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1. DSD PRESENTATION
ON
VIRTUAL AND CACHE
MEMORY
Submitted By – Animesh Jain

2. What is Cache Memory?
Cache memory is a type of
memory that lies between the CPU
(Central Processing Unit) and the RAM
(Random Access memory).

3. The purpose of cache memory is to reduce the
memory access time of the CPU from the RAM.
The cache memory is much faster than RAM. So
access time on cache is much lesser than the
access time on RAM. But the cost of memory
used for cache memory is much higher than the
cost of memory used for RAM, and hence, the
capacity of cache memory is very small.
The type of memory used for cache memory is
called SRAM ( Static Random Access Memory).

6. Whenever the CPU wants to access memory, it first
checks whether what it needs resides in the cache
memory. If yes, it would be able to access it with the
least latency. If it does not reside in cache, then the
requested content would be copied from RAM to the
cache and then only the CPU will access it from the
cache. Here, when copying content from the cache, not
only the content in the requested memory address but
also the nearby content is copied to cache. So, the next
time there is a high probability for a cache hit to occur as
most computer programs access nearby data or last
accessed data most of the times. So due to the cache,
the average memory latency is reduced.

8. In CPU, there are three types of
caches: Instruction cache to store program
instructions, Data cache to store data items,
and the Translation Look-aside Buffer to store
memory mappings. For data cache, generally,
there are multi-level caches. That is, there are
several caches as L1, L2 and L3. L1 cache is
the fastest but smallest cache memory that is
closest to the CPU. L2 cache is slower than L1,
but larger than L1 and resides after the L1
cache. Because of this hierarchy a better
average memory access time can be achieved
at a less cost.

9. What is Virtual Memory?
Virtual memory is a memory management
technique used in computers systems. There is
no hardware called virtual memory, but it is a
concept that uses RAM and the hard disk to
provide a virtual address space for programs.
First RAM is divided into chunks
called pages and they are identified by physical
memory addresses. In the hard disk, a special
portion is reserved where, in Linux, it is called
the swap and, in Windows, it is called a page
file. When a program is started, it is given a
virtual address space that can be even larger
than the actual physical memory.

10. Virtual memory space is also divided into
chunks called pages and each of this virtual
memory page can be mapped to a physical
page. The table called page table keep track of
this mapping. When the physical memory runs
out of space, what is done is, certain physical
pages are pushed to that special portion in the
hard disk. When any page pushed to the hard
disk is needed again, it is brought to the
physical memory by putting another selected
page from the physical memory to the hard
disk.

12. What is the difference between
Cache Memory and Virtual Memory?
• Cache memory is a type of memory used
for improving the main memory access time.
It is a faster type of memory that resides
between CPU and RAM to reduce the
average memory access latency. Virtual
memory is a memory management method
where it is a concept that lets programs get
its own virtual memory space, which is even
larger than the real physical RAM available.

13. • Cache memory is a type of hardware
memory that actually exists physically. On
the other hand, there is no hardware called
virtual memory as it is a concept that uses
RAM, hard disk, Memory management unit,
and software to provide a virtual type of
memory.
• Cache memory management is done fully
by hardware. Virtual memory is managed by
the operating system (software).

14. • Cache memory lies between RAM and the
processor. Data transfers involve RAM, cache
memory, and the processor. Virtual memory, on
the other hand, involves transfer of data
between RAM and hard disk.
• Cache memories take small sizes such
as Kilobytes and Megabytes. Virtual memory,
on the other hand, involves huge sizes that
take gigabytes.
• Virtual memory involves data structures such
as page tables that store mapping between
physical memory and virtual memory. But this
type of data structures is not necessary for
cache memory.

15. Summary:
Cache Memory vs Virtual Memory
Cache memory is used for improving the main memory
access time while virtual memory is a memory
management method. Cache memory is an actual
hardware, but there is no hardware called virtual
memory. RAM, hard disk, and various other hardware
together with the operating system produces the concept
called virtual memory to provide large and isolated
virtual memory spaces to each program. The content in
the cache memory is managed by hardware while the
content in the virtual memory is managed by the
operating system.

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REFERENCE

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