Computer Memory Info FAQ DDR SDRAM Dual Channel DDR

Memory Info and FAQ

Chipset
This line tells you what kind of chipset your motherboard uses. Your chipset enables many of the devices in your computer (processor, memory, keyboard, mouse, etc.) to communicate with one another. Unlike processors and memory, chipsets are an integral part of a motherboard and generally cannot be upgraded.

Accepts PC133 SDRAM
This line tells you whether or not your system will accept PC133 synchronous dynamic random access memory (SDRAM). SDRAM delivers bursts of data at very high speeds using an interface that is synchronized to the CPU clock. PC133 SDRAM meets Intel's requirements for use with 133MHz motherboards.

In general, PC133 SDRAM can also be used with a 100MHz or 66MHz front side bus; however, your memory will only operate as fast as the slowest "link" in your system. For example, if you install a PC133 module in a system with a 100MHz FSB or in a system containing a 100MHz module, the PC133 module will operate at 100MHz.

Accepts SDRAM 100MHZ
This line tells you whether or not your system will accept 100MHz synchronous dynamic random access memory (SDRAM). SDRAM delivers bursts of data at very high speeds using an interface that is synchronized to the CPU clock. PC100 SDRAM is a particular type of 100MHz SDRAM that meets Intel's requirements for use with 100MHz motherboards.

In general, 100MHz SDRAM can also be used with a 66MHz front side bus; however, your memory will only operate as fast as the slowest "link" in your system. For example, if you install a PC100 module in a system with a 66MHz FSB or in a system containing a 66MHz module, the PC100 module will operate at 66MHz.

Accepts SDRAM 66MHZ
This line tells you whether or not your system will accept 66MHz synchronous dynamic random access memory (SDRAM). SDRAM delivers bursts of data at very high speeds using an interface that is synchronized to the CPU clock. 66MHz SDRAM is used in systems that have a 66MHz front side bus.

Accepts Registered SDRAM
This line tells you whether or not your system will accept registered SDRAM. Registered modules contain a register that delays all information transferred to the module by one clock cycle. Like buffered modules, registered modules are typically used only in servers and other mission-critical systems where it is extremely important that the data is properly handled.

Accepts EDO
This line tells you whether or not your system will accept extended data out (EDO) memory. Enhancements its addressing system allow EDO to operate 10 to 15% faster than FPM; however, it is not as fast as SDRAM.

Accepts Fast Page Mode
This line tells you whether or not your system will accept fast page mode (FPM) memory. FPM is the oldest type of memory that Crucial sells. In the FPM scheme, information from the same row of DRAM can be accessed an infinite number of times after supplying the row address only once.

Max EDO/FPM
This line tells you the maximum amount of EDO or FPM memory (in megabytes) that your motherboard will recognize. The total memory on all the modules installed in your system cannot exceed this amount.

Accepts DDR
This line tells you whether or not your system will accept double data rate (DDR) SDRAM. DDR SDRAM is the most recent addition to Crucial Technology's memory offerings. It reads information on both the rising and falling edge of the CPU's clock cycle, roughly doubling the speed of memory processing over standard SDRAM.

PC1600 DDR SDRAM is used in systems with a 100MHz front side bus. PC2100 DDR SDRAM is used in systems with a 133MHz front side bus.

Accepts ECC
This line tells you if your motherboard will accept error checking and correcting (ECC) modules. ECC modules have an extra chip that detects if the data was correctly read or written by the memory module. If the data wasn't properly written, the extra chip will correct it in many cases (depending on the type of error). Non-ECC (also called non-parity) modules do not have this error-detecting feature.

If you plan to use your system as a server or a similar mission-critical type machine, it is to your advantage to use ECC. If you plan to use your PC for regular home, office, or gaming applications, you are better off with non-ECC. Current technology DRAM is very stable and memory errors are rare, so unless you have a need for ECC, you are better served with non-ECC SDRAM or DDR SDRAM.

Accepts Parity
This line tells you if your motherboard will accept parity modules. Parity modules have an extra chip that detects if data was correctly read or written by the memory module, depending on the type of error. However, unlike an ECC module, a parity module will not correct the error.

If you plan to use your system as a server or a similar mission critical type machine, it is to your advantage to use parity. If you plan to use your PC for regular home, office, or gaming applications, you are better off with non-parity. Current technology DRAM is very stable and memory errors are rare, so unless you have a need for parity, you are better served with non-parity DRAM.

Accepts RDRAM (Rambus)
This line tells you if your motherboard will accept Rambus (RDRAM) memory. Rambus is a proprietary memory of Rambus Inc., and manufacturers who produce it are required to pay a royalty. Rambus and SDRAM modules are not interchangeable and do not fit into the same size slots.

