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Understanding Computer Memory
By Stephen Bucaro
Understanding computer memory is important because your
computer's memory is where the action is. Sure your
computer's central processing unit (CPU) may be the
computer's brain, but what good is a brain without memory?
Because memory is such an important part of the computer,
it has been redesigned and tweaked over the years to make
it store more data and work faster. That has resulted in
all kinds of technical terms and contractions. In this
article, I attempt to clear up some of these terms and
give you a basic understanding of computer memory.
When you work with an application on your computer, it is
loaded into "main memory". There are other kinds of memory
besides main memory. For example, there is "flash BIOS"
memory and "cache" memory. In fact, I could argue that a
computer consists mainly of memory. Even the CPU consists
mainly of a type of memory called "registers". In this
article, I focus on main memory.
Memory is actually an array of "cells". Each cell holds
a binary bit, either 0 or 1. The cells are arranged in
rows of 32 cells to store each "word". A word might
contain the bits to code a single number or a single
character. Each word can be accessed by it's address. A
computer does not have to access the addresses in
numerical order. It can store or retrieve a word from any
address at random, hence the term "random access memory"
(RAM).
Because a computer needs millions of memory cells, they
have to be cheap. The cheapest memory cell is an extremely
tiny capacitor. In reality "0" means no electric charge
is stored in the cell and "1" means an electric charge is
stored in the cell.
A tiny capacitor can store an electric charge, but that
charge dissipates off within a few milliseconds. To
prevent loss of the data, the computer uses circuitry
that repeatedly scans all the memory cells and refreshes
the charge in any cells that contain a "1". This circuitry
operates between accesses of the memory by the CPU and
other components. This continuous recharging is referred
to as "dynamic" refresh, hence the term "dynamic" RAM
(DRAM).
I am always surprised at how many people who work with
computers don't understand what a "bus" is. It's very
simple. You need a wire to connect to a memory cell.
Instead of a wire, a circuit board uses a copper "trace"
inside or on the surface of the circuit board. To access
a word of data, you need 32 parallel traces. You also
need a few traces to carry the signals that control
whether you are reading or writing the memory. This
describes the "data bus".
To access a word of data, you need to provide the address
of the word. It requires 64 parallel traces for the
"address bus". There are many other parallel groups of
traces on the circuit board that conduct various
communication and control signals, each of them is
referred to as a "bus". The most important signal tracing
around the circuit board is the system clock.
The system clock is the "heart beat" of the system. It is
a continuous stream of electric pulses at a very high
frequency. If you have a 1.2 GHz computer system, then
your system clock pulses come at a rate of 1,200,000,000
per second. Memory can be accessed on the rising edge of
the system clock. In other words, memory reading and
writing is synchronized to the system clock, hence the
term "synchronous" DRAM (SDRAM).
SDRAM comes in a 168-pin dual inline memory module (DIMM)
package. It comes in two speeds, 100 MHz (PC100) and 133
MHz (PC133). It comes in 3.3 volt and 5.0 volt versions.
Some SDRAM contain extra circuitry called "error checking
and correction" (ECC) that stores extra bits along with
the data. The extra bits are used to verify the integrity
of the data and correct any errors.
New systems use a type of memory that can access data on
the rising edge and the falling edge of the system clock
pulses. This means your computer can access the data in
memory at double the rate, hence the term "Double Data
Rate SDRAM (DDR-SDRAM).
DDR-SDRAM comes packaged in a 168-pin DIMM. It comes in
two speeds, 266 MHz (PC2100) and 333 MHz (PC2700). If your
motherboard has more than one memory module slot, and you
have only one memory module, you might be required to
install the single DIMM in slot 1.
Some Intel motherboards use Rambus DRAM (RDRAM). RDRAM
uses a proprietary communications channel to access
memory. RAMBUS is packaged in a 184-pin RAMBUS In-line
Memory Module (RIMM). There are two connectors on an
Intel Rambus motherboard, each must contain either a RIMM
or a C-RIMM. A C-RIMM (continuity RIMM) does not contain
memory. It is a module that provides a path pass-through
for the signals.
If you have an older computer, its main memory may use
"fast page memory" (FPM) or "extended data output" (EDO)
memory. Some of these used a special "parity bit" to
validate the integrity of the data. These older types of
memory came packaged in a 30 pin single inline memory
module (SIMM) or a 72 pin SIMM.
If you want to upgrade your computer's memory, make sure
you are using the correct type of memory for your
motherboard. It's important to study the manual that came
with your motherboard to determine exactly what type and
speed of memory is required.
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