Dynamic RAM: Definition & Types

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  • 0:05 What is DRAM?
  • 3:05 Types of DRAM
  • 5:57 Lesson Summary
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Lesson Transcript
Instructor: Paul Zandbergen

Paul has a PhD from the University of British Columbia and has taught Geographic Information Systems, statistics and computer programming for 15 years.

Dynamic random-access memory (DRAM) is a type of storage that is widely used as the main memory for a computer system. In this lesson, learn why it is one of the most important characteristics of a computer's performance.

What Is DRAM?

You may have experienced this scenario. You are working on your computer, start a new document, and save it on your hard drive. You start typing away and about 20 minutes later, the power to your computer is disconnected - perhaps a power failure or somebody tripped over your power cord. Your screen goes black, and you have to power up your computer again. When you open the document you were working on, it contains none of your work from the last 20 minutes - gone! Let's learn what happened and why it is important to understand how your computer's main memory works.

The main memory of most computers consists of dynamic random-access memory, or DRAM. It is considered a type of random-access memory (RAM), which means that information stored on it can be accessed in a random order. This makes it a really fast type of storage and therefore, ideal for use as the main memory for a computer system. In contrast, information stored on a hard disk drive needs to be read in a particular order, which makes it much slower.

Information in DRAM is stored using an integrated circuit which contains transistors and capacitors. A single pair of one transistor and one capacitor stores one bit of information. The capacitor can hold a low charge (also called discharged) or a high charge (also called charged), which correspond to 0 and 1, respectively. The transistor acts as a switch to read the state of the capacitor or to change its state. Capacitors slowly leak their charge, and therefore, the capacitor charge needs to be refreshed regularly by restoring the charge. That is the reason this type of RAM is called dynamic. The other most common type of RAM is called static random-access memory, or SRAM, because it uses only transistors and does not need to be refreshed.

While your computer is running, the CPU uses DRAM as the main memory to store large amounts of information that need to be accessed quickly. Consider the document you were working on. This document is stored as a file on your hard disk drive, but while you are working on it, the file is loaded into the main memory so you can work faster. Any changes you make are stored only in the main memory. Once you save your work, those changes to the file get written back to your hard drive.

When your computer is running and the main memory is being used, the capacitors that make up your DRAM are being refreshed constantly, typically every 64 milliseconds. This happens without you noticing anything and is simply built into how DRAM works. So what happens when the power is disconnected? All the capacitors lose their charge immediately, and all the information stored in DRAM is lost. In the case of your document, all changes you made since the last time you saved the file only existed in the main memory, and those changes are now gone. The file on your hard disk drive is the version you last saved and does not contain those recent changes.

Types of DRAM

The basic architecture of DRAM using pairs of transistors and capacitors has changed very little since it was invented in the 1970s. A few things have changed, however. DRAM typically comes in the form of modules: integrated circuits mounted on small plastic boards with metal pins to connect to slots on the motherboard. This interface has changed a lot over the years. For example, earlier DRAM modules used single in-line memory modules (SIMM) while more modern versions use dual in-line memory modules (DIMM). The number of pins has also increased, from 30 pins in earlier modules to 240 pins in modern versions.

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