DIMM Socket Count
This line tells you how many dual inline memory modules (DIMMs) can be installed in your system at once. A DIMM consists of a number of memory components (usually black) that are attached to a printed circuit board (usually green). The gold or tin pins on the bottom of the DIMM provide a connection between the module and a socket on a larger printed circuit board. The pins on the front and back of a DIMM are not connected, providing two lines of communication paths between the module and the system.

DIMMs come in several sizes. 168-pin DIMMs, the most common size for PCs, are approximately 5.375" long and 1.375" high. 100-pin DIMMs, the most common size for printers, are approximately 3.5" long and 1.25" high. 184-pin DIMMs, used for DDR SDRAM, are approximately 5.375" long and 1.375" high. While 184-pin DIMMs and 168-pin DIMMs are about the same size, 184-pin DIMMs have only one notch within the row of pins while the 168-pin DIMMs have two notches.

Sockets per DIMM Bank
This line tells you how many DIMMs must be installed at the same time. In most cases, DIMMs are installed individually. However, if there were two sockets per bank, you would need to install two modules at the same time. In this scenario, if you wanted to add 64MB, you would need to purchase two 32MB modules and install them together.

72-pin SIMM Socket Count
This line tells you how many 72-pin single inline memory modules (SIMMs) can be installed in your system. A SIMM consists of a number of memory components (usually black) that are attached to a printed circuit board (usually green). The gold or tin pins on the bottom of the SIMM provide a connection between the module and a socket on a larger printed circuit board. The pins on the front and back of a SIMM are connected, providing a single line of communication paths between the module and the system.

Each 72-pin SIMM provides a 32-bit data path, so they can be installed singly in 32-bit systems (486 models) but must be installed in pairs in 64-bit systems (Pentium and Athlon models). 72-pin SIMMs are approximately 4.25" long and 1" high, though the heights may vary. They have one notch on the bottom left and one notch in the center of the module.

Sockets per 72-pin SIMM Bank
This line tells you how many 72-pin SIMMs must be installed at the same time in your system. In general, they can be installed singly in 32-bit systems (486 models) but must be installed in pairs in 64-bit systems (Pentium and Athlon models). If you have two sockets per bank, you would need to install two modules at the same time. In this scenario, if you wanted to add 64MB, you would need to purchase two 32MB modules and install them together.

30-pin SIMM Socket Count
This line tells you how many 30-pin single inline memory modules (SIMMs) can be installed in your system. A SIMM consists of a number of memory components (usually black) that are attached to a printed circuit board (usually green). The gold or tin pins on the bottom of the SIMM provide a connection between the module and a socket on a larger printed circuit board. The pins on the front and back of a SIMM are connected, providing a single line of communication paths between the module and the system.

Each 30-pin SIMM provides an 8-bit data path, so they must be installed in banks of 4 in order to communicate with 32-bit systems (such as 486 models). All 30-pin SIMMs use FPM memory technology. 30-pin SIMMs are approximately 3.5" long and .75" high, though the heights may vary. They have a single notch on the bottom left to ensure that they are installed correctly.

PC1600 DDR SDRAM is used in systems with a 100MHz front side bus. PC2100 DDR SDRAM is used in systems with a 133MHz front side bus.

Sockets per 30-pin SIMM Bank
This line tells you how many 30-pin SIMMs must be installed at the same time in your system. In general, 30-pin SIMMs are installed in banks of 4. That means that if you wanted to add 64MB, you would need to purchase four 16MB modules and install them together.

Buffering
This line tells you whether your system takes buffered, unbuffered, or registered modules. Unbuffered modules are the most common. In unbuffered memory, the chipset controller deals directly with the memory. There is nothing between the chipset and the memory as they communicate. Buffered modules contain a buffer to help the chipset cope with the large electrical load required when the system has a lot of memory. Registered modules are unbuffered modules that contain a register that delays all information transferred to the module by one clock cycle. Buffered and registered modules are typically used only in servers and other mission-critical systems where it is extremely important that the data is properly handled.

DDR and SDRAM modules can be registered or unbuffered; EDO and FPM modules can be buffered or unbuffered.

Max Unbuffered SDRAM
This line tells you the maximum amount of unbuffered memory (in megabytes) that your motherboard will recognize. The total of all the modules installed in your system cannot exceed this amount. In unbuffered SDRAM, the chipset controller deals directly with the memory. There is nothing between the chipset and the memory as they communicate.

Max Registered SDRAM
This line tells you the maximum amount of registered memory (in megabytes) that your motherboard will recognize. The total of all the modules installed in your system cannot exceed this amount. Registered modules contain a register that delays all information transferred to the module by one clock cycle. is usually done on modules with a lot of memory to help ensure that the data is properly handled.

