Difference Between ROM, RAM, And Storage Capacity?

Because they're measured in the same units, users often confuse a computer's RAM, ROM, and storage capacity. This article explores their differences in laymen's terms.

These days, it's hard to function without knowing how to use a computer. They've become indispensable tools in most schools, at many jobs, and even at home. It seems that there are few machines, from telephones to cars to cash registers, that can't be hooked up to computers in order to make them more efficient. And if computers aren't already complex enough to strain our brains, the folks who work with them seem to have a fondness for impenetrable acronyms: RAM, ROM, CPU, DIMM, SIMM, IDE, CMOS, BIOS, TSR, MHz -- the list goes on. Plus, they have a penchant for using the same units of measurement for different things.

The most glaring example of the latter is the use of the term "byte" and its larger relatives to describe three different (if related) issues: RAM, ROM, and storage capacity. All are important to understand if you really want to know how your computer works, and unfortunately it's easy to get them confused. The most common error is to assume that the RAM and the disk drive storage capacity are the same; they're not, and ROM is something else altogether. All are memory of a sort, but not the same kinds of memory. The purpose of this article is to clarify the differences between them, in language that's easy to understand.

Let's start with basics: the units used to measure the different flavors of computer memory. A byte (B) consists of a grouping of eight binary digits ("bits"), and is typically considered the smallest addressable unit of data. A byte is usually enough to indicate a single character in a file -- say, a letter or a number. A grouping of 1024 bytes is called a kilobyte (KB); 1000 KB, or 1,024,000 bytes, equals a megabyte (MB). Larger units include the gigabyte (GB), which equals 1,000 MB, and the terabyte (TB), which is way up there at 1000 GB -- 1,024,000,000,000 bytes. In the old days (before 1990), KB were usually sufficient for discussing the capacity of an everyday computer. Back then, a computer with a MB ("meg") of memory or storage capacity was a manmade wonder right up there with the Pyramids. No more: MB and GB are necessary now, and it seems that terabyte-level computers are just around the corner.

So if the compugeeks of the world are capable of creating computers of such power and complexity, why did they decide to use the same units of measure for different things? The answer is twofold. First off, the usage got entrenched in the industry early on, and is now impossible to root out. Secondly (and most importantly), the phenomena the units measure is quite similar, whether you're speaking of RAM, ROM, or capacity. Bytes, kilobytes, megabytes, and gigabytes always describe computer memory. The memory, however, is used for different things.

RAM is short for Random Access Memory, and comes from hardware components wired into or attached to the motherboard, the main circuit board of your computer. RAM is used to run certain basic programs and functions that your computer needs to operate correctly, and functions only while the computer is receiving power. Programs you're using are written in RAM temporarily while the computer is processing them. Think of RAM as a playing field, a large open area where your programs function. Each program takes up a certain amount of space; the field can accommodate one or several different programs at one time, but its capacity is limited. When you shut down a program, it disappears from RAM and (ideally) the space it occupied can be reused. Sometimes some operating systems, including Microsoft Windows, won't relinquish the RAM space even when a program is closed. However, because stuff in the RAM is retained only while the computer is powered up, turning it off will always clear the RAM.

If you want a larger playing field in real life, you have to add onto the field by acquiring more property. With RAM, you do this by adding additional memory. In most cases, this memory comes in the form of "RAM sticks," small rectangular cards studded with memory modules. These fit in special slots in the motherboard. Single Inline Memory Modules (SIMMs) are still used, but Dual Inline Memory Modules (DIMMs) are becoming the standard.

ROM is an acronym for Read Only Memory, a type of unchangeable memory residing in chips on your motherboard. ROM contains the bare minimum of instructions needed to start your computer. Because it's used for critical functions, it can't be removed short of ripping it out of the motherboard; adding to it is just as difficult. Think of it as analogous to municipal utilities, such as gas and electricity. If you want a different configuration, you'll have to "move on" to a different motherboard or computer. Incidentally, the term "ROM" is also used, not entirely correctly, when referring to some kinds of storage media that can't be modified, such as CD-ROMs.

The term "storage capacity" is most often used to describe disk drives, which tend to be permanent, though many forms of storage media are removable: the various types of floppy disks, high-capacity Zip disks, CD-ROMs, and tape cartridges, to name the most common types. To extend the real-estate analogy used previously, your storage memory -- also known as secondary storage -- can be thought of as a series of warehouses, some of them mobile, where you can store programs. Programs come in various sizes, from a few KB on up to several hundred MB. A particular storage "warehouse," such as a disk drive, has a finite amount of space in which to store programs. Depending upon its capacity, any given storage warehouse might be packed tight, or it might contain one tiny program stuck off in a corner. If one particular "warehouse" gets full, you can always construct or bring in another.

That's it in a nutshell. Basically, RAM is the size of your playing field, and can be increased as you purchase more "real estate"; ROM is equivalent to your utilities, the hardwired bare necessities needed to operate your computer, and is fixed in size; and storage capacity can be thought of as warehouses of various size, some of them mobile, which can be trucked in or constructed as circumstances warrant. It would take a book the size of a dictionary to cover everything about RAM, ROM, and storage, but hopefully this article will provide you with the basics you need to cut through any initial confusion. Good luck -- when it comes to computers, you'll need it!

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