPU. |
| If H1 and H2 are the Hit Ratios and T1 and T2 are the access time of L1 and L2 memory levels respectively then the Average Memory Access Time can be calculated as: T=(H1*T1)+((1-H1)*H2*T2 | If H1 and H2 are the Hit Ratios and T1 and T2 are the access time of L1 and L2 memory levels respectively then the Average Memory Access Time can be calculated as:
T=(H1*T1)+((1-H1)*H2*(T1+T2) |
NOTE:
- By default the memory structure of Computer Systems is designed with Hierarchical Access Memory Organization. It is so because in this type of memory organization the average access time is reduced due to locality of references.
- Simultaneous access Memory organization is used for the implementation of Write Through Cache.
- In both types of memory organization, the Hit Ratio of last memory level is always 1.
Conclusion
In conclusion, Simultaneous Access Memory Organization is a technique that allows CPU to check for the required data in all levels of memory at the same time. While Hierarchical Access Memory Organization is a technique that allows CPU to access only level 1 of memory for searching the required data. If not found, then it will fetch data from higher levels to level 1 using locality of reference. In this way, both techniques can be used as per their requirement so that system performance can be enhanced.
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