Computer Memory: A Brief Survey of Technologies

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Below is a (very) brief cheat-sheet of descriptions of the most commonly used memory technology and specifications today:

Dual In-line Memory Modules (DIMM) -- A Dual In-line Memory Module (DIMM) is actually not a type of memory; but rather, simply a number of memory components placed onto a circuit board with 240 pins which provide an interface to the memory bus. Memory types like DRAM utilize this form factor (Memory Module Picture, 2005).
Opposed to a Single In-Line Memory Modules (SIMM), which provides a 32bit gateway; DIMMs provide a 64-bit gateway. DIMMs are commonly used on Intel based computers.

Static RAM (SRAM) -- Static RAM is the second most common type of memory used in computers today. Data stored in it is considered volatile; in that, it loses its data once it loses power. SRAM is larger than DRAM and consists of six transistors. It is also more expensive, uses more electricity, and is less dense; however, SRAM is faster than DRAM. Its name refers to the fact that bits stored electronically in its cells do not require periodic refreshing. It is commonly used in small amounts for high-speed purposes such as the memory cache of CPUs (Englander, 2003).

Dynamic random access Memory (DRAM) -- Dynamic RAM is most popular type of memory used in computers’ main memory. Like SRAM, it is volatile; in that, the data stored in its cells is lost when power to it is cut; however, unlike SRAM, the bits stored electronically in its cells require periodic refreshing (Englander, 2003). DRAM is smaller than SRAM and consists of a single capacitor and transistor. It is less expensive than SRAM and also requires less electricity. It is also denser than SRAM; in that, more bits of information can be stored on a smaller integrated circuit than SRAM.

Enhanced Synchronous DRAM (ESDRAM). -- Enhanced Synchronous DRAM is essentially standard DRAM, but accompanied by a small cache of faster SRAM. Functioning conceptually like a CPU cache, this small amount of SRAM is used to improve latency times associated with generic DRAM. This cache also allows for a larger bus to be placed between the SRAM and DRAM, effectively increasing the speed of DRAM even during cache-misses. (Memory: Evolution or Revolution)

Double Data Rate 2 (DDR2) -- Double Data Rate RAM (DDR2) is essentially built upon the same memory specification as DDR SDRAM; however it is faster than generic DDR SDRAM by way of its ability to process twice as many signals. (Mellor, C. 2004). This is accomplished by through a differential signal technology which has a 4-bit pre-fetch cache opposed to DDR SDRAM’s 2-bit, resulting in an ability to read and write data twice as fast for each clock-cycle. Like DDR SDRAM, DDR2 is an example of volatile memory.

Double Data Rate Synchronous Dynamic RAM (DDR SDRAM) -- Double Data Rate Synchronous Dynamic RAM is essentially SDRAM with the capabilities of performing twice as many operations by submitting a signal at the top and bottom of each clock cycle (Gervasi, 2001). DDR SDRAM is volatile, in that must be powered in order to retain the data stored in it.

Double Data Rate 3 (DDR3) -- Double Data Rate RAM (DDR3) is essentially the same as DDR2; however, the one big difference is that it has an 8-bit pre-fetch cache opposed to DDR2’s 4-bit and DDR SDRAM’s 2-bit. This resulting in an ability to read and write data four times as fast as DDR SDRAM for each clock-cycle. Another important difference is that DDR3 consumes less power than DDR2 and DDR SDRAM. Like DDR SDRAM, DDR3 is an example of volatile memory (Mellor, C. 2007).

NAND Flash Memory -- NAND, which stands for “Not AND”, is a type of solid-state memory which persists data electronically without mechanical assistance (i.e. moving parts) like standard platter based hard drives do. NAND memory is also non-volatile; in that, it does not require power to retain the data stored on it. (“NAND vs. NOR”, 2005) Because of NAND flash memory’s lack of random-access capabilities, it has most commonly been used in secondary memory applications; opposed to main memory. Due to its non-volatility, higher density, lower cost, and faster write times relative to NOR flash memory, it is ideal for the use as storage in mobile devices such as digital cameras, MP3 players, and USB pen drives.

NOR Flash Memory -- NOR Flash memory, is a type of solid-state memory, which like NAND flash memory, persists data electronically without the use of mechanical parts. Also like NAND Flash memory, it is non-volatile; in that, it does not require power to retain the data stored on it.’
It can be used as ordinary non-volatile secondary storage; however; due to its random access and fast read capabilities, it is well suited to be used as main memory in portal devices such as PDAs and cell phones. (“NAND vs. NOR”, 2005)

Zero capacitor RAM (ZRAM) -- Zero capacitor RAM, is similar to DRAM; in that, it contains only one transistor; however, as its name suggests, it does use a capacitor to form a memory cell as DRAM does (Technology Overview). Its configuration, consisting of only a single transistor, is the smallest possible implementation of a memory cell. This configuration makes is much more dense than SRAM and DRAM. Innovative Silicon, the innovators of ZRAM, refers to it as “ultra-dense memory.” Like both SRAM and DRAM, ZRAM is volatile; in that, it requires constant power in order to maintain the data stored in its cells. Although not as fast as SRAM, ZRAM is faster than DRAM, is smaller, and uses less electricity than either type.

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