RAM is an acronym for Random Access Memory. RAM is basically a form of electronically "remembering" something, and being able to access it randomly. This means that any piece of information can be read or written to until either it is removed or the power is taken away. There are 2 basic types of Random Access Memory. There is Static RAM and Dynamic RAM. These 2 basic forms of RAM operate in very differenet ways, and both have their advantages and disadvantages. Almost all personal computers have both types of memory.

SRAM is static in nature. It is composed of 4 transistors and 2 resistors. Unlike dynamic RAM, SRAM does not need to be refreshed, because both reading from SRAM is not distructive and there is no problem with a capacitor leaking.

DRAM is very unstable, so it is called Dynamic. DRAM, depending on what type, has to be refreshed approximately every 64ms, or 15.6 times per second. The reason for this lies in DRAM's composition. All DRAM is made up of memory cells. These cells are composed of 1 capacitator and one transistor. Capacitors by nature hold electrons. A capacitor that is full of electrons is a considered on or having the value of 1, and an empty capacitor is considered to be off, or having the value of 0.

When a capacitor is charged, it powers the gate of the transistor. The gate of the transistor controls how much electrical current is allowed to travel from the emitter, also know as the drain, to the collector, also know as the source. If the capacitor has a charge, than the gate will allow electrons to move across the transistor. If the capacitor does not have a charge, than very few electrons will be able to traverse the transistor. Since the gate isn't 100% efficient, electrons will slowly leak out of the capacitor until it is empty. This would be bad because the cell has lost it's value, and therefor to prevent this from happening the capacitor needs to be refreshed continuously.

At least every 64ms, the capacitor has to be recharged, called "refresh". The capacitor also has to be recharged when ever it is read from, because reading from it discharges the capacitor. If the capacitor is not refreshed, it will loose it's electrical charge and the cell will have a value of 0.

We haven't gotten into why we need both SRAM and DRAM. SRAM is good because it is fast, has low latency, and doesn't need to be refreshed. This memory takes approximately 2-3 cycles until it can have the information output. SRAM is bad because it is large, and therefore expensive, and it requires more power to operate, and therefore it preduces a lot of heat. That is why we need DRAM. DRAM is simple, small and space effient. DRAM may be slower and have a longer latency than SRAM, but it certainly is still very usefull. Todays DRAM can take from 2 to 9 seconds of latency until information is output. SRAM is good for low amouts of memory, anything even over 4MB is very bulky. SRAM is good for internal memory in processors, and cache, but DRAM is best for main system memory, where the average computer has need for 32MB, and some high end computers need more than 8 gigabytes. DRAM is used where it's small size and power effieciency outwiegh its slowness compaired to SRAM.

SRAM is almost as best as we can make it. We currently don't have the technology to mass produce SRAM small enough to replace DRAM. That is why DRAM is still used in computers. Unlike SRAM, it has taken DRAM many decades to evolve into what we use in our computers, and there is even new DRAM types in the distant future.

RAM Basics | DRAM Evolution