How Does The RAM Work?

May 7, 2009

RAM – The Random Access Memory of the computer or laptop, is abbreviated as RAM. This is a temporary memory and will lose its contents once the machine is powered off. All programs and applications that run on a laptop or notebook PC at a particular instant of time, are loaded into the RAM. In other words, the computer can run any application or program only if the code for that particular application or program is transferred from any other storage media (hard drive or a CD or DVD) to the RAM.

14091_157_21 How Does The RAM Work? The RAM consists of several memory units or memory cells. Each of these cells can store a single bit of data (i.e. either a ‘1′ or a ‘0′). Each basic cell usually consists of a transistor and a capacitor. The capacitor is an electronic component that is capable of storing electrons or charge. When a ‘1′ needs to be stored, the capacitor is charged or filled with electrons. When a ‘0′ needs to be stored, it is discharged. The transistor is employed to store the charge in the capacitor or to discharge it.

There are basically 2 types of RAMs:-
1) Dynamic RAM (DRAM) – The dynamic RAM consists of a transistor + capacitor type architecture as mentioned above. The capacitor cannot hold the charge for long and automatically keeps getting discharged. Hence it has to be refreshed every now and then. So just as the capacitor is discharging, it is recharged again. This is done at a particular clock rate. This refreshing actually slows down the working of the RAM.

2) Static RAM (SRAM) – Unlike DRAM, the static RAM does not lose its charge. It uses more number of transistors and other electronic components to retain its charge.

dram-memory-cell How Does The RAM Work? Thus, the SRAM is more stable than the DRAM. It is more faster too, because it does not require the refreshing to be done. However, since there are more number of components used to build the SRAM, it is more expensive and occupies more space, when compared to DRAM. Usually, the SRAM is used for the Cache of the CPU, where instructions are fetched in advance and stored for quick access and the DRAM is used for the regular active program instructions storage.

RAMs can further be classified depending on the way they access the required data from the memory cells. Some of them are:-

FPM DRAM - Fast page mode dynamic random access memory. Every bit is searched one by one for the required one. When found, it is read, and the search for the next bit is then started again.

EDO DRAM – Extended Data-out dynamic random access memory. Every bit is searched one by one here too. However, as soon as the required bit is found, the search for the next bit begins, while the reading of the first bit is still happening.

SDRAM – Synchronous dynamic random access memory. The speed of the DRAM (for finding and reading the bits) is synchronised to the clock of the data bus and the processor. The data bus carries data between the processor and the DRAM. Basically, the clock of the computer, helps in timing the processor, the data bus and the SDRAM.

DDR SDRAM – Dual Data Rate SDRAM. The SDRAM is made to be in sync with the system clock, transferring data to the CPU at every rising edge of the clock wave. DDR SDRAM sends data to the CPU in the rising as well as falling edge of the clock wave. Hence the speed is doubled in DDR SDRAM, when compared to normal SDRAM. We can say that 2 bits are fetched per cycle in DDR.

DDR2 SDRAM – Double Data Rate two SDRAM. This type of DRAM also fetches data at the rise and fall of the clock cycle. However, the data bus and the memory are designed to be twice as much faster as the processor. Thus 4 bits are fetched per cycle when DDR2 SDRAM is used.

ram_chip How Does The RAM Work? There are many differences between DDR and DDR2 SDRAMs. First and foremost, they are not compatible with each other, as their packaging differs. DDR2 SDRAM consumes less power and requires lesser voltage when compared to DDR SDRAM. The bandwidth for DDR2 is obviously more than that of DDR.

Noise elimination circuits are used in both SDRAMs. While these circuits are integrated into the motherboard in the case of DDR, they are built into the memory chip itself in the case of DDR2. Thus, the noise is eliminated nearer to its origin in DDR2. This is referred to as on-die termination (ODT). The pre-fetch buffers, along with this ODT feature and OCD (off chip driver), which helps maintain signal integrity, work to improve frequency of operation. However, there are latencies introduced as well. Latencies are the delays involved between the finding of the data and making it available for use. The latency involved in DDR2 is greater than that of DDR. But since the frequency of operation is more for the former, and considering all other positive aspects, it proves to be more efficient. DDR2 SDRAM is the most commonly used RAM in the laptop, netbooks and notebook PCs these days.

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