RAM (Random-access Memory)

RAM (Random-access Memory) consist of memory modules that are are installed on the motherboard that contain microchips which hold data and programs while the CPU processes both. Data in RAM is only held temporarily and will be lost when the PC is turned off. In the past, RAM chips were soldered directly onto the motherboard. Today, all RAM comes in modules of either SIMMs, DIMMs or RIMMs.

RAM Modules

Some standard module RAM types include:

Single in-line Pin Package (SIP)
Dual in-line Package (DIP)
Single in-line memory module (SIMM)
Dual in-line memory module (DIMM)
Rambus (proprietary)
Small outline DIMM (SO-DIMM). Used in laptop PCs.
Small outline RIMM (SO-RIMM)

There are two main categories of RAM. Static RAM (SRAM) and dynamic RAM (DRAM). DRAM needs to be refreshed (refreshed refers to rewriting data to the chip) every few milliseconds. Static RAM will hold data as long as it has power and provides a memory cache.

SRAM

SRAM does not need data to be constantly rewritten. This saves CPU time that would be required by DRAM which gives SRAM better performance over DRAM. SRAM is more expensive than DRAM. Due to the price of SRAM most PCs have little SRAM and a lot of DRAM.

DRAM

DRAM is always stored in either DIMM, RIMM or SIMM modules and plug directly into the motherboard.

SIMMs

The speed of SIMMs is measured in nanoseconds. This speed depends on the access time. Access time is the amount of time it takes to refresh chips and time the CPU takes to receive a value from a request. The smaller the access time, the faster the speed of the RAM will be.

DIMMs

DIMMs are rated by speed and also the amount of memory they contain. The connectors on DIMMs have either 168 or 184 pins.

Types of DIMMs

72-pin-DIMMs
144-pin-DIMMs
168-pin-DIMMs
184-pin-DIMMs
240-pin-DIMMs

RIMMs

Direct Rambus DRAM (sometimes just called Direct RDRAM or RDRAM) is named after Rambus, Inc., the company that developed it. RDRAM uses RIMM technology. RIMMs require continuity, which means all RDRAM sockets must be filled. A C-RIMM is used as a placeholder for empty sockets and ensures continuity through all the sockets. C-RIMMs contain no actual memory chips.

DDR (Double-Data-Rate) SDRAM

DDR SDRAM has double the bandwidth per cycle of regular SDRAM. It accomplishes this by by sending data on both the rising and falling edge of the clock cycle without increasing the speed of the front side bus. DDR-2 replaced DDR SDRAM and DDR-2 will soon be superseded by DDR-3. They all operate on the same principles, but the newer DDR-2/3 run on higher clock frequencies.

Volatile RAM

Most types of RAM are volatile. What this means is that unlike some other methods of storage such as hard drives, RAM loses its data when the computer is powered down. There are several types of non-volatile RAM currently being developed that will preserve data while powered down.