different types of memory
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Computer Memory
Somnath Chaudhuri
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9th April2011
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Introduction
Hardware refers to the physical equipment used for theinput, processing, output and storage activities of acomputer system.
Central processing unit (CPU) manipulates the data andcontrols the tasks performed by the other components.
Primary storage internal to the CPU; temporarily stores
data and program instructions during processing.
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How the CPU Works
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How the CPU Works (Continued)
Binary form: The form in which data and instructions can beread by the CPU only 0s and 1s.
Machine instruction cycle: The cycle of computer processing,whose speed is measured in terms of the number of instructionsa chip processes per second.
Clock speed: The preset speed of the computer clock that timesall chip activities, measured in megahertz and gigahertz.
Word length: The number of bits (0s and 1s) that can beprocessed by the CPU at any one time.
Bus width: The size of the physical paths down which the dataand instructions travel as electrical impulses on a computerchip.
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What is Memory
Main memory consists of a number ofstorage locations, each of which is
identified by a unique address
The ability of the CPU to identify
each location is known as its
addressability
Each location stores a word i.e. the
number of bits that can be processed
by the CPU in a single operation.
Word length may be typically 16, 24,
32 or as many as 64 bits.
A large word length improves system
performance, though may be less
efficient on occasions when the full word
length is not used
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Computer Memory
Two basic categories of computer memory:
Primary storage andsecondary storage.
Primary stores small amounts of data and information
that will be immediately used by the CPU.
Secondary stores much larger amounts of data and
information (an entire software program, for example)
for extended periods of time.
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Primary Storage
Primary storage ormainmemory stores three types ofinformation for very brief periods of time:
Data to be processed by the CPU;
Instructions for the CPU as to how to process the data;
Operating system programs that manage variousaspects of the computers operation.
Primary storage takes place in chips mounted on thecomputers main circuit board, called the motherboard.
Four main types of primary storage: register, random
access memory (RAM), cache memory and read-onlymemory (ROM).
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Types of Primary Storage
Registers: registers are part of the CPU with the leastcapacity, storing extremely limited amounts ofinstructions and data only immediately before andafter processing.
Random access memory (RAM): The part ofprimary storage that holds a software program andsmall amounts of data when they are brought fromsecondary storage.
Cache memory: A type of primary storage where thecomputer can temporarily store blocks of data usedmore often.
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Types Primary Storage (Continued)
Read-only memory (ROM): Type of primarystorage where certaincritical instructions aresafeguarded;the storage is nonvolatileand retainsthe instructions whenthe power to the computeris turned off.
Flash memory: A form ofrewritable read-onlymemory thatis compact, portable, and requireslittle energy.
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Random Access Memory (RAM)
holds its data as long as the computer is switched on
All data in RAM is lost when the computer is switched off
Described as being volatile
It is direct access as it can be both written to or read from in any order
Its purpose is to temporarily hold programs and data for processing.
In modern computers it also holds the operating system
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Types ofRAM
1. Dynamic Random Access Memory (DRAM)
Contents are constantly refreshed 1000 times per second
Access time 60 70 nanoseconds
2. Synchronous Dynamic Random Access Memory (SDRAM)
Quicker than DRAM Access time less than 60 nanoseconds
3. Direct Rambus Dynamic Random Access Memory (DRDRAM)
New type of RAM architecture
Access time 20 times faster than DRAM
More expensive
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4. Static Random Access Memory (SRAM)
Doesnt need refreshing
Retains contents as long as power applied to the chip
Access time around 10 nanoseconds
Used forcache memory
Also fordate and time settings as powered by small battery
Types ofRAM
5. Video Random Access memory
Holds data to be displayed on computer screen
Has two data paths allowing READ and WRITE to occur at the same time
A systems amount of VRAM relates to the number of colours and resolution
A graphics card may have its own VRAM chip on board
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Read only memory (ROM)
ROM holds programs and data permanently even when computer isswitched off
Data can be read by the CPU in any order so ROM is also direct access
The contents of ROM are fixed at the time of manufacture
Stores a program called the bootstrap loaderthat helps start up the
computer
Access time of between 10 and 50 nanoseconds
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Types ofROM
3. Electrically Erasable Programmable Read Only Memory (EEPROM)
Reprogrammed electrically without using ultraviolet light
Must be removed from the computer and placed in a special machine to do this
Access times between 45 and 200 nanoseconds
4. Flash ROM
Similar to EEPROM
However, can be reprogrammed while still in the computer
Easier to upgrade programs stored in Flash ROM
Used to store programs in devices e.g. modems
Access time is around 45 90 nanoseconds
5. ROM cartridges
Commonly used in games machines
Prevents software from being easily copied15
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Caches
The rate of data fetching by the CPU from the main memory isabout 100 times faster than from secondary memory. But
there is also a mismatch between main memory and CPU.
