RAM types

RAM types
The traditional RAM type is DRAM (dynamic RAM). The other type is SRAM (static RAM).
SRAM continues to remember its content, while DRAM must be refreshed every few milli
seconds. DRAM consists of micro capacitors, while SRAM consists of off/on switches.
Therefore, SRAM can respond much faster than DRAM. SRAM can be made with a rise time
as short as 4 ns. It is used in different versions in L2 cache RAM (for example pipe line Burst
Cache SRAM).
DRAM is by far the cheapest to build. Newer and faster DRAM types are developed
continuously. Currently, there are at least four types:
l   FPM (Fast Page Mode)
l   ECC (Error Correcting Code)
l   EDO (Extended Data Output)
l   SDRAM (Synchron Data RAM)
A brief explanation of DRAM types:
FPM was the traditional RAM for PC's, before the EDO was introduced. It is mounted in SIMM
modules of 2, 4, 8, 16, or 32 MB. Typically, it is found in 60 ns or 70 ns versions. 60 ns is the
fastest and the one to use. You cannot mix different speeds on the same Pentium system
board.
EDO is an improvement of FPM RAM. Data are read faster. By switching from FPM to EDO,
one can expect a performance improvement of 2 to 5 percent. EDO RAM are usually sold in 60
ns versions. A 50 ns version is available at higher cost.
ECC RAM is a special error correcting RAM type. It is especially used in servers.
SDRAM is the newest RAM type for PC's. It comes only in 64 bit modules (long 168 pin
DIMM's). SDRAM has a rise time of only 8-12 ns. The performance improvement over EDO
RAM is only 5 percent running at 66 MHZ, but at 100 MHZ it will prove a lot better.
RAMBUS (RDRAM) is a future RAM type. Intel and others have great expectations from this
type.