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Memory Hierarchy

Introduction Memory unit is an essential component in any digital computer as it is needed for storing programs and data.

A computer is equipped with a hierarchy of memory subsystems,some internal to the system (directly accessible by the processor),and some external (accessible by the processor via an I/Omodule)

External memory consists of peripheral storage devices, such asdisk and tape, that are accessible to the CPU via I/O controllers.External memory can also be referred as secondary memory or auxiliary memory .

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I nternal memory is equated with main memory. But thereare other forms of internal memory like CPU requires its own

local memory in the form of registers. Internal memory isalso called as main memory/Primary memory .

Main memory can be classified as1) Volatile:

-- RAM (Random Access Memory)-- RAM is working memory. Data can be read or written inRAM with the help of address location and when the data is

no longer needed we can use the storage location for writingagain.-- Contents of volatile memory are vanished when power supply is switched off.

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T here are three key characteristics of memory :

COST CAPACITY ACCESS TIMEThe relationship between them is as follows:

G reater capacity, smaller cost per bit

G reater capacity, greater access time

Smaller access time, greater cost per bitThe overall goal of using a memory hierarchy is to obtain thehighest possible average access speed while minimizing totalcost of entire memory system.

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Memory hierarchy

L0:

L1:

L2:

L3:

L4:

Registers

Cache

Magnetic disk

Magnetic tape

Main Memory Larger,slower,

and cheaper (per byte)

Smaller,faster,and

costlier (per byte)

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C ontemporary memory hierarchy

Registers

Cache

Main Memory

Magnetic disk

Magnetic tape Optical disk

Disk cache

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In both the above figures as we go down following occurs :

a. Decreasing cost/bit b. Increasing capacity

c. Increasing access time

d. Decreasing frequency of access of memory by the CPU

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Random-Access Memory (RAM)Random-Access Memory (RAM)K ey features

RAM is packaged as a chip. Basic storage unit is a cell (one bit per cell). Multiple RAM chips form a memory.Static RAM (SRAM) Each cell stores bit with inverter, transistor circuit.

Retains value indefinitely, as long as it is kept powered. Relatively insensitive to disturbances such as electrical

noise. Faster and more expensive than DRAM. Access time is about 10 nsDynamic RAM (DRAM) Each cell stores bit with a capacitor and transistor. Value must be refreshed every 10-100 ms. Sensitive to disturbances.

Slower and cheaper than SRAM. Access time is about 50 ns

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Advanced DRAM Organization

Enhanced DRAM Cache DRAM Synchronous DRAM Rambus DRAM

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Enhanced DRAMSimplest of new DRAM architectures

Developed by RamtronIntegrates a small SRAM cache onto a DRAM chipRefresh operation can be conducted in parallel with cacheread operationIt has separate read path and write path so it enables asubsequent read access to cache in parallel with thecompletion of write operation.

Cache DRAMDeveloped by MitsubishiSimilar to EDRAM, includes a larger SRAM cache thanthe EDRAMSRAM on CDRAM can be used as either true cache or asa buffer to support the serial access to a block of data.Buffer stores most recently accessed data.

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Synchronous DRAMJointly developed by no of companies

It exchanges data with the processor synchronized to anexternal clock signalIt runs at speed equivalent to that of processor / memory

bus.Data moves in and out from DRAM under the control of system clock.It has dual bank internal architectureSDRAM includes important key features like Moderegister and associated control logic. It provides a

mechanism to customize SDRAM according to systemneeds.It performs best when transferring large blocks of dataserially, e.g. in word processing, multimedia etc.

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Rambus DRAMDeveloped by Rambus

RDRAM chips are vertical packages, with all pins on oneside.fastest current memory technologies used by PCs.

Normally SRAM can deliver data at a maximum speed of about 100 MHz, RDRAM transfers data at up to 800MHzRDRAM is used with Pentium III Xeon processors andmore recently it is being used with Pentium 4 processors

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T ype of Main Memory

Semiconductor chips are used for main memory M emoryType

Category Erasure Write M echanism

Volatility

RAM Read-writememory

Electrically, byte level

Electrically Volatile

ROM Read-onlymemory

Not possible

Masks

Non-Volatile

PROM

Electrically

EPROM

Read-mostlymemory

UV light,

chip levelFlashmemory

Electrically, block-level

EEPROM Electrically,

byte level