memory module types, SIMM, DIMM, SODIMM, RIMM, DDR, SDRAM
Home About Us Reference Product Service Sitemap

Memory module types: SIMM, DIMM, SODIMM, RIMM, DDR, SDRAM

Let's first have a look at some of the terms as follows:

Dynamic RAM

Dynamic RAM (DRAM) stores data for only a tiny fraction of a second before losing it. To maintain stored data, the system must constantly refresh DRAM, which exacts a performance penalty and limits its speed. Typical DRAM provides 60 ns (nanosecond) access, but is inexpensive and consumes relatively little power.

Static RAM

Static RAM (SRAM) automatically maintains its contents as long as power is applied to it, without requiring refresh. SRAM provides access times an order of magnitude faster than DRAM, but is expensive and power-hungry.

Main memory

The bulk of a PC's memory uses DRAM (modern systems use a variant called SDRAM, described in the next section) and is called main memory. It is large typically 256 MB to 1 GB or morebut too slow to keep up with a modern CPU. Main memory is where the CPU stores programs and data that it will soon need. Main memory functions as a buffer between the CPU and disk.

Cache memory

Cache memory is a small amount of fast SRAM that buffers access between the CPU and main memory.

Primary cache memory

Primary cache memory, also called Level 1 cache or L1 cache, is typically 16 to 128 KB of very fast memory on the same chip as the CPU itself. L1 cache size and efficiency are major factors in CPU performance. The amount and type of L1 cache is determined by the CPU you use, and cannot be upgraded.

Secondary cache memory

L1 cache is not large enough to eliminate the speed disparity between processors and main memory. Secondary cache memory, also called Level 2 cache or L2 cache, bridges that gap with a reasonable compromise between cost and performance. L2 cache is a part of the CPU package (or of the CPU substrate itself) on all modern processors, including the Intel Pentium 4 and Celeron, and the AMD Athlon 64 and Sempron. Modern processors have L2 cache memory sizes ranging from 128 KB to 2 MB.

Synchronous DRAM

Synchronous DRAM, also called SDRAM, began shipping in 1996 and was commonly used on PCs until 2001. Unlike older and now obsolete asynchronous forms of memory, SDRAM shares a common clock reference with the CPU. The CPU and memory are slaved together, allowing the CPU to transfer data to and from memory whenever it wishes to do so, rather than requiring the CPU to await an arbitrary window. SDRAM speeds are specified in MHz rather than in nanoseconds, as was true of earlier forms of memory.


Relative to standard SDRAM, Double Data Rate SDRAM (DDR-SDRAM) doubles the amount of data transferred per clock cycle, and thereby effectively doubles peak memory bandwidth. DDR-SDRAM is an evolutionary improvement of standard SDRAM, which is now sometimes called Single Data Rate SDRAM or SDR-SDRAM to differentiate it. Because DDR-SDRAM costs essentially the same to produce as SDR-SDRAM, it quickly obsoleted SDR-SDRAM.

The chips used to produce a DDR-SDRAM memory module, called a DIMM (Dual In-line Memory Module) are named for their operating speed. For example, 100 MHz chips are double-pumped to 200 MHz, and so are called DDR200 chips. Similarly, chips that operate at 133 MHz are called DDR266 chips, those that operate at 166 MHz are called DDR333 chips, and those that operate at 200 MHz are called DDR400 chips.

Unlike SDR-SDRAM DIMMs, which are designated by their chip speeds, DDR-SDRAM DIMMs are designated by their bandwidth.

Their data path is 64 bits (8 bytes) wide. So, for example, a DDR-SDRAM DIMM that uses DDR200 chips transfers 8 bytes 200 million times per second, for a total bandwidth of 1,600 million bytes/second and is called a PC1600 DIMM. Similarly, DDR-SDRAM DIMMs that use DDR266 chips are labeled PC2100, those that use DDR333 chips are labeled PC2700, and those that use DDR400 chips are labeled PC3200.

The falling price of PC3200 DDR-SDRAM modules quickly obsoleted slower forms of DDR-SDRAM, although PC2700 modules remain in limited distribution. The limited availability of PC2100 and slower forms of DDR-SDRAM is not an issue for systems that use those slower variants, because PC3200 memory is backward-compatible with slower variants.


