It is called "Dynamic Random Access Memory" because it must be refreshed constantly (many times per second) so it can retain the information stored in the memory. Its memory cells are based on tiny electronic "capacitors" that store electrical charges representing binary values. These capacitors lose their charges, so they have to be recharged constantly.

DRAM comes in FPM, EDO and SDRAM types. FPM DRAM is much slower than other types of RAM such as: SRAM, EDORAM, SDRAM, etc.; however, it is much cheaper!

A DRAM chip is usually a Dual Inline Pin (DIP) chip connected directly to a SIMM chip (or a DIMM chip) as shown:

Courtesy Kingston Technology Corporation.

A SIMM chip often contains 8 or 9 DRAM chips per SIMM. An 8-pin SIMM chip SIMM is non-parity RAM, and a 9-pin SIMM chip is "parity" RAM. An 8-chip SIMM is a non-parity chip with 32 bits, a 9-chip SIMM is a parity chip with 36 bits. However, many SIMMs come with differing amounts of DRAM chips. I have seen some with DRAM chips on both sides of the SIMMs, and some with only 2 or 4 DRAM chips on the SIMM. So, you cannot always tell the capacity just by the number of DRAM chips on the SIMM.

DRAM is the RAM of choice because it costs less and is physically smaller than SRAM or other types of memory. DRAM is too slow for most cache memory. SRAM is used more as "cache" than as RAM.

SIMM and DIMM chips:

Single In-Line Memory Modules (SIMMs) come in 30 and 72 pin versions. They hold a number of DRAM chips (which may be DRAM or EDORAM). They are normally either 60ns or 70ns in speed.

Dual In-Line Memory Modules (DIMMs) come on 168-pin chips but are usually filled with SDRAM chips.. They will not fit in most machines sold before 1996-7. You have to have the 168-pin slots. The DIMMs on the market that I have seen have SDRAM chips, with a speed of "10 ns". WOW!! And, There are basically 3 types of DRAM:

1. Fast-Page Mode (FPM)
2. Extended Data Out (EDO RAM)
3. Synchronous DRAM (SDRAM)

Fast-Page Mode DRAM. (FPM)

FPM DRAM was used mostly in the 386 and 486 models of computer. It was one of the earliest evolutions of DRAM. FPM usually comes on 30 and 72 pin SIMMs, and its speeds are 60ns and 70ns.

Extended Data Out (EDO).

EDO RAM is a hopped up version of FPM. It allows a CPU to start reading a new piece of data in RAM, even before it completes the last read of data from RAM. It is 10-15% faster than FPM. The speed I have seen advertised is 50, 60, and 70ns.

Synchronous DRAM (SDRAM).

Is the newest (as of early 1997) marketed version of DRAM. It has a clock connecting the memory to the system clock, which controls the operations of the microprocessors different chips. It synchronizes the input and output processes. It is much, much faster than EDO RAM. The SDRAM I have seen is mounted on 168 pin DIMM chips, and the speed was 10ns! Only a couple of types of SRAM can match or surpass this speed!

Just remember this: You cannot upgrade, in most cases, from FPM to EDO to SDRAM. So, the type that is in your PC is what you are stuck with. You can buy more of what you are stuck with, but at a cost.