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MemoryMemory

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What is memory and What is memory and Why it is requiredWhy it is required A computer uses A computer uses

memory to hold memory to hold temporary temporary instructions and data instructions and data needed to complete needed to complete tasks.tasks.

This enables the This enables the computer’s Central computer’s Central Processing Unit, to Processing Unit, to access instructions access instructions and data stored in and data stored in memory very quickly. memory very quickly.

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Difference between Difference between Memory and storageMemory and storage The term memory refers to the amount of RAM The term memory refers to the amount of RAM

installed in the computer, whereas the term installed in the computer, whereas the term storage refers to the available amount of hard storage refers to the available amount of hard disk capacity. disk capacity.

Another important difference between memory Another important difference between memory and storage is that the information stored on a and storage is that the information stored on a hard disk remains intact even when the computer hard disk remains intact even when the computer is turned off. However, any data held in memory is turned off. However, any data held in memory is cleared when the computer is turned off. is cleared when the computer is turned off.

The computer memory holds any changes you The computer memory holds any changes you make to a document until you save the changes make to a document until you save the changes to a disk. If anything interrupts the computer’s to a disk. If anything interrupts the computer’s operation— such as a power outage or system operation— such as a power outage or system error— any changes made, but not saved, are error— any changes made, but not saved, are lost.lost.

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Types of MemoryTypes of Memory

DDRDDR DDR-2DDR-2

Further it is classified intoFurther it is classified into Registered Registered UnbufferedUnbuffered FBDFBD

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What is DDR memoryWhat is DDR memory

DDR stands for DDR stands for DDouble ouble DData ata RRateate DDR reads data on both the rising DDR reads data on both the rising

and falling edges of the clock signal.and falling edges of the clock signal. The double data rate architecture is The double data rate architecture is

essentially a 2n prefetch essentially a 2n prefetch architecture, with an interface architecture, with an interface designed to transfer two data words designed to transfer two data words per clock cycle at the I/O pins.per clock cycle at the I/O pins.

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DDR MemoryDDR Memory

Number of Pin=184Number of Pin=184

Voltage Requirement=2.5VVoltage Requirement=2.5V

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DDR-2 MemoryDDR-2 Memory

Number of Pins=240Number of Pins=240 Voltage=1.8VVoltage=1.8V The I/O buffers are clocked at

twice the core frequency. In order to satisfy the output, DDR I prefaces 2 bits, DDR II needs to prefects four bits with every read command and pipeline them for time multiplexed output.

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Registered MemoryRegistered Memory

On Registered DIMM's the address and control signals from On Registered DIMM's the address and control signals from your motherboards chipset get intercepted and buffered by your motherboards chipset get intercepted and buffered by two or three register chips on the module .two or three register chips on the module .

Store and forward method is implemented.Store and forward method is implemented.

Because the address and control signals only have to go to Because the address and control signals only have to go to the register chips on a Registered DIMM, instead of each the register chips on a Registered DIMM, instead of each and every RAM chip on all the memory modules installed in and every RAM chip on all the memory modules installed in your computer, the load on your motherboard chipset's your computer, the load on your motherboard chipset's address and control pins is greatly decreased.address and control pins is greatly decreased.

This decrease in load allows the motherboard's chipset to This decrease in load allows the motherboard's chipset to drive the address and control signals quickly enough to drive the address and control signals quickly enough to remain stable when a large amount of memory is installed. remain stable when a large amount of memory is installed.

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Registered MemoryRegistered Memory

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Unbuffered MemoryUnbuffered Memory

Memory controller drives the Memory controller drives the memory directly.memory directly.

Suitable for Gaming applicationSuitable for Gaming application

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Single Channel and Single Channel and Dual Channel MemoryDual Channel Memory

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To illustrate the difference between single- and dual-channelTo illustrate the difference between single- and dual-channel

In this illustration, single-channel memory is like a funnel that feeds data to the processor engine through a single pipe.Data is transferred 64 bits at a time.

Peak Bandwidth is calculated as:Memory Speed x Number of bytes transferred per channel x Number of Channels(its data rate) (8 Bytes or 64 bits) (one or two)For PC3200, also known as DDR400, modules on a dual-channel motherboard,Peak Bandwidth = (400 MHz) x (8 Bytes) x (2 Channels)= 6400 Megabytes per second (MB/s)or 6.4 Gigabytes per second (GB/s)

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How to calculate how How to calculate how much memory is in a much memory is in a modulemodule A module consists of a group of A module consists of a group of

chips. If you add together the chips. If you add together the capacities of all the chips on the capacities of all the chips on the module, you get the total module, you get the total capacity of the module. capacity of the module.

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COMPONENT CAPACITY EXPRESSION CAPACITY UNITS EXAMPLE

Chips Chip Density Mbit (megabits) 64Mbit

Memory Modules Module Capacity MB (megabytes) 64MB

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It's easy to calculate the capacity of a It's easy to calculate the capacity of a memory module if you know the memory module if you know the capacities of the chips on it. capacities of the chips on it.

If there are eight 64Mbit chips, it's a If there are eight 64Mbit chips, it's a 512Mbit module. 512Mbit module.

The capacity of a module is described The capacity of a module is described in megabytes, not megabits, you have in megabytes, not megabits, you have to convert bits to bytes. to convert bits to bytes.

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What are the What are the components present in a components present in a memory modulememory module PCBPCB DRAMDRAM SPD - SPD - When a computer system boots

up, it must “detect” the configuration of the memory modules in order to run properly. Parallel Presence Detect is the traditional method of relaying the required information by using a number of resistors. PPD is the method SIMMs and some DIMMs use to identify themselves. Serial Presence Detect uses an EEPROM (Electrically Erasable Programmable Read-Only Memory) to store information about the module.

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DDR/DDR-2 DDR/DDR-2 ComparisonComparison

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Error Correction Code TechniqueError Correction Code Technique

It is a more comprehensive method of data-It is a more comprehensive method of data-integrity checking that can detect and correct integrity checking that can detect and correct single-bit errors. Due to price competition, it is single-bit errors. Due to price competition, it is becoming more common among personal becoming more common among personal computer manufacturers not to use data computer manufacturers not to use data integrity checking. They are eliminating the integrity checking. They are eliminating the need for more expensive parity memory, for need for more expensive parity memory, for example, to lower the price of their computers. example, to lower the price of their computers. (This trend is complemented by the increased (This trend is complemented by the increased quality of memory components available from quality of memory components available from certain manufacturers and, as a result, the certain manufacturers and, as a result, the relative infrequency of memory errors.)relative infrequency of memory errors.)