CL=3, CL=2, and CL=2, 2-clock
In our Memory Selector, the CAS latency of our parts is designated with "CL=3," "CL=2," or "CL=2, 2-clock." (You may see this written elsewhere as "CL2, etc." or "CAS=2, CAS=3, etc.") CAS latency is the amount of time it takes for your memory to respond to a command. It only affects the initial burst of data. Once data starts flowing, latency has no effect.

Latency is measured in terms of clock cycles. For example, a CL=2 part requires two clock cycles to respond, while a CL=3 part requires three clock cycles. Thus, CL=2 parts complete the initial data access a little more quickly than CL=3 parts. However, a clock cycle for a systems with a 100MHz front side bus is only 10 nanoseconds (10 billionths of a second), so you probably won't be able to tell the difference between a CL=2 and a CL=3 part.

ECC or Non-parity? You may have to decide whether you want ECC or non-parity. ECC can find and correct some memory errors, but it comes with a performance price-it will slow your system by about 2%. Fortunately, memory errors are rare in today's memory chips, so most average users don't have a need for ECC. If you're planning to use your system as a server or other "mission-critical" machine, we recommend ECC. If you're looking for maximum speed, we recommend non-parity.

What is ECC SDRAM? ECC (error correction code) SDRAM is memory that is able to detect and correct some SDRAM errors without user intervention. ECC SDRAM replaced parity memory which could only detect, but not correct, SDRAM errors.

What are Parity and ECC (Error Checking and Correction)? Early on, RAM was not as stable a solution as it is today. Irregularities could cause the data in memory to corrupt or alter in ways that often led to a system crash or hard disk data damage. This problem was first solved with Parity RAM. Through additional or modified chips, it added an additional bit to each byte of RAM which verified the validity of each byte. If the data did not check out properly, your computer would typically halt to avoid further problems. ECC added a further process to the cycle. Instead of merely checking the bytes, it can correct most errors with an extra bit. It is fairly popular with the CAD crowd, as it helps maintains strict accuracy. For most consumers, however, it is not necessary due to the low rate of errors in today's memory, and actually involves a slight performance hit.

What causes SDRAM errors? Per Dell, "Memory errors are characterized as hard or soft. Hard errors are caused by defects in the silicon or metalization of the SDRAM package, and are usually permanent once they manifest. Soft errors are caused by charged particles or radiation, and are transient. In the past, soft errors were primarily caused by alpha particles, but that failure mode has been mostly eliminated today by strict quality control of the packaging material by SDRAM vendors. Currently the primary source of soft errors in SDRAM is electrical disturbance caused by cosmic rays, which are very high-energy subatomic particles originating in outer space."

What happens when a SDRAM crash occurs? When main memory crashes, all data in memory is lost. The larger the amount of main memory on the computer, the greater the possibility of nonrecoverable data loss.

What kind of errors can ECC SDRAM correct? Most ECC SDRAM can correct single bit errors, and detect, but not correct larger errors. Thus, errors greater in size than 1 bit will still crash the computer.

Chipkill was invented to augment ECC DRAM. Large server manufacturers have implemented additional error correcting hardware capabilities with a technology known as Chipkill. Per Dell, "Chipkill correct is the ability of the memory system to withstand a multibit failure within a SDRAM device, including a failure that causes incorrect data on all data bits of the device. These methods rely on the chip set and hardware architecture of the system and cannot be achieved through software upgrades."

So what is the possibility of data loss? The data shown below illustrates the results of an IBM analysis comparing server outages due to memory failures of parity, ECC and Chipkill-equipped servers.

In summary, the following outage rates were identified:
A 32MB parity memory-equipped server received
   7 outages per 100 servers over 3 years.
The 1GB ECC memory-equipped server received
   9 outages per 100 servers over 3 years.
The 4GB Chipkill-equipped server received
   6 outages per 10,000 servers over 3 years.

It can be seen that the Chipkill equipped server had a failure rate of a magnitude of over 10 times lower than regular ECC SDRAM. Also, remember that the more system memory a computer has, the more likely it will crash due to a memory error.

What about speed? I could find no conclusive evidence that ECC SDRAM performed any slower than non-ecc SDRAM. Both Dell and IBM stated in their referenced articles there was no speed penalty to use a Chipkill enhanced server instead of an ECC memory equipped server without Chipkill.

So who should buy ECC SDRAM? First, the average user should be frequently saving data to their hard drive, so the likelihood of catastrophic memory failure should be small and therefore ECC memory would be overkill. Second, if you are thinking of running a server, you definitely want to have a working RAID disk array, as your hard drives are much more likely to fail then your memory. Third, if you want to run a server, there is no reason not to have ECC memory if your motherboard supports it. Currently ECC SDRAM only costs a little bit more than regular SDRAM.