By early 2003, the original DDR-SDRAM technology was fast approaching its limits. As AMD and Intel transitioned to higher FSB speeds, DDR-SDRAM has been hard pressed to keep pace. Mainstream DDR-SDRAM tops out at PC3200. Dual-channel DDR chipsets (which combine the bandwidth of paired memory modules) using PC3200 memory limit peak bandwidth to 6,400 MB/s. That matches the bandwidth requirements of a processor with a 64-bit (8-byte) wide memory channel operating with an 800 MHz FSB, such as mainstream Pentium 4 models, but as new processors are introduced, even dual-channel DDR-SDRAM will be unable to keep up with increases in processor bandwidth.

The long-term solution is DDR2 SDRAM. DDR2 incorporates a series of evolutionary improvements on standard DDR technology, including increased bandwidth, lower voltage (1.8V versus the 2.5V of DDR), lower power consumption, and improved packaging. Just as DDR-SDRAM doubled bandwidth over SDR-SDRAM when running at the same clock rate, DDR2-SDRAM doubles bandwidth over DDR-SDRAM by doubling the speed of the electrical interface. DDR2 DIMMs use a new 240-pin connector that is incompatible with the 184pin DDR-SDRAM and earlier connectors.


DDR3 SDRAM, double-data-rate three synchronous dynamic random access memory, is a random access memory interface technology used for high bandwidth storage of the working data of a computer or other digital electronic devices. DDR3 is part of the SDRAM family of technologies and is one of the many DRAM (dynamic random access memory) implementations.

DDR3 SDRAM is an improvement over its predecessor, DDR2 SDRAM, and the two are not compatible. The primary benefit of DDR3 is the ability to transfer at twice the data rate of DDR2 (I/O at 8กม the data rate of the memory cells it contains), thus enabling higher bus rates and higher peak rates than earlier memory technologies. In addition, the DDR3 standard allows for chip capacities of 512 megabits to 8 gigabits, effectively enabling a maximum memory module size of 16 gigabytes.

With data being transferred 64 bits at a time per memory module, DDR3 SDRAM gives a transfer rate of (memory clock rate) กม 4 (for bus clock multiplier) กม 2 (for data rate) กม 64 (number of bits transferred) / 8 (number of bits/byte). Thus with a memory clock frequency of 100 MHz, DDR3 SDRAM gives a maximum transfer rate of 6400 MB/s.

DDR3 is a DRAM interface specification; the actual DRAM arrays that store the data are the same as in any other type of DRAM, and have similar performance.

Rambus RDRAM

Rambus RDRAM is a proprietary RAM standard developed jointly by Intel and Rambus. Rambus RDRAM is packaged in modules called RIMMs, which is a tradename rather than an acronym.

There are three types of RDRAM memory, called Base Rambus, Concurrent Rambus, and Direct Rambus. The first two are obsolete, and were used only in devices like game consoles. All RDRAM memory used in PCs is Direct Rambus memory. RDRAM has been made in four speeds, designated PC600, PC700, PC800, and PC1066, although only PC800 and PC1066 remain available. As with DDR-SDRAM, RDRAM modules are named according to their bandwidth, but with a difference. RDRAM uses a 16-bit or 18-bit data path (versus 64-bit for SDRAM) to transfer two bytes at a time, versus the 8-byte bandwidth of DDR-SDRAM. Accordingly, PC800 RDRAM has a bandwidth of only 1.6 GB/s and PC1066 2.133 GB/s, much lower than that of DDR-SDRAM, let alone DDR2-SDRAM.


SODIMM, small outline dual in-line memory module, is a type of computer memory built using integrated circuits.

SODIMMs (also written SODIMMs) are a smaller alternative to a DIMM, being roughly half the size of regular DIMMs. SODIMMS are often used in systems which have space restrictions such as notebooks, small footprint PCs (such as those with a Mini-ITX motherboard), high-end upgradable office printers, and networking hardware like routers.

SODIMMs have 72, 100, 144, 200 or 204 pins. The 72 and 100 pin packages supports 32-bit data transfer, while the 144, 200 and 204 pin packages support 64-bit data transfer. This compares to regular DIMMs that have 168, 184, or 240 pins, all supporting 64-bit data transfer.

The following provides you with more information of the types of memory module:

30 Pin SIMM: The 30 Pin SIMM was the first generation of the SIMM memory family. They are typically found in older Intel 286 and 386 desktop computer systems. They come in both 8 bit and 9 bit (parity) configurations, with memory ranges of 256K to 8 megabyte, 60ns to 80ns and are 5 Volts only. The DRAM types supported are mostly Page Mode and Nibble Mode DRAM which comes in both DIP, PLCC and SOJ packages.