CPU can process the data 10 times faster than the main
memory. Which limits the performance of the CPU due to
mismatch in CPU and main memory speed. So Cache memory
act as a buffer b/w main memory and CPU.
Cache: A smaller, high speed storage device used to increase
the speed of processing by making current programs and data
available to the CPU at a rapid rate. The basic characteristic ofcache memory is its fast access time.
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Small amount of memory typically 256 or 512 kilobytes
Temporary store for often used instructions
Level 1 cache is built within the CPU (internal)
Level 2 cache may be on chip or nearby (external)
Faster for CPU to access than main memory
Cache memory
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The operation of cache memory
1. Cache fetches data
from next to current
addresses in main
memory
2. CPU checks to see
whether the next
instruction it requires is in
cache
3. If it is, then the
instruction is fetched from
the cache a very fast
position
4. If not, the CPU has to
fetch next instructionfrom main memory - a
much slower process
Main
Memory
(DRAM)
CPU
Cache
Memory
(SRAM)
= Bus connections 18
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Virtual memory
Uses backing storage e.g. hard disk as a temporary location for
programs and data where insufficient RAM available
Swaps programs and data between the hard-disk and RAM as the
CPU requires them for processing
A cheap method of running large or many programs on a computer
system
Cost is speed: the CPU can access RAM in nanoseconds but hard-
disk in milliseconds
Virtual memory is much slower than RAM
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Secondary Storage
Memory capacity that can store very large amounts of
data for extended periodsof time.
It is nonvolatile.
It takes much more time to retrieve data because ofthe electromechanical nature.
It is cheaper than primary storage.
It can take place on a variety of media
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Secondary storage
Sequential access Direct access
Magnetic tape
Magnetic disk Optical disk
Floppy disk Hard disk CD-ROM WORM
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Secondary Storage (Continued)
Hard disk: A form of secondary storage that stores data onplatters divided into concentric tracks and sectors, which can be
read by a read/write head that pivots across the rotating disks.
Floppy disk: A form of easily portable secondary storage onflexible disks; also called floppy disks.
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Optical storage devices: A form of secondary storage inwhich a laser reads the surface of a reflective plastic platter.
Compact disk, read-only memory (CD-ROM): A form ofsecondary storage that can be only read and not written on.
Digital video disk (DVD): An optical storage device used tostore digital video or computer data.
Fluorescent multilayer disk (FMD-ROM): An opticalstorage device with much greater storage capacity than DVDs.
Secondary Storage (Continued)
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Memory Hierarchy
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An Example Memory Hierarchy
registers
on-chip L1
cache (SRAM)
main memory
(DRAM)
local secondary storage(local disks)
Larger,
slower,
and
cheaper
(per byte)
storage
devices
remote secondary storage
(distributed file systems, Web servers)
Local disks hold files
retrieved from disks on
remote network servers.
Main memory holds disk
blocks retrieved from local
disks.
off-chip L2
cache (SRAM)
L1 cache holds cache lines retrieved
from the L2 cache memory.
CPU registers hold words retrieved from
L1 cache.
L2 cache holds cache lines retrievedfrom main memory.
L0:
L1:
L2:
L3:
L4:
L5:
Smaller,faster,
and
costlier
(per byte)
storage
devices
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