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PC memory types (RAM)

[tech: PC ]
author: Lyberty; last updated: April 2004

RAM -- Random Access Memory

DRAM -- Dynamic RAM

Dynamic random access memory (DRAM) is the most common kind of random access memory (RAM) for personal computers and workstations. Memory is the network of electrically-charged points in which a computer stores quickly accessible data in the form of 0s and 1s. Random access means that the PC processor can access any part of the memory or data storage space directly rather than having to proceed sequentially from some starting place. DRAM is dynamic in that, unlike static RAM (SRAM), it needs to have its storage cells refreshed or given a new electronic charge every few milliseconds. Static RAM does not need refreshing because it operates on the principle of moving current that is switched in one of two directions rather than a storage cell that holds a charge in place. Static RAM is generally used for cache memory, which can be accessed more quickly than DRAM.
DRAM stores each bit in a storage cell consisting of a capacitor and a transistor. Capacitors tend to lose their charge rather quickly; thus, the need for recharging. A variety of other RAM interfaces to the computer exist, such as EDO RAM and SDRAM.

RDRAM -- Rambus DRAM

If you are using RDRAM, make sure that all memory sockets of a channel are filled with either a memory chip or a continuity module.
RDRAM often has to be installed in pairs of the same type of memory chips.

RDRAM devices may be configured into single-, dual- or quad-channel RIMM modules. For dual-channel or quad-channel (4-channel) RDRAM chipsets and motherboards, memory module upgrades should be in matched pairs. For instance, to add 512 MByte of memory into a dual or 4-channel system, two matched 256 MByte modules should be inserted.

32-bit RIMM modules, such as RIMM 4200, 4800, and 6400 modules, can be upgraded singly on dual channel systems.

Q: DIMM (Dual Inline Memory Module) or RIMM (Rambus Inline Memory Module)?
A: RIMM is the trademarked name for a Direct Rambus memory module.
RDRAM is available only in RIMM packages (such as 184-pin RIMMs).
RIMMs look similar to DIMMs, but have a different pin count. (Typically DIMMs are 168-pin, while RIMMs are 184-pin.) RIMMs transfer data in 16-bit chunks.

For more information, see the Rambus F.A.Q.

Example of RDRAM speeds include PC 800 (PC-800 or PC800) and PC 1066 (PC-1006 or PC1066).

Dual Channel RDRAM -- Dual-channel PC800 RDRAM, as found in Intel's 850 Pentium 4 chipset, provides a higher bandwidth than even PC2700 DDR-SDRAM (DDR333).
[view chart]


SDRAM (synchronous DRAM) is a generic name for various kinds of dynamic random access memory (DRAM) that are synchronized with the clock speed that the microprocessor is optimized for. This tends to increase the number of instructions that the processor can perform in a given time. The speed of SDRAM is rated in MHz rather than in nanoseconds (ns). This makes it easier to compare the bus speed and the RAM chip speed. You can convert the RAM clock speed to nanoseconds by dividing the chip speed into 1 billion ns (which is one second). For example, an 83 MHz RAM would be equivalent to 12 ns.

Example: PC-133 CL2 SDRAM = 133MHz (SDRAM, PC133 • CL=2 • Unbuffered • Non-parity • 133MHz • 3.3V )


DDR SDRAM -- Double Data Rate SDRAM

DDR SDRAM is synchronous dynamic RAM (SDRAM) that can theoretically improve memory clock speed to at least 200 MHz*. It activates output on both the rising and falling edge of the system clock rather than on just the rising edge, potentially doubling output. It's expected that a number of Socket 7 chipset makers will support this form of SDRAM.

When released DDR SDRAM memory was about twice as expensive as conventional SDRAM memory.

*Synchronous DRAM speed is measured in MHz rather than nanoseconds (ns). You can convert the RAM clock speed to nanoseconds by dividing the chip speed into 1 billion ns (which is one second). For example, an 83 MHz RAM would be equivalent to 12 ns.

PC-4000 = DDR-500
PC-3200 = DDR-400 *
PC-2700 = DDR-333 (DDR333)*
PC-2100 = DDR-266 (DDR266)
PC100 = PC-100 DDR RAM was sometimes called PC-1600 SDRAM because of its data bandwidth (transfer capacity) of 1.6GB per second.
*comes in 184-pin DIMM and 200-pin SODIMM formats

The fiirst official platform with DDR SDRAM support was released in October 2000.

DDR2 SDRAM -- Double Data Rate SDRAM

DDR2 PC2-3200 = DDR2-400
DDR2 PC2-4300 = DDR2-533

Figure: Marketing-based comparison of DDR types, in GigaBytes per second.
Figure: Marketing-based comparison of DDR types, in GigaBytes per second.


RAM used to be availabe in 72 pin SIMM's, but are now (as of April 2004) most commonly found as 168 pin DIMM's.

Examples of 168 pin DIMM SDRAM include PC 66 (PC-66), PC 100 (PC-100), and PC 133 (PC-133).





How to identify different sizes of computer memory RAM?
Tom's Hardware: RAM GUIDE
the Crucial memory page | Crucial: Rambus and DDR explained
"OCZ Attempts to Step Up the Clock Pace with DDR500 (PC-4000) Module" [Jan. 2004]

another old page on computer memory types

DVD Information (includes DVD-RAM)