The RAMs (known as Random Access Memory according to ABBREVIATIONFINDER) is one of the most important parts of computers as they are on them that the processor stores the data with which it is dealing. This type of memory has an extremely fast data recording process, when compared to the various types of ROM memory. However, the recorded information is lost when there is no more electricity, that is, when the computer is turned off, and is therefore a type of volatile memory.
There are two types of RAM technology that are widely used: static and dynamic, that is, SRAM and DRAM, respectively. There is also a newer type called MRAM. Here is a brief explanation of each type:
– SRAM (Static Random-Access Memory – Static RAM): this type is much faster than DRAM memories, but it stores less data and has a high price if we consider the cost per megabyte. SRAM memories are often used as a cache;
– DRAM (Dynamic Random-Access Memory – Dynamic RAM): memories of this type have high capacity, that is, they can hold large amounts of data. However, access to this information is usually slower than access to static memories. This type also usually has a much lower price when compared to the static type;
– MRAM (Magnetoresistive Random-Access Memory – Magneto-resistive RAM): MRAM memory has been studied for some time, but it was only in the last few years that the first units appeared. It is a type of memory somewhat similar to DRAM, but which uses magnetic cells. Thanks to this, these memories consume less energy, are faster and store data for a long time, even in the absence of electricity. The problem with MRAM memories is that they store very little data and are very expensive, so they are unlikely to be widely adopted.
Aspects of RAM memory operation
DRAM memories are formed by chips that contain a huge amount of capacitors and transistors. Basically, a capacitor and a transistor together form a memory cell. The first has the function of storing electrical current for a certain time, while the second controls the passage of this current.
If the capacitor is current storage, it has a bit 1. If it is not, it has a bit 0. The problem is that the information is kept for a short period of time and, so that there is no loss of data from the memory, a component of the memory controller is responsible for the refresh function , which consists of rewriting the cell contents from time to time. Note that this process is performed thousands of times per second.
Refreshing is a solution, but accompanied by “side effects”: this process increases energy consumption and, consequently, increases the heat generated. In addition, the speed of access to memory ends up being reduced.
SRAM memory, in turn, is quite different from DRAM and the main reason for this is the fact that it uses six transistors (or four transistors and two resistors) to form a memory cell. In fact, two transistors are responsible for the control task, while the others are responsible for electrical storage, that is, for the formation of the bit.
The advantage of this scheme is that refreshing is no longer necessary, making SRAM memory faster and less energy consuming. On the other hand, as its manufacture is more complex and requires more components, its cost ends up being extremely high, making the construction of a computer based only on this type too expensive. That is why its most common use is as a cache, because small amounts of memory are required for this.
As DRAM memories are more common, they will be the focus of this text from this point on.
Various memory technologies have been (and are) created over time. It is thanks to this that, periodically, we find faster memories, with greater capacity and even memories that demand less and less energy. Here is a brief description of the main types of RAM:
– FPM (Fast-Page Mode): one of the first RAM memory technologies. With the FPM, the first reading of the memory has an access time longer than the following readings. This is because, in fact, four read operations are performed in a row, instead of just one, in an xyyy type scheme, for example: 3-2-2-2 or 6-3-3-3. The first reading ends up taking longer, but the next three are faster. This is because the memory controller works only once with the address of a line (RAS) and then works with a sequence of four columns (CAS), instead of working with a RAS signal and a CAS signal for each bit. FPM memories used SIMM modules, both 30 and 72 ways;
– EDO (Extended Data Output): the successor to the FPM technology is EDO, which highlights the ability to allow a memory address to be accessed at the same time that a previous request is still in progress. This type was applied mainly in SIMM modules, but it was also found in 168-way DIMM modules. There was also a similar technology, called BEDO (Burst EDO), which worked faster because it had less access time, but was hardly used, as it had a higher cost because it was owned by the Micron company. In addition, it was “overshadowed” by the arrival of SDRAM technology;
– SDRAM (Synchronous Dynamic Random Access Memory): FPM and EDO memories are asynchronous, which means that they do not work synchronously with the processor. The problem is that, with faster and faster processors, this started to become a problem, as the processor often had to wait too long to access the data in memory. SDRAM memories, in turn, work synchronously with the processor, avoiding delay problems. Based on this technology, we started to consider the frequency with which memory works to measure speed. Then arose the memories SDR SDRAM (Single Data Rate SDRAM), which could work with 66 MHz, 100 MHz and 133 MHz (also called PC66, PC100 and PC133, respectively). Many people refer to this memory only as “SDRAM memories” or even as “DIMM memories”, because of its module. However, the SDR appellation is the most appropriate;
– DDR SDRAM (Double Data Rate SDRAM): DDR memories have a significant evolution compared to the SDR standard, because they are capable of handling twice as much data in each clock cycle (SDR memories work only with one operation per cycle). Thus, a DDR memory that works at the frequency of 100 MHz, for example, ends up doubling its performance, as if it worked at the rate of 200 MHz. Visually, it is possible to easily identify them in relation to SDR modules, because the latter contains two divisions at the bottom, where your contacts are, while DDR2 memories have only one division.
– DDR2 SDRAM : as the name implies, DDR2 memories are an evolution of DDR memories. Its main feature is the ability to work with four operations per clock cycle, therefore, double the previous standard. The DDR2 modules also have only one division at the bottom, however, this opening is slightly more sideways.
– DDR3 SDRAM: DDR3 memories are, obviously, an evolution of DDR2 memories. Again, the number of operations per clock cycle is doubled, this time, eight. A novelty here is the possibility of using Triple-Channel. Learn more about this type in this article on DDR3 ;
– Rambus (Rambus DRAM): Rambus memories are named because they were created by Rambus Inc. and arrived on the market with the support of Intel. They are different from the SDRAM standard, as they only work with 16 bits at a time. On the other hand, Rambus memories work with a frequency of 400 MHz and with two operations per clock cycle. However, they had disadvantages, very high latency rates, high heating and higher cost. Rambus memories have never been widely accepted in the market, but they have not been a total fiasco either: they were used, for example, in the Nintendo 64 game console. Interestingly, Rambus memories work in pairs with “empty modules” or “blind combs”. This means that for each Rambus module installed, an “empty module” has to be installed in another slot. This technology ended up losing space for DDR memories.