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The system clock is
"heart beat" of 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 rising edge of system clock. In other words, memory reading and writing is synchronized to system clock, hence 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 data. The extra bits are used to verify integrity of data and correct any errors.
New systems use a type of memory that can access data on rising edge and falling edge of system clock pulses. This means your computer can access data in memory at double rate, hence 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 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 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 integrity of 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 correct type of memory for your motherboard. It's important to study manual that came with your motherboard to determine exactly what type and speed of memory is required.
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