RAM

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Introduction

RAM (Random Access Memory) has quite a simple task for such a confusing name. For one random access is not referring to the RAM being random but the user being random in nature. For instance, do you know what you are going to exactly do three seconds from now with the computer? Well no, you will never move the mouse or press a button the same way or at the same time every time you use the computer. Below you will see different RAM types that have evolved through the years with the increase in speed, hardware, and error correction & detection demands.

Species of RAM

The DIPP (Dual Inline Pin Package) is what you see on the RAM sticks them selves, and at one time you had to install them individually on the motherboard with the notch indicating which pin is to go into its proper socket. This system was far from user friendly considering that you needed to know a little about electronics so that you don't install the RAM in the wrong direction. This system soon lost ground when people realized that there was a better way that could ensure that no mistakes could happen.

The 30-pin SIMM (Single Inline Memory Module) was an improvement to the original design because the SIPP's were placed on a circuit board and called a SIMM. This soon proved to be superior for reasons you might have already guessed, you could install four SIPP boards on the computer in little time, you could not install them in the wrong way because of a pimple notch, and if care was taken, you could never break the pins off of the SIPP's.

72-pin SIMM (Single Inline Memory Module) was yet another improvement to the RAM evolution and was introduced in the 486 era. The main purpose of the 72-pin SIMM over the 30-pin SIMM was that mother board space was becoming hard to come by with the RAM sometimes taking ¼ of the motherboard.

72-pin SIMM's freed up a lot of space and circuitry while once again making it easier to buy and install RAM. Although only 2 sticks of RAM are needed to fill a bank, there are some 4 different types of this RAM all which make purchasing and troubleshooting old systems harder and will take some research on the motherboard.

168-pin DIMM 168-pin DIMM (Dual Inline Memory Module) RAM is one of the largest leaps forward in technology. When the computer calls upon RAM it needs information in 64k blocks called banks but 30-pin SIMM's can only transfer 16k blocks

per stick, 72-pin SIMM's can only deliver 32k per stick while the 168-pin DIMM can deliver 64k per stick. This is not the only feature that makes it so great, previous RAM used a system that was insulated from the system bus through the chipset. Now RAM is directly in contact with the main bus which allows the RAM to be synchronous with the CPU and allow react speeds of 10ns rather then the fastest SIMM's around 60ns.

Breeds of RAM

Now that you may have a grasp on what the different types of RAM look like, it is also important that you know what the different breeds are. Through the years companies have tried to give the user the best product for their buck while keeping the cost at a competitive price, sometimes this has worked and sometimes it has not.

SRAM (Static Random Asses Memory) is the ultimate type of RAM, it is made up of transistors that form high speed flip-flops that also come at a high cost. Each flip-flop is made up of 18 transistors that make up each logical bit that is to be stored, now multiply this number by the thousands and soon you will realize that there are a lot of transistors in such a little space. This method takes up a lot of manufacturing time with a low yield of storage per area. Only supper computers, graphic industries, and serious computer lovers enjoy the speed of this RAM, the rest of us have deled with DRAM.

DRAM (Dynamic Random Access Memory) is a solution to the high cost of SRAM. While SRAM is made up of transistors, DRAM is made up of capacitors that act like little batteries that need to be recharged every millisecond. The concept behind D-RAM is that capacitors are devices that can hold electrical charge through electrostatic charge, the larger the capacitor the more of a charge it can hold. Since these capacitors are infinitely small, they can only hold a charge for a small amount of time and is why they have to be refreshed every so often. The tradeoff is that this RAM is cheep but is also slow and is as subject to more errors which is why there is a larger variety of D-RAM.

Sub-Breeds of DRAM

One of the oldest types of RAM for the PC is FPMRAM (Fast Page Mode Random Access Memory) which offered little assistance to make DRAM more like SRAM, it has an access time of 120ns in it's early years to 60ns just before it was finally retired.

Parity (error detection) RAM was not such a great success as it was a stepping stone for other types of RAM. The Parity scheme was one that looked at all the bits that were to be transferred and added them all up, if the result was even then the parity bit was not set, if all the bits added together and the result was odd then the parity bit was set. The computer would know if any data was lost by parity bit, if it did not match the data being sent then the system would lock up. This scheme was almost useless because parity RAM was more expensive, locked up the system if an error occurred , and if an even number of bits were changed with data corruption then the computer would never know. People soon began to wonder if this scheme was more harmful then helpful and as a result this RAM died out very fast. Parity chips can be identified by the number of chips on the RAM stick, parity RAM has an odd number of DIPP's on board while ECC, EDO, and FPM RAM all have an even number of DIPP chips.

ECC RAM (Error Correction Code Random Access Memory) uses parity to not only detect errors but to correct them with the use of special algorithms. This RAM is still in use today but is not talked about much with the introduction of SDRAM.

EDO (Extended Data Out) RAM not only used its own scheme of error correction but also used a variety of FPM RAM that keeps track of where the last call in memory was located. When the CPU asks for more information, there is a 92% chances that the next called to RAM is the next location in RAM. This phenomenon is known as the "principle of locality" and delivers a 40% increase in performance in speed.

SDRAM (Synchronous Dynamic RAM) is proprietary to the DIMM slot which takes advantage of the system clock to be synchronized with the CPU. It is buffered with a technology that knows what information is needed next so it can deliver it in a burst of data. Also it has a bandwidth of 64k which allows these chips to fulfill the needs of one bank. This means that you only need to install one DIMM chip to fill a bank, which is needed to boot the computer up.