The capacitor in a dynamic RAM memory cell is like a leaky bucket. It needs to be refreshed periodically or it will discharge to 0. This refresh operation is where dynamic RAM gets its name. Dynamic RAM has to be dynamically refreshed all of the time or it forgets what it is holding. The downside of all this refreshing is that it takes time and slows down the memory.
In this article, you’ll learn all about what RAM is, what kind you should buy and how to install it.
Memory is made up of bits arranged in a two-dimensional grid.
In this figure, red cells represent 1s and white cells represent 0s. In the animation, a column is selected and then rows are charged to write data into the specific column.
Memory cells are etched onto a silicon wafer in an array of columns (
bitlines) and rows ( wordlines). The intersection of a bitline and wordline constitutes the address of the memory cell.
DRAM works by sending a charge through the appropriate column (CAS) to activate the transistor at each bit in the column. When writing, the row lines contain the state the capacitor should take on. When reading, the sense-amplifier determines the level of charge in the capacitor. If it is more than 50 percent, it reads it as a 1; otherwise it reads it as a 0. The counter tracks the refresh sequence based on which rows have been accessed in what order. The length of time necessary to do all this is so short that it is expressed in
nanoseconds (billionths of a second). A memory chip rating of 70ns means that it takes 70 nanoseconds to completely read and recharge each cell.
Memory cells alone would be worthless without some way to get information in and out of them. So, the memory cells have a whole support infrastructure of other specialized circuits. These circuits perform functions such as:
Identifying each row and column ( row address select and column address select)
Keeping track of the refresh sequence ( counter)
Reading and restoring the signal from a cell ( sense amplifier)
Telling a cell whether it should take a charge or not ( write enable)
Other functions of the memory controller include a series of tasks that include identifying the type, speed and amount of memory and checking for errors.
Static RAM works differently from DRAM. We’ll look at how in the next section.
Static RAM uses a completely different technology. In static RAM, a form of flip-flop holds each bit of memory (see How Boolean Logic Works for details on flip-flops). A flip-flop for a memory cell takes four or six transistors along with some wiring, but never has to be refreshed. This makes static RAM significantly faster than dynamic RAM. However, because it has more parts, a static memory cell takes up a lot more space on a chip than a dynamic memory cell. Therefore, you get less memory per chip, and that increases its price.
Static RAM is fast and expensive, and dynamic RAM is less expensive and slower. So static RAM is used to create the CPU’s speed-sensitive cache, while dynamic RAM forms the larger system RAM space.
Memory chips in desktop computers originally used a pin configuration called
dual inline package (DIP). This pin configuration could be soldered into holes on the computer’s motherboard or plugged into a socket that was soldered on the motherboard. This method worked fine when computers typically operated on a couple of megabytes or less of RAM, but as the need for memory grew, the number of chips needing space on the motherboard increased.
The solution was to place the memory chips, along with all of the support components, on a separate
printed circuit board (PCB) that could then be plugged into a special connector ( memory bank) on the motherboard. Most of these chips use a small outline J-lead (SOJ) pin configuration, but quite a few manufacturers use the thin small outline package (TSOP) configuration as well. The key difference between these newer pin types and the original DIP configuration is that SOJ and TSOP chips are surface-mounted to the PCB. In other words, the pins are soldered directly to the surface of the board, not inserted in holes or sockets.
Memory chips are normally only available as part of a card called a
module. When you shop for memory, on many of the modules you can see the individual memory chips.
In the next section we’ll look at some other common types of RAM.
Types of RAM
An engineer holds a DDR SDRAM memory chip.
The following are some common types of RAM:
SRAM: Static random access memory uses multiple transistors, typically four to six, for each memory cell but doesn’t have a capacitor in each cell. It is used primarily for cache.
DRAM: Dynamic random access memory has memory cells with a paired transistor and capacitor requiring constant refreshing.
FPM DRAM: Fast page mode dynamic random access memory was the original form of DRAM. It waits through the entire process of locating a bit of data by column and row and then reading the bit before it starts on the next bit. Maximum transfer rate to L2 cache is approximately 176 Mbps.