72 Pin SIMM: The 72 Pin SIMM was the second generation of the SIMM family. They are typically found in the Intel 486, 486DX, 586 and some early Pentium desktop computer systems. They come in both 32 bit and 36 bit (parity) configurations, with memory ranges of 4, 8, 16 and 32 megabytes. These were normally produced in two voltage, 5V and 3.3V, and theirs speeds ranged from 60ns to 70ns. Both Fast Page Mode (FPM) and Extended Data Out (EDO) were standard DRAM types offered in 72 pin configurations. Dram chip packaging consisted of DIP, PLCC and SOJ packages.

168 Pin DIMM: The 168 PIN DIMM is what is found in most desktop computers today. Early on there were three memory types offered in the 168 Pin DIMM form factor, FPM, EDO and Synchronous DRAM (SDRAM). Their configurations include 64bit, 72bit and 80Bit, ECC and Non-ECC, and memory sizes included 16, 32, 64, 128, 256, 512 and 1,024 megabytes. Available voltages included 3.3v and 5v for FPM and EDO, and 3.3v for SDRAM. Features added to later versions of the 168 Pin DIMM family were the inclusion of a serial EEPROM called the "Serial Presence Detect" which contains information about the module type. Some of the later versions of the SDRAM modules (usually higher cost modules) could reach a maximum frequency of 150Mhz. Present day SDRAM technology has passed this 150 MHz range. Later generations of the 168 Pin DIMM became available as Registered DIMM's or Buffered DIMM's for high-end workstations and servers and Un-buffered DIMM's for most personal computers. There are many variants to be found with 168 Pin DIMM's, as they are the current popular form factor. The post popular of these today are the PC 100 and PC 133 varieties.

100 Pin DIMM: 100 Pin DIMMs are primarily used in printers. It comes in both 64 bit and 72bit ECC configurations, and is available in memory ranges of 16 to 128 megabytes. The SDRAM chips used on the 100 Pin DIMM are typically SDRAM in TSOP packaging. Both FPM and EDO were/are used in 100 Pin DIMM.

184 Pin RIMM: The 184 pin RIMM is used on motherboards using the latest Intel i820/i840 chipsets and is referred to as Rambus. The 184 pin RIMM module comes in both 16bit and 18bit ECC configurations, operating frequencies of 600MHz, 700MHz and 800 MHz and are available with memory ranges of 64 through 512 megabytes. The Rambus packaging is referred to as the Ball Grid Array (BGA) form factor. The Rambus modules only require 2.5 volts. There is a 1 GHz Rambus chip under development that was slated for release in 2001.

DDR SDRAM: It is the newest of the memory types under development that will be available in the 184 Pin DIMM form factor. The principle difference between conventional SDRAM and DDR SDRAM is its ability to read/write data on both edges of a clock, therefore resulting in faster data transfer. DDR SDRAM will be available in two frequencies, 200Mhz and 266Mhz, and operational voltage is 2.5v. There is speculation (these have not been released to manufacturing as of this writing) that these modules will be available in both 64 bit and 72bit ECC configurations, with memory ranges of 64 megabytes through to 1 gigabyte. The DDR SDRAM chips used are of the TSOP package and DDR RAM is essentially twice as fast as SDRAM.

72 Pin SODIMM's are typically found in Pentium II Laptop computer. They come in 64 bit configurations, with memory ranges of 8, 16 and 32 megabyte. These modules are designed around lower power consumption and only one voltage is offered, FPM and EDO at 3.3v. FPM and EDO are the standard DRAM types offered and the packaging is referred to as SOJ.

144 Pin SODIMM's are typically used in PC 66 and PC 100 SDRAM Laptop compatible computers. It comes in both 64 bit and 72bit ECC versions and the memory ranges were 16 to 256 megabyte. The SDRAM chips used in 144 Pin SODIMM's are typically SDRAM in the form of TSOP packaging and available in 3.3v only.

The 200 Pin SODIMM module comes in both 64 bit and 72bit ECC configurations, with memory ranges of 64 megabyte through 512 megabyte. DDR SODIMM modules are slated for use in next-generation DDR Laptop applications. The DDR SDRAM chips used on the 200 DIMM are typically in TSOP packaging.

©1994 - 2010 Edusoftmax Inc. All rights reserved. Questions? Comments?    Visitors: