The memory SDRAM standard or Synchronous Dynamic Random Access Memory standard is designed to operate at higher speeds by simultaneously keeping two sets of memory addresses open to allow for alternate data transfer on each set of addresses. The pipelining capability of SDRAM allows for the identification of the exact clock cycle that the requested data will be available, ensuring that the CPU doesn't have to wait between memory accesses.
Synchronous DRAM or SDR SDRAM is essentially having the synchronous interface, which means that the internal clock, bus clock and I/O are all the same. If you have PC133 SDRAM, the internal clock, bus clock, and I/O are all 133 MHz rated. The single data rate implies that the SDRAM can only perform a single read/write command in a clock cycle.
Double data rate (DDR) SDRAM is the second generation, and it offers greater bandwidth compared to the previous generation. That means that the memory module improves performance by doubling the transfer rate without necessarily increasing the frequency of the clock.
DDR SDRAM is the first generation of the DDR memory and offers double the transfer rates of SDR SDRAM. DDR has a prefetch buffer of 2 bit. The effective transfer rate of the DDR is anything between 266 and 400 MT/s. It explains why you have the options of DDR266 and DDR400.
DDR2 SDRAM (Double Data Rate 2 SDRAM) is the second generation of the DDR which has the improved bus signal functionality to operate the external data bus at double the speed of DDR SDRAM. DDR2 has prefetch buffer of 4 bit but the clock speed at 133 to 299 MHz is similar to that of DDR. However, the transfer rate of DDR2 is anything between 533 and 800 MT/s and improved I/O bus signal. At ALLHDD you can find bulk packs with storage capacity of 8GB and 16GB for a great price.
DDR3 SDRAM is the third generation and has a transfer rate of 800 to 1600 MT/s with a prefetch buffer of 8 bit. DDR3 memory helps reduce power consumption by 40% at 1.5V compared to DDR2 at 1.8 V. The power consumption of a DDR is a high 2.5V. The automatic self-refresh (ASR) and self-refresh temperature (SRT) functionalities allow the memory chip to control the refresh rate as per the temperature variation.DDR4 SDRAM (Double Data Rate Fourth SDRAM) is the fourth generation of DDR with a higher transfer rate and lower operating voltage at 1.2 V. DDR4 will achieve transfer rates of 2133 to 3200 MT/s. The outstanding feature of DDR4 is the four new bank groups technology that allows single-handed operation at each group. The DDR memory chip will process 4 data within a single clock cycle offering greater efficiency compared to DDR3. Other functions on DDR4 memory are CA parity, cyclic redundancy check (CRC), and data bus inversion (DBI) that improve signal integrity and stability of data transmission and access. We have different DDR4 memory chips for increased performance and more effective memory.