EDO DRAM: Extended data-out dynamic random access memory does not wait for all of the processing of the first bit before continuing to the next one. As soon as the address of the first bit is located, EDO DRAM begins looking for the next bit. It is about 5-20 percent faster than FPM DRAM. Maximum transfer rate to L2 cache is approximately 264 Mbps.
SDRAM: Synchronous dynamic random access memory takes advantage of the burst mode concept to greatly improve performance. It does this by staying on the row containing the requested bit and moving rapidly through the columns, reading each bit as it goes. The idea is that most of the time the data needed by the CPU will be in sequence. SDRAM is about 5 percent faster than EDO RAM and has a transfer rate of 0.8-1.3 megatransfers per second (MT/s). It was developed in 1988.
DDR SDRAM: This is the next generation of SDRAM. Double data rate synchronous dynamic RAM is just like SDRAM except that is has higher bandwidth, meaning greater speed. Its transfer rate is 2.1-3.2 MT/s. DDR was released in 2000 and has advanced three subsequent generations. DDR2 (2003) has a transfer rate of 4.2-6.4 MT/s and DDR3 (2007) transfers data at 8.5-14.9 MT/s. The most recent generation in widespread use is DDR4, launched in 2014. Its transfer rate is 17-21.3 MT/s. These standards are set by the Joint Electron Device Engineering Council (JEDEC), an organization made up of electronics companies. JEDEC released its specification for DDR5 in July 2020. RAM manufacturer Micron believes the new standard will increase performance by 87 percent when compared with a DDR4 module.
RDRAM: Rambus dynamic random access memory is a radical departure from the previous DRAM architecture. Designed by Rambus, RDRAM uses a Rambus in-line memory module (RIMM), which is similar in size and pin configuration to a standard DIMM. What makes RDRAM so different is its use of a special high-speed data bus called the Rambus channel. RDRAM memory chips work in parallel to achieve a data rate of 800 MHz, or 1,600 Mbps or higher. Since they operate at such high speeds, they generate much more heat than other types of chips. To help dissipate the excess heat Rambus chips are fitted with a heat spreader, which looks like a long thin wafer. Just like there are smaller versions of DIMMs, there are also SO-RIMMs, designed for notebook computers.
Credit Card Memory: Credit card memory is a proprietary self-contained DRAM memory module that plugs into a special slot for use in notebook computers.
PCMCIA Memory Card: Another self-contained DRAM module for notebooks, cards of this type are not proprietary and should work with any notebook computer whose system bus matches the memory card’s configuration. They are rarely used nowadays.
CMOS RAM: CMOS RAM is a term for the small amount of memory used by your computer and some other devices to remember things like hard disk settings. This memory uses a small battery to provide it with the power it needs to maintain the memory contents.
VRAM: VideoRAM, also known as multiport dynamic random access memory (MPDRAM), is a type of RAM used specifically for video adapters or 3-D accelerators. The "multiport" part comes from the fact that VRAM normally has two independent access ports instead of one, allowing the CPU and graphics processor to access the RAM simultaneously. Located on the graphics card, VRAM comes in a variety of formats, many of which are proprietary. The amount of VRAM is a determining factor in the resolution and color depth of the display. VRAM is also used to hold graphics-specific information such as 3-D geometry data and texture maps. True multiport VRAM tends to be expensive, so many graphics cards use SGRAM (synchronous graphics RAM) instead. Performance is nearly the same, but SGRAM is cheaper.
A close-up of the memory RAM circuit board.
The kinds of board and connector used for RAM in desktop computers have evolved over the past few years. The first types were proprietary, meaning that different computer manufacturers developed memory boards that would only work with their specific systems.
SIMM, which stands for single in-line memory module. This memory board used a 30-pin connector and was about 3.5 x 0.75 inches in size (about 9 x 2 cm). In most computers, you had to install SIMMs in pairs of equal capacity and speed. This is because the width of the bus is more than a single SIMM.
For example, you would install two 8-megabyte (MB) SIMMs to get 16 megabytes total RAM. Each SIMM could send 8 bits of data at one time, while the system bus could handle 16 bits at a time. Later SIMM boards, slightly larger at 4.25 x 1 inch (about 11 x 2.5 cm), used a 72-pin connector for increased bandwidth and allowed for up to 256MB of RAM. SIMM was used from the early 1980s to early 2000s.
As processors grew in speed and bandwidth capability, the industry adopted a new standard in
dual in-line memory module (DIMM). DIMMs range in capacity and can be installed singly instead of in pairs.
Some brands of laptop computers use RAM based on the
small outline dual in-line memory module (SODIMM) configuration. SODIMM cards are small, about 2 x 1 inch (5 x 2.5 cm) and have 144 or 200 pins. Capacity ranges from 2 to 32GB per module. Some sub-notebook computers use even smaller DIMMs, known as MicroDIMMs. The industry has been moving to low-power DDR4 modules in thinner and lighter laptops, because they use less energy and are more compact. Unfortunately, they must be soldered into place, meaning the average user can’t replace the original RAM.
Most memory available today is highly reliable. Most systems simply have the memory controller check for errors at startup and rely on that. Memory chips with built-in error-checking typically use a method known as parity to check for errors. Parity chips have an extra bit for every 8 bits of data. The way parity works is simple. Let’s look at even parity first.
When the 8 bits in a byte receive data, the chip adds up the total number of 1s. If the total number of 1s is odd, the parity bit is set to 1. If the total is even, the parity bit is set to 0. When the data is read back out of the bits, the total is added up again and compared to the parity bit. If the total is odd and the parity bit is 1, then the data is assumed to be valid and is sent to the CPU. But if the total is odd and the parity bit is 0, the chip knows that there is an error somewhere in the 8 bits and dumps the data. Odd parity works the same way, but the parity bit is set to 1 when the total number of 1s in the byte are even.
The problem with parity is that it discovers errors but does nothing to correct them. If a byte of data does not match its parity bit, then the data are discarded and the system tries again. Computers in critical positions need a higher level of fault tolerance. High-end servers often have a form of error-checking known as
error-correction code (ECC). Like parity, ECC uses additional bits to monitor the data in each byte. The difference is that ECC uses several bits for error checking — how many depends on the width of the bus — instead of one. ECC memory uses a special algorithm not only to detect single-bit errors, but actually correct them as well. ECC memory will also detect instances when more than one bit of data in a byte fails. Such failures are very rare, and they are not correctable, even with ECC.
The majority of computers sold use nonparity memory chips. These chips do not provide any type of built-in error checking, but instead rely on the memory controller for error detection.
How Much RAM Do You Need?
It’s been said that you can never have enough money, and the same holds true for RAM, especially if you do a lot of graphics-intensive work or gaming. Next to the CPU itself, RAM is the most important factor in computer performance. If you don’t have enough, adding RAM can make more of a difference than getting a new CPU!
If your system responds slowly or accesses the hard drive constantly, then you need to add more RAM. If you are running Windows 10, Microsoft recommends 1GB as the minimum RAM requirement for the 32-bit version, and 2GB for 64-bit. If you’re upgrading to Windows 11, you’ll need at least 4GB. If you’re using a Mac with MacOS 11 (Big Sur) you’ll also need 4GB.
Linux has a reputation for working happily on systems with low system requirements, including RAM. Xubuntu, one popular low-requirement Linux distribution, requires a mere 512MB RAM. Xubuntu uses the lightweight Xfce desktop environment, which also works with other Linux distributions. Of course, there are distributions of Linux that have higher system requirements.
No matter what operating system you use, remember the minimum requirements are estimated for normal usage — accessing the internet, word processing, standard home/office applications and light entertainment. If you do computer-aided design (CAD), 3-D modeling/animation or heavy data processing, or if you are a serious gamer, then you will need more RAM. You may also need more RAM if your computer acts as a server of some sort (webpages, database, application, FTP or network).
Another question is how much VRAM you want on your video card. Almost all cards that you can buy today have at least 12-16MB of RAM. This is normally enough to operate in a typical office environment. You should probably invest in a higher-end graphics card if you want to do any of the following:
Play realistic games
Capture and edit video
Create 3-D graphics
Work in a high-resolution, full-color environment
Design full-color illustrations
When shopping for video cards, remember that your monitor and computer must be capable of supporting the card you choose.
How to Install RAM
An engineer installs RAM into the motherboard socket.
Most of the time, installing RAM is a very simple and straightforward procedure. The key is to do your research. Here’s what you need to know:
How much RAM you have
How much RAM you wish to add
Where it goes
RAM is usually sold in densities with multiples of 2 gigabytes: 2, 4, 8, 16, 32. In other words, the module is the same standard size, but there can be different amounts of memory on the same board. For example, if your computer has 8GB and you want 16GB total RAM, you would want to buy a module with an 8GB density.
Once you know how much RAM you want, check to see what
form factor (card type) you need to buy. You can find this in the manual that came with your computer or you can contact the manufacturer. It’s important to realize that your options depend on the design of your computer. Most computers sold for normal home/office use have DIMM slots. High-end systems are moving to RIMM technology, which will eventually take over in standard desktop computers as well. Since DIMM and RIMM slots look a lot alike, be very careful to make sure you know which type your computer uses. Putting the wrong type of card in a slot can cause damage to your system and ruin the card.
You will also need to know what type of RAM is required. Some computers require very specific types of RAM to operate. For example, your computer may only work with 60ns-70ns parity EDO RAM. Most computers are not quite that restrictive, but they do have limitations. For optimal performance, the RAM you add to your computer must also match the existing RAM in speed, parity and type.
Additionally, some computers support dual-channel RAM configuration either as an option or as a requirement. Dual-channel RAM modules are installed in matched pairs, so if there is a 512MB RAM card installed, there is another 512MB card installed next to it. When dual channel is an optional configuration, installing RAM in matched pairs speeds up the performance of certain applications.
Your computer is only configured to accept so much memory. There is a limited number of memory slots, and depending on your machine, you may be limited to an 8GB density module even if the manufacturer makes a 16 or 32GB module. Or, in some cases, your computer may allow you to upgrade the RAM that was installed in the factory. If you had a machine that came with 4GB of replaceable RAM, but that machine could accept 16GB, you could buy two 8GB modules and swap out the 4GB module.
Some manufacturers — both computer and memory — offer a wizard on their websites where you can enter your computer’s model to help you find what type of memory you’ll need to install. Check the system settings on your machine to find out how much memory is installed. Once you know how many slots there are and how much memory it can accept, you can decide how much memory to buy. Some manufacturers solder the base memory in place, but otherwise you may be able to uninstall a smaller RAM card and replace it with a larger one.
Knowing your computer’s configuration beforehand will help you avoid frustration when you buy memory. Finding out you can’t use what you bought after you’ve opened your computer up can be very, very annoying.
Before you open your computer, check the end-user license agreement to make sure you won’t be voiding the warranty in the process. Some manufacturers seal the case and request that the customer have an authorized technician install RAM. If you’re ready to open the case, turn off and unplug the computer. Ground yourself by using an anti-static pad or wrist strap to discharge any static electricity. Depending on your computer, you may need a screwdriver or nut-driver to open the case. Some desktop systems come in tool-less cases that use thumbscrews or a simple latch. Laptops are often more challenging.
The actual installation of the memory module does not normally require any tools. RAM is installed in a series of slots on the motherboard known as the
memory bank. The memory module is notched at one end so you won’t be able to insert it in the wrong direction.
For SIMMs and some DIMMs, you install the module by placing it in the slot at approximately a 45-degree angle, then pushing it forward until it is perpendicular to the motherboard and the small metal clips at each end snap into place. If the clips do not catch properly, check to make sure the notch is at the right end and the card is firmly seated. Many DIMMs do not have metal clips; they rely on friction to hold them in place. Again, just make sure the module is firmly seated in the slot. Read your motherboard’s manual for directions.
Once the module is installed, close the case, plug the computer back in and power it up. When the computer starts the POST ("Power On Self Test"), it should automatically recognize the memory, but it may take a couple of reboots for that to happen. That’s all there is to it!
Lots More Information
How to Add RAM to Your Desktop
How to Add RAM to Your Laptop
How Computer Memory Works
What is the Difference between Static and Dynamic RAM?
Does Adding More RAM to Your Computer Make it Faster?
More Great Links
Tom’s Hardware Guides: RAM Guide
Crucial Memory Buying Guide