SDRAM is short for synchronous DRAM, a type of DRAM that runs in synchronization with the memory bus. SDRAM delivers information in very high-speed bursts using a high-speed, clocked interface. SDRAM removes most of the latency involved in asynchronous DRAM because the signals are already in synchronization with the motherboard clock.

Like EDO RAM, your chipset must support this type of memory for it to be usable in your system. Starting in 1996 with the 430VX and 430TX, most of Intel's chipsets began to support industry-standard SDRAM, making it the most popular type of memory for new systems into 2001.

SDRAM performance is dramatically improved over that of FPM or EDO RAM. Because SDRAM is still a type of DRAM, the initial latency is the same, but overall cycle times are much faster than with FPM or EDO. SDRAM timing for a burst access would be 5-1-1-1, meaning that four memory reads would complete in only eight system bus cycles, compared to eleven cycles for EDO and fourteen cycles for FPM. This makes SDRAM almost 20% faster than EDO.

Besides being capable of working in fewer cycles, SDRAM is also capable of supporting up to 133MHz (7.5ns) system bus cycling. As such, most new PC systems sold from 1998 to 2000 have included SDRAM memory.

SDRAM is sold in DIMM form and is often rated by megahertz speed rather than nanosecond cycling time, which was confusing during the change from FPM and EDO DRAM.

To meet the stringent timing demands of its chipsets, Intel created specifications for SDRAM called PC66, PC100, and PC133. To meet the PC100 specification, 8ns chips usually are required. Normally, you would think 10ns would be considered the proper rating for 100MHz operation, but the PC100 specification calls for faster memory to ensure all timing parameters are met.

At one time, some module manufacturers sold modules they claim are "PC150" or "PC166" even though those speeds do not exist as official JEDEC or Intel standards and no chipsets or processors officially support these speeds. Normally, these modules actually use hand-picked 7.5ns (133MHz) or 7.0ns (143MHz) rated chips that can run overclocked at 150MHz or 166MHz speeds. In essence, PC150 or PC166 memory would more accurately be called PC133 memory that has been tested to run at overclocked speeds not supported by the original chip manufacturer. This specially selected overclockable memory was sold at a premium to enthusiasts who want to overclock their motherboard chipsets, thereby increasing the speed of the processor and memory bus.

DDR SDRAM

Double data rate (DDR) SDRAM memory is a JEDEC-created standard that is an evolutionary upgrade of standard SDRAM in which data is transferred twice as quickly. Instead of doubling the actual clock rate, DDR memory achieves the doubling in performance by transferring twice per transfer cycle: once at the leading (falling) edge and once at the trailing (rising) edge of the cycle. This effectively doubles the transfer rate, even though the same overall clock and timing signals are used.

DDR found initial support in the graphics card market and since then has become the mainstream PC memory standard. As such, DDR SDRAM is supported by all the major processor, chipset, and memory manufacturers.

DDR SDRAM first came to market during 2000, but it didn't really catch on until 2001 with the advent of mainstream motherboards and chipsets supporting it. DDR SDRAM uses a new DIMM module design with 184 pins

DDR DIMMs come in a variety of speed or throughput ratings and normally run on 2.5 volts. They are basically an extension of the standard SDRAM DIMMs redesigned to support double clocking, where data is sent on each clock transition (twice per cycle) rather than once per cycle as with standard SDRAM. To eliminate confusion with DDR, regular SDRAM is often called single data rate (SDR). As you can see, the raw chips are designated by their speed in megatransfers per second, whereas the modules are designated by their approximate throughput in megabytes per second.

DDR2 SDRAM

JEDEC and its members began working on the DDR2 specification in April 1998, and published the standard in September 2003. DDR2 chip and module production actually began in mid-2003 (mainly samples and prototypes), and the first chipsets, motherboards, and systems supporting DDR2 appeared in mid-2004.

DDR2 SDRAM is simply a faster version of conventional DDR-SDRAM memory: It achieves higher throughput by using differential pairs of signal wires to allow faster signaling without noise and interference problems. DDR2 is still double data rate just as with DDR, but the modified signaling method enables higher speeds to be achieved with more immunity to noise and cross-talk between the signals. The additional signals required for differential pairs add to the pin countDDR2 DIMMs have 240 pins, which is more than the 184 pins of DDR. The original DDR specification tops out at 400MHz, whereas DDR2 starts at 400MHz and goes up to 1000MHz and beyond

In addition to providing greater speeds and bandwidth, DDR2 has other advantages. It uses lower voltage than conventional DDR (1.8V versus 2.5V), so power consumption and heat generation are reduced. Because of the greater number of pins required on DDR2 chips, the chips typically use fine-pitch ball grid array (FBGA) packaging rather than the thin small outline package (TSOP) chip packaging used by most DDR and conventional SDRAM chips. FPGA chips are connected to the substrate (meaning the memory module in most cases) via tightly spaced solder balls on the base of the chip.

Volume production of DDR2 chips and modules started in the latter part of 2003, with supporting chipsets and motherboards arriving in mid-2004. Variations of DDR2 such as G-DDR2 (Graphics DDR2) are being used in some of the higher-end graphics cards as well. Although all the major memory and Intel-compatible chipset makers support DDR2, the major holdout through 2005 was AMD, whose Athlon 64 and Opteron processor families include integrated DDR memory controllers.

Starting in mid-2006, AMD will also support DDR2 with redesigned versions of the Athlon 64, Sempron, and Opteron processor families.

DDR2 DIMMs resemble conventional DDR DIMMs but have more pins and slightly different notches to prevent confusion or improper application. For example, the different physical notches prevent you from plugging a DDR2 module in to a conventional DDR (or SDR) socket. DDR2 memory module designs incorporate 240 pins, significantly more than conventional DDR or standard SDRAM DIMMs.