DRAM Memory Guide
Last Updated: 06-24-2010 , Posted: 01-14-2005
A Quick Reference to System Memory
Often referred to as main memory RAM is the acronym for Random Access Memory. It is a type of computer memory that can, as its name implies, be accessed randomly. That is, any byte of memory can be accessed without touching the preceding bytes. RAM is the "working memory" storage area within the computer. All data on the computer is stored on the hard drive, but in order for the CPU to work with the data during normal operations, the data the computer uses and works with is read into the working memory, which is the RAM chips.
There are two different types of RAM; DRAM (Dynamic Random Access Memory) and SRAM (Static Random Access Memory). The two types differ in the technology they use to hold data, with DRAM being the more common type. In terms of speed, SRAM is faster. DRAM needs to be refreshed thousands of times per second while SRAM does not need to be refreshed, which is what makes it faster. DRAM supports access times of about 60 nanoseconds, SRAM can give access times as low as 10 nanoseconds.
Despite SRAM being faster, it is not as commonly used as DRAM because it is so much more expensive. Both types of RAM are volatile, meaning that they lose their contents when the power is turned off.
This reference will provide general information on the two types of RAM and provide an overview on the common modules of each type. As DRAM is the main system memory used in home and office PCs, being cheaper and more common than SRAM, we will focus on DRAM.
Types of SRAM
Async SRAMAsync SRAM is an older type of SRAM. It is asynchronous, meaning that it works independently of the system clock.
Sync SRAMSync SRAM is synchronized with the system clock, and increased speed.
Pipeline Burst SRAMPipeline Burst SRAM is the most common type of SRAM. It is able to operate at bus speeds higher than 66MHz.
Types of DRAM
FPM DRAM - Fast Page Mode DRAM [View Webopedia Definition]FPM DRAM is only slightly faster than regular DRAM. This used to be the main type of memory used in PCs but was eventually replaced by EDO RAM, due to its slow speed. FPM DRAM, is now considered to be obsolete. It was mainly used in the older 386 and 486 computers. It is not suitable for memory buses over 66 MHz.
EDO DRAM - Extended Data Out DRAM [View Webopedia Definition]EDO DRAM provided a better performance increase over FPM DRAM. EDO RAM cannot operate on a bus speed faster than 66MHz. With a need for speed, BEDO DRAM was introduced.
BEDO DRAM - Burst EDO DRAM [View Webopedia Definition]Burst EDO DRAM is a type of EDO DRAM that can process four memory addresses in one burst. BEDO DRAM can only stay synchronized with the CPU clock for short periods (bursts). It is faster than it's predecessor, EDO DRAM.
SDRAM - Synchronous DRAM [View Webopedia Definition]Short for Synchronous DRAM, a type of DRAM that can run at much higher clock speeds than conventional memory. SDRAM actually synchronizes itself with the CPU's bus. SDRAM is the new memory standard for modern PCs.
NOTE: When looking at SDRAM The number following "PC" indicates the speed of the system's frontside bus. (example: The PC100 SDRAM is designed for systems equipped with a 100 MHz frontside bus.)
RDRAM - Rambus DRAM [View Webopedia Definition]Short for Rambus DRAM, a type of memory (DRAM) developed by Rambus, Inc. Whereas the fastest current memory technologies used by PCs (SDRAM) can deliver data at a maximum speed of about 100 MHz, RDRAM transfers data at up to 800 MHz. RDRAM (and DDR-SDRAM) are the two technologies expected to replace SDRAM.
DDR SDRAM - Double Data Rate [View Webopedia Definition]Short for Double Data Rate-Synchronous DRAM, a type of SDRAM that supports data transfers on both edges of each clock cycle (the rising and falling edges), effectively doubling the memory chip's data throughput. DDR-SDRAM also consumes less power, which makes it well-suited to notebook computers. DDR-SDRAM is also called SDRAM II. and DDRAM. DDR-SDRAM and RDRAM are the two technologies expected to replace SDRAM.
NOTE: When looking at DDR memory, the number following "PC" indicates the module's total bandwidth. (example: PC1600 DDR memory is designed for systems equipped with a 100 MHz frontside bus. The number 1600' refers to the module's bandwidth; the quantity of data that it transfers in one second, of 1.6 GB per second.
DDR2 SDRAMDDR2 SDRAM is the next step up from DDR SDRAM. DDR2 SDRAM offers new features and functions that enable higher clock and data rate operations. DDR2 transfers 64 bits of data twice every clock cycle. DDR2 SDRAM memory is not compatible with current DDR SDRAM memory slots.
