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The Ultimate Memory Guide

More about Memory Technologies

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More about memory technologies

DRAM chips come primarily in three forms: DIP (Dual In-line Package), SOJ (Small Outline J-lead), and TSOP (Thin, Small Outline Package). Each is designed for specific types of applications.

DIP Integrated Circuit

SOJ DRAM Package TSOP DRAM Package

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SIMM module identification

SIMMs, just like the DRAM chips that comprise them, are specified in terms of depth and width, which indicate the SIMM's capacity and whether or not it supports parity. Here are some examples of popular 30- and 72-pin SIMMs. Note that the parity SIMMs are distinguished by the `x9' or `x36' format specification.




Note that the parity SIMMs are distinguished by the "x 9" or "x 36" format specifications. This is because parity memory adds a parity bit to every 8 bits of data. So, a 30-pin SIMM provides 8 data bits per cycle, plus a parity bit, which equals 9 bits;

72-pin SIMMs provide 32 bits per cycle, plus 4 parity bits, which equals 36 bits.





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Refresh

A memory module is made up of electrical cells. The refresh process recharges these cells, which are arranged on the chip in rows. The refresh rate refers to the number of rows that must be refreshed.

Two common refresh rates are 2K and 4K. The 2K components are capable of refreshing more cells at a time and they complete the process faster; therefore, 2K components use more power than 4K components.

Other specially-designed DRAM components feature self refresh technology, which enables the components to refresh on their own -- independent of the CPU or external refresh circuitry. Self refresh technology, which is built into the DRAM chip itself, reduces power consumption dramatically. It is commonly used in notebook and laptop computers.

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3.3-volt versus 5-volt

Computer memory components operate at either 3.3 volts or 5 volts. Until recently, 5 volts was the industry standard. Making integrated circuits, or ICs, faster requires a reduced cell geometry, that is, a reduction in the size of the basic `building blocks.' As components become smaller and smaller, the cell size and memory circuitry also become smaller and more sensitive. As a result, these components cannot withstand the stress of operating at 5 volts. Also, 3.3-volt components can operate faster and use less power.

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Composite versus noncomposite modules

The terms composite and noncomposite refer to the number of chips used on a given module. The term noncomposite describes memory modules that use fewer chips. For a module to work with fewer chips, those chips must be higher in density to provide the same total capacity. This table summarizes the primary differences between composite and noncomposite modules.

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EDO Memory

Extended Data Output, or EDO memory, is one of a series of recent innovations in DRAM chip technology. On computer systems designed to support it, EDO memory allows a CPU to access memory 10 to 15 percent faster than comparable fast-page mode chips. Computers designed to take advantage of the EDO speed advantage include those that feature Intel's Triton chip.

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Synchronous DRAM

Synchronous DRAM is a new DRAM technology that uses a clock to synchronize signal input and output on a memory chip. The clock is coordinated with the CPU clock so the timing of the memory chips and the timing of the CPU are in `synch.' Synchronous DRAM saves time in executing commands and transmitting data, thereby increasing the overall performance of the computer.

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Cache memory

Cache Memory is a special high-speed memory designed to accelerate processing of memory instructions by the CPU. The CPU can access instructions and data located in cache memory much faster than instructions and data in main memory. For example, on a typical 100-megahertz system board, it takes the CPU as much as 180 nanoseconds to obtain information from main memory, compared to just 45 nanoseconds from cache memory. Therefore, the more instructions and data the CPU can access directly from cache memory the faster the computer can run.

Types of cache memory include primary cache (also known as Level 1 [L1] cache) and secondary cache (also known as Level 2 [L2] cache). Cache can also be referred to as internal or external. Internal cache is built into the computer's CPU, and external cache is located outside the CPU.

Primary cache is the cache located closest to the CPU. Usually, primary cache is internal to the CPU, and secondary cache is external. Some early-model personal computershave CPU chips that don't contain internal cache. In these cases the external cache, if present, would actually be the primary (L1) cache.

Earlier we used the analogy of a room with a work table and a set of file cabinets to understand the relationship between main memory and a computers hard disk. If memory is like the work table that holds the files you're working on making them easy to reach, cache memory is like a bulletin board that holds the papers you refer to most often. When you need the information on the bulletin board you simply glance up and there it is.



Cache memory is like a bulletin board that makes the work at the memory "work table" go even faster.


Memory is like a work table that makes immediate work easily accessible.





You can also think of cache memory as a worker's tool belt that holds the tools and parts needed most often. In this analogy, main memory is similar to a portable tool box and the hard disk is like a large utility truck or a workshop.

The "brain" of a cache memory system is called the cache memory controller. When a cache memory controller retrieves an instruction from main memory, it also takes back the next several instructions to cache. This occurs because there is a high likelihood that the adjacent instruction will also be needed. This increases the chance that the CPU will find the instruction it needs in cache memory, thereby enabling the computer to run faster.

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