Helping people with computers... one answer at a time.
The difference between the various types of things we might call "memory" is a fundamental concept that's often confused. I'll review the basics.
Memory, hard disk, and RAM: I get a surprising number of questions that show a misunderstanding between these most basic of computer terms.
The most common mix-up is that people believe they are the same. While they're not, I can see how it could be easy to confuse them at a conceptual level. So, let me explain these three terms.
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RAM
RAM: Random Access Memory
RAM stands for Random Access Memory. RAM is a collection of electronic circuitry where your computer's programs and data are stored when it is running.
As you can see to the right, RAM is one or more small circuit boards containing several integrated circuits where the actual microscopic circuitry lives. Each computer typically comes with a certain amount of RAM installed. It also has a limit to how much total RAM it can contain.
The machine I'm using as I type originally came with two gigabytes (roughly two billion bytes) of RAM, and has since been upgraded to its maximum capacity of eight gigabytes. This maximum is a hardware limitation of the main computer circuit board or "mother board" into which the RAM modules are inserted. If I wanted to exceed this limit, this computer would need a new motherboard, although most folks would simply get a new computer with greater capacity.
The one characteristic that differentiates RAM from other types of data storage is that it requires power to retain its contents. Remove power from RAM and it forgets everything that it had contained.
Hard Disk
An Opened Hard Disk
A Hard Disk Drive (HDD), or sometimes just hard disk or hard drive, uses physical platters coated with magnetic material to store data. As the disk spins underneath read/write heads, the changes in magnetic polarization can be read by the read head, or they can be set or written by the write head.
A hard disk is a physical device that typically resides in your computer, although it's very common to have external hard drives connected