This RAM, That RAM....which is which?
RAM, otherwise known as Random Access Memory, can be one of the most confusing parts of the computer to deal with. With so many variations of memory on the market today, how do you know which one to consider? Which type of RAM is best for you?
This article briefly explains the different assortment of RAM types available on the market today. SDRAM, DDR SDRAM, EDO, FPM and a few others are explained, hopefully relieving the confusion that is on many people's minds concerning which RAM is which?
SIMM, RIMMs, DIMMs, too many -IMMs for me to handle, but what exactly do each of these memory types do? SIMM, which stands for Single In-line Memory Module, are soldered onto a modular PCB (Printed Circuit Board) generally referred to as a 'stick'. This "stick of RAM" goes into the socket on the system board. SIMMs can supply up to 32-bits of data. Most of the models used are all known as "72 pin SIMMs" which refers to the 32-bit data transfer, and coincidentally the 72-pin socket they fit into. There are also "30 pin SIMMs" which have an 8-bit data transfer and fit into 30-pin sockets. Both types of SIMM are long out of date.
DIMM memory module
DIMM, Dual In-line Memory Module, are in fact a lot like SIMM's in that they are both installed vertically into memory expansion slots. One main differences between the two types of memory is that on a SIMM pins on opposite sides of the board are tied together to form one electrical contact. Whereas on a DIMM, opposite pins remain isolated to form two separate contacts. DIMMs come in 168 pin modules and provide 64-bits of data transfer. The 168-pin DIMM module is much longer than the 72-pin SIMMs, producing twice the speed of transfer.
Rambus RIMM Module
RIMM, Rambus Memory Module, is a trademark name for the Direct Rambus Memory Module. RIMMs also look similar to the SIMM model, but with a few distinct characteristics. A two byte-wide data channel is used resulting in a peak data transfer rate of 1.6 Gigabytes per second. This seems to heat-up the chips quite easily. In order to prevent the chips from overheating, Rambus installs "heat spreaders" on the module to protect the chips from overheating. RIMMs are available in 184-pin format.
Now that we covered the basics of the -IMM memory category, we can get into the memory chips you are much more likely to be familiar with. Memory modules such as DRAM, SDRAM, DDR SDRAM, EDO, and FPM are more than likely the types installed in our systems today.
The question remains though, what do they do for your computer?
From IMM's to other acronyms
DRAM or Dynamic RAM, is a type of RAM that only holds its data if it's constantly accessed by the "refresh circuit." By the way in which the cells are constructed, the reading action refreshes the contents of the memory. This refreshing action is why it's called Dynamic RAM. Conventional DRAM chips are not used in new computer systems any more; they have been replaced with such types as DDR RAM and SDRAM.
FPM DRAM, Fast Page Mode, is a bit faster than conventional DRAM, which nonetheless is slow. FPM works by sending the row address once for many accesses to memory in locations near each other, improving access time. Although most PCs still accept FPM chips, no one in their right mind would purchase such a low-end product. It's only suitable for memory buses that operate under the rate of 66MHz. Don't bother with FPM unless you already have it installed, and if you have it installed..UPGRADE!
EDO DRAM, Extended Data Out DRAM, was the most popular form of DRAM technology. It's a bit faster than the FPM DRAM due to the tweak in which the memory access works. In reality, EDO DRAM and FPM DRAM are too similar to bother with. Both were extremely popular in their time, but their time was over in a heartbeat, especially with EDO DRAM. EDO is not suitable for any bus rates over 75MHz just like FPM is not suitable for any bus rates over 66MHz. EDO offers just a 10% increase in speed over FPM, which is too little to waste your time with today. Again, don't waste your valuable money with EDO, save it for something incredible, something amazing, such as SDRAM!
SDRAM, Synchronous DRAM, is more than likely what you have in your computer right now. Introduced in 1997 originally for 66MHz bus speeds, SDRAM has soared with popularity over the years. SDRAM is designed to synchronize itself with the timing of the CPU. The memory controller knows the exact clock cycle when the requested data will be ready, making sure the CPU doesn't have to wait between memory accesses. SDRAM also has interleaving and burst modes which makes retrieval even faster than it already is. The most common bus speeds available for SDRAM are 66, 100, and 133 MHz. Although they have 150MHz chips available, you really don't have a need for a chip with that high of a speed..at least yet.
DDR SDRAM, Double Data Rate SDRAM, is the newest generation of memory chip available. The name explains the beauty of this baby, double the speed! DDR SDRAM functions the same as SDRAM, but doubles the bandwidth of the memory by transferring data twice per cycle! 100 MHz chips turn into 200MHz, 133 MHz goes up to 266MHz, and so on. Now you can easily find chipsets that support these memory chips which operate at 200MHz and 266MHz bus speeds. DDR SDRAM is more expensive then SDRAM, but what do you expect? Double the speed and no increase on value? I didn't think so. DDR SDRAM will be the popular choice soon enough, might as well get a jump-start, right?
RAM can be an awfully confusing aspect of the PC. You must know which one to use; otherwise you might be getting the backend of your potential speed. Lets face it, FPM and EDO are out of the game entirely because no one needs them anymore. We had our fun with them, but they can't handle today's system requirements.
SDRAM is great! With support for bus speeds of up to 150MHz, SDRAM will be in household computers for a while, and the users will be satisfied. But like the generation we are in now, technologies rise too fast to comprehend, and DDR SDRAM will quickly be next in line for mass popularity. Enjoy SDRAM technologies for now as it will shortly be time to move towards a newer RAM technology and leave SDRAM long forgotten.