Unlike DDR SDRAM, regular SDRAM has two notches in the pin-side.
Synchronous Dynamic Random Access Memory (SDRAM) and Double Data Rate (DDR) SDRAM are very closely related. The modules look similar and perform similar functions on compatible main boards. SDRAM is the older of the two technologies. DDR SDRAM is a higher-performance RAM. The two types of memory are not compatible and cannot be mixed.
Physical Differences
The overall form of the two memory modules is similar. Both are the same length and are often the same height. Both memory modules have a similar number of interface pins to transfer data to the computer's main board. Because the two memory modules are not compatible, subtle changes were made when DDR SDRAM was released. SDRAM has two notches in-line with the pins. One of the notches is in the center of the module. The other is toward one of the sides. DDR SDRAM, on the other hand, only has one notch. The notch on DDR SDRAM is not in the center of the module, it is slightly off-center.
Bus Tie
SDRAM and DDR SDRAM are both tied to the Front Side Bus (FSB) speed of the computer. The clock rate of the bus is tied to the clock rate of the processor's FSB. For example, if the computer has a 133 MHz FSB, the memory interfaces with the main board at 133 MHz. If a computer has a 266 MHz FSB, the memory interfaces at 266 MHz. The interface clock rate for the two types of memory is the same, although the memory is accessed in very different ways.
Data Rate
DDR SDRAM can handle twice as many requests for information as SDRAM. It can do this because it allows access at both the top of the clock cycle and the bottom. SDRAM allows memory access only at the top of the cycle. If you picture the clock cycle as a sine wave, SDRAM allows access only at the top of the wave. DDR SDRAM allows access both at the top of the wave and the bottom, thus allowing the "doubling" that Double Data Rate implies.
Tags: allows access, clock rate, Data Rate, memory modules, SDRAM allows