these slides includes the memory hierarchy, and the details of memory installed on particular computer system

1. Operating System
Unit - 2
MEMORY HIERARCHY IN OPERATING SYSTEM BY RAM K PALIWAL
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2. Memory Hierarchy
The design constraints on a computer’s memory can be summed up by three
questions:
1. How much?
2. How fast?
3. How expensive?
As might be expected, there is a tradeoff among the three key characteristics of
memory: namely,
• Capacity
• Access time
• Cost.
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3. Memory Hierarchy
A variety of technologies are used to implement memory systems,
and across this spectrum of technologies, the following relationships
hold:
• Faster access time, greater cost per bit
• Greater capacity, smaller cost per bit
• Greater capacity, slower access speed
The way out of this dilemma is to not rely on a single memory
component or technology, but to employ a memory hierarchy.
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4. Memory
Hierarchy
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5. Memory Hierarchy
A typical hierarchy is illustrated in last diagram. As one goes down
the hierarchy, the following occur:
◦ a. Decreasing cost per bit
◦ b. Increasing capacity
◦ c. Increasing access time
◦ d. Decreasing frequency of access to the memory by the processor
Thus, smaller, more expensive, faster memories are supplemented
by larger, cheaper, slower memories. The key to the success of this
organization decreasing frequency of access at lower levels.
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6. Memory Hierarchy
This Memory Hierarchy Design is divided into 2 main types:
External Memory or Secondary Memory –
Comprising of Magnetic Disk, Optical Disk, Magnetic Tape i.e.
peripheral storage devices which are accessible by the processor via
I/O Module.
Internal Memory or Primary Memory –
Comprising of Main Memory, Cache Memory & CPU registers. This is
directly accessible by the processor.
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7. Memory Hierarchy Interaction
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8. Memory Access Methods
Each memory type, is a collection of numerous memory locations. To
access data from any memory, first it must be located and then the
data is read from the memory location. Following are the methods to
access information from memory locations:
◦ Random Access: Main memories are random access memories, in which each
memory location has a unique address. Using this unique address any
memory location can be reached in the same amount of time in any order.
◦ Sequential Access: This methods allows memory access in a sequence or in
order.
◦ Direct Access: In this mode, information is stored in tracks, with each track
having a separate read/write head.
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9. Memory Hierarchy
Cache Memory
The data or contents of the main memory that are used again and again by CPU, are stored in
the cache memory so that we can easily access that data in shorter time.
Hit Ratio
The performance of cache memory is measured in terms of a quantity called hit ratio. When the
CPU refers to memory and finds the word in cache it is said to produce a hit. If the word is not
found in cache, it is in main memory then it counts as a miss.
The ratio of the number of hits to the total CPU references to memory is called hit ratio.
Hit Ratio = Hit/(Hit + Miss)
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10. References
1. Operating System Concept ninth Edition by Abraham Silberschatz and Galvin
2. Operating Systems, internals and Design Principles, sixth edition by William Stalling
3. https://www.studytonight.com/computer-architecture/memory-organization
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