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
The DIP Style DRAM package was extremely popular when it was common for
memory to be installed directly on the computer's system board. DIP's are
"through-hole" components, which means they install in holes extending into
the surface of the printed circuit board. DIP's can be soldered in place or
placed in sockets.
SOJ DRAM Package
SOJ and TSOP packages are surface-mount components -- that is, they mount
directly onto the surface of the printed circuit board.
TSOP DRAM Package
TSOP and SOJ gained in status with the advent of the SIMM. Of the two, the
SOJ package is by far the most popular.
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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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