SDRAM, RDRAM & DDR/DDR2 SDRAM - A Price Comparison
* Price samples courtesy of SharkyExtreme.com
Buying DRAMDRAM technology changes and advances quite quickly. Trying to figure out which type of memory you need for an upgrade can be quite confusing to the average user. First, you need to know what type of memory is supported by the system chipset of your motherboard. This will be indicated on the documentation/manual included with your motherboard. If this is not available to you, most motherboard manufactures will have the information available on their Web site. Additionally, some Web sites like crucial.com will have a memory selector which allows you to input your system information into a form and it will return a list of suitable memory modules you can upgrade with. Unless cost is a major factor, you will want to go with the fastest type of memory supported by the motherboard. If cost is too much of an issue you can then look at the next step down and so on until you reach a happy medium between price and speed.
Due to advances in technology, over and under production runs, along with rampant competition in the memory retail market, prices on system memory are constantly fluctuating. A good way to save money or perhaps to purchase faster memory on a budget is to check memory prices online where there is "worldwide" competition for your upgrade dollars. One site to help with this task is PriceWatch — here you can search for a specific type of memory and see the prices hundreds of online vendors are selling this memory for. Once you have located the vendor with a price that meets your budget needs, you can then visit Reseller Ratings, an excellent website which users customer's experiences to report on the validity of an online vendor. This combined with watching Weekly Memory Price Guides (such as the one located at SharkyExtreme) will help you to better understand memory price trends, so you can buy the fastest memory supported by your motherboard — at a price you can afford!
More Memory Terminology
SIMMAcronym for single in-line memory module, a small circuit board that can hold a group of memory chips. A SIMM has a 32-bit path to the memory chips. Typically, SIMMs hold up to eight (on Macintoshes) or nine (on PCs) RAM chips. On PCs, the ninth chip is often used for parity error checking. Unlike memory chips, SIMMs are measured in bytes rather than bits. SIMMs are easier to install than individual memory chips. SIMMs come in 30-pin and 72-pin varieties.
DIMMShort for dual in-line memory module, a small circuit board that holds memory chips. A DIMM has a 64-bit path to the memory chips. Because the Pentium processor requires a 64-bit path to memory you can install memory one DIMM at a time. DIMMs come as 168-pin (SDRAM DIMM) or 184-pin (DDR DIMM) that is used to provide DDR SDRAM memory for many desktop computers. DDR DIMM will not fit into a standard SDRAM DIMM slot.
RIMMThe memory module used with RDRAM chips. It is similar to a DIMM package but uses different pin settings. Rambus trademarked the term RIMM as an entire word. It is the term used for a module using Rambus technology. It is sometimes incorrectly used as an acronym for Rambus Inline Memory Module. RIMMs have 184 pins. Rambus memory modules will only fit motherboards and systems especially designed for RIMMs, despite having the same number of pins as DDR DIMMs.
CAS - column address strobeAbbreviated as CAS, a signal, or strobe, sent by the processor to a DRAM circuit to activate a column address. DRAM stores data in a series of rows and columns, similar in theory to a spreadsheet, and each cell where a data bit is stored exists in both a row and a column. A processor uses CAS and RAS (row address strobe) signals to retrieve data from DRAM. When data is needed, the processor activates the RAS line to specify the row where the data is needed, and then activates the CAS line to specify the column. Combined, the two signals locate the data stored in DRAM. When seen in memory information, you will usually spot the reference "CAS2" or "CAS3". The number following CAS (2 or 3)represents the number of clock cycles before a DRAM column can be accessed. CAS2 or CAS3 are also referred to as CL2 or CL3
refreshTo recharge a device with power or information. For example, dynamic RAM needs to be refreshed thousands of times per second or it will lose the data stored in it.
bank(n.) The area of a motherboard that contains slots for memory modules. Memory banks are typically double sided (allowing for single- or double-sided memory modules), and the banks in the slots are numbered. Memory banks are organized into units representing the minimum number of memory chips that must work in tandem.
ECC memoryShort for Error-Correcting Code memory, a type of memory that includes special circuitry for testing the accuracy of data as it passes in and out of memory.
access timeDRAM (dynamic random access memory) chips for personal computers have access times of 50 to 150 nanoseconds (billionths of a second). Static RAM (SRAM) has access times as low as 10 nanoseconds. Ideally, the access time of memory should be fast enough to keep up with the CPU. If not, the CPU will waste a certain number of clock cycles, which makes it slower.
main memoryRefers to physical memory that is internal to the computer. The word main is used to distinguish it from external mass storage devices such as disk drives. Another term for main memory is RAM.
registered memoryA memory module that contains registers that hold the data for one clock cycle before it is moved on to the motherboard. This process increases the reliability of high-speed data access. Registered memory modules are typically used only in servers and other mission-critical systems where it is extremely important that the data is properly handled.
buffered memoryBuffered memory contains a buffer to assist the chipset deal with the large electrical load required when the system has a lot of memory. Much like registered modules, buffered modules are typically used in servers and other mission-critical systems where it is extremely important that the data is properly handled.
unbuffered memoryUnbuffered memory deals directly with the chipset controller with nothing in between as they communicate.
parityParity memory modules have an extra chip that will detect if data was correctly read or written by the memory module. Unlike ECC, parity will not correct the error.