3 .0 CO M PUT ER M EM O RY
3 . 1 M A I N M E M O RY
3 . 2 C AC H E M E M O RY
3 . 3 I N T E R N A L P RO C E S S O R M E M O RY
3 . 4 V I RT UA L S TO R AGE
Aspects Volatile Memory Non – volatile memory
Definition Memory that loses its
contents when the computer
or hardware device loses
power.
Memory that keep its content
even if the power is lost
Example RAM – is why if your
computer freezes or reboots
when working on a program,
you lose anything that hasn’t
been saved.
EPROM - erasable
programmable read-only
memory
Others NVRAM – non volatile random
access memory
is programmable read-only
memory (programmable ROM)
that can be erased and re-used
MEMORY VS STORAGE
RAM is memory / primary
memory
Hard disk – storage / secondary
storage
Hard Drive space is always more
than RAM
• Cache hit -Data found in cache. Results in data transfer at maximum speed.
• Cache miss- Data not found in cache. Processor loads data from M and copies into
cache. This results in extra delay, called miss penalty.
• Hit ratio = percentage of memory accesses satisfied by the cache.
• Miss ratio = 1-hit ratio
• Cache is partitioned into lines (also called blocks). Each line has 4-64 bytes in it.
During data transfer, a whole line is read or written. Each line has a tag that
indicates the address in M from which the line has been copied.
HOW MEMORY IS USED ???• When a program such as your Internet browser is open, it is loaded from your hard drive
and placed into RAM, which allows that program to communicate with the processor at
higher speeds.
• Anything you save to your computer, such as a picture or video, is sent to your hard drive
for storage.
Types of RAMEDO
RAM
SDRAM
DDR
RAM
DDR2 RAM
DDR3 RAM
DDR4 RAM
3.1 MAIN MEMORY
• Refers to physical memory that is internal to the computer. (RAM)
• The word main is used to distinguish it from external mass storage devices such
as disk drives.
• Programs and data copied from secondary memory into main memory where the
processor can interact with them.
• A copy remains in secondary memory
• One megabyte of memory is enough to hold approximately one million (106)
characters of a word processing document.
• Nothing permanent is kept in main memory.
3.2 CACHE MEMORY
• Term cache refers to hidden
• = CPU memory
• Is RAM that a computer microprocessor can access more quickly than it can access regular RAM.
• The function of the cache memory is to reduce latency of MM data access
• The Cache is a memory that stores temporary data, in "silent mode" to upper level of
utilization, for a quick reusing.
• Locality of reference, also known as the principle of locality, is a term for the
phenomenon in which the same values, or related storage locations, are frequently accessed,
depending on the memory access pattern
Temporal
Data which have been used
recently have high likelihood of
being used again.
Spatial
If a data is referenced, it is
very likely that nearby data
will be accessed soon
Fully Associative Cache
Directory Data
Data attribute bits
/ state Data address ADD
(address of memory block)
2 fields
2 arrays
“No restriction on mapping
from M to C.” Associative
search of tags is expensive.
Feasible for very small size
caches only.
A given memory block
can be mapped into one
and only cache line.
• Time Average memory access time = Hit time + Miss rate x Miss penalty
• Assume that
• Hit time = 5 ns (nano seconds)
• Miss rate = 10%
• Miss penalty = 100 ns (nano seconds)
• The average memory access time = 15 ns.
• Better performance at a cheaper price.
DIRECT MAPPED CACHE
Advantage Disadvantage
No need of expensive
associative search
Miss rate may go
up due to
possible increase
of mapping
conflicts
Each M-block can now be mapped into any one
of a set of N C-blocks. The sets are predefined.
Let there be K blocks in the cache. Then
N = 1 Direct-mapped cache
N = K Fully associative cache
Most commercial cache have N= 2, 4, or 8.
Cheaper than a fully associative cache.
Lower miss ratio than a direct mapped cache.
ADDRESS TRANSLATION: AN EX AMPLE
Main memory size = 2 KB
Block size = 8 bytes
Cache size = 64 bytes
Set size = 2
No. of sets in cache = 4
To locate an M-block in cache, check
the tags in the set S = (M-block) mod
(number of sets) i.e. the index field.
CACHE MEMORY LEVEL EXPL AINED
• Cache memory is fast and expensive. Traditionally, it is categorized as "levels" that describe its
closeness and accessibility to the microprocessor:
• Level 1 (L1) cache is extremely fast but relatively small, and is usually embedded in the
processor chip (CPU).
• Level 2 (L2) cache is often more capacious than L1; it may be located on the CPU or on a
separate chip or coprocessor with a high-speed alternative system bus interconnecting the
cache to the CPU, so as not to be slowed by traffic on the main system bus.
• Level 3 (L3) cache is typically specialized memory that works to improve the performance
of L1 and L2. It can be significantly slower than L1 or L2, but is usually double the speed of
RAM. In the case of multicore processors, each core may have its own dedicated L1 and L2
cache, but share a common L3 cache. When an instruction is referenced in the L3 cache, it is
typically elevated to a higher tier cache.
• By keeping as much of this information as possible in SRAM, the computer avoids
accessing the slower DRAM, making the computer perform faster and more efficiently.
Today, most computers come with L3 Cache or L2 cache, while older computers
included only L1 cache. Below is an example of the Intel i7 processor and its shared L3
cache.
• Cache, Internet browser cache, or temporary Internet files with an Internet
browser, like Google Chrome, Firefox, or Internet Explorer, is used to improve how
fast data loads while browsing the Internet.
• In most cases, each time you open a web page, the page and all its files are sent to
the browser's temporary cache on the hard drive. If the web page and files contained
on that web page (e.g. pictures) need to load again and have not been modified, the
browser opens the page from your cache instead of downloading the page again.
• Cache saves lots of time, especially if you use a modem, and can also help save on
bandwidth for the website owner.
• Anyone concerned about their privacy or wants to hide pages they have visited on
the computer can clear their Internet cache (history).
• Like memory caching, disk caching is used to access commonly accessed data.
However, instead of using high-speed SRAM, a disk cache uses conventional main
memory.
• The most recently accessed data from a disk is stored in a memory buffer. When a
program needs to access data from the disk, it first checks the disk cache to see if the
data is there.
• Disk caching can dramatically improve the performance of applications because accessing
a byte of data in RAM can be thousands of times faster than accessing a byte of data on a
hard drive.
• A cache server is a computer or network device set up to store web pages that have
been accessed by users on a network. Any user trying to access a web page stored on the
cache server is sent the stored version, instead of downloading the web page again.
• Cache servers help reduce network and Internet traffic congestion, as well as save the
company on bandwidth costs.
INTERNAL PROCESSOR MEMORY
• Typically refers to main memory, but may also refer to ROM and flash memory.
• In either case, internal memory generally refers to chips rather than disks or
tapes.
• RAM – A series of chips on the motherboard
VIRTUAL STORAGE
• A system for storing computer data in more than one place so that it can be used by any
computer connected to the system, for example over the internet
• Virtual memory
Answer ALL questions.
1. What is computer memory?
2. Write the full name for the acronym given:
ROM: .......................................................................
RAM: .......................................................................
3. State the characteristic of ROM.
4. State the characteristic of RAM.
5. What is cache memory?
6. What is the function of virtual storage?
7. Give TWO examples of virtual storage.
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