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Types of DRAM

 

FPM DRAM - Fast Page Mode DRAM
An improvement over straight DRAM. It treats contiguous blocks of memory as pages. The premise behind this is that if a needed piece of data is pulled from a certain location in secondary storage, most likely the next piece you need will come right after the previous piece. Getting back to the RAS and CAS, another shortcut to speed up
RAM was to initially use a RAS and CAS, and then each subsequent read only needed a CAS. Why? Well, because of the spreadsheet type layout (except that the information is stored sequentially horizontally) you are already on the right row, so to get the next piece of information you only need to move over a column.

• 60 to 70 ns
• 30MHz Max bus speed, a standard Pentium has a 66MHz bus speed, so you can see how FPM DRAM is behind the power curve.

EDO DRAM - Extended Data Out DRAM
EDO RAM is FPM RAM (They are architecturally the same) with exception to the buffering circuitry. The data coming off an EDO chip is valid longer than FPM RAM. This all has to do with timing and polling cycles from the CPU. The data is valid longer, so the CPU doesn't have to re-request the same data again, hence the extended data out
name.

• 45, 50, 60 and 70 ns
• 10 to 15 percent faster than FPM DRAM
• 66MHz bus speed maximum

Many motherboards support both FPM and EDO SIMMs. Use one or the other, but do not mix them.

 

BEDO DRAM - Burst Extended Data Out DRAM
Think of this as a caching type system, except it's better. The CPU sends a burst to the RAM with requests for the data it needs. This burst contains four elements. The first element is a request for the data it needs now, and the next three are requests for data it will need next. This is better than caching, because the CPU knows exactly what data it will need next, as opposed to an educated guess performed by caching algorithms.

• 66MHz bus speed maximum
• Only supported by VIA 580UP, 590UP and 680UP chipsets.

 

SDRAM - Synchronous DRAM
The RAM is synched to the system clock, and it allows for two pages of memory to be opened simultaneously.

• 6, 7, 10 and 12 ns
• 100MHz bus, but older SDRAM chips are limited to 60MHz bus.

Ok, that covers the DRAM section. These are the chips that make up your system memory which is also referred to as your primary storage. The next section deals with RAM used for a different but similar purpose.

DRAM	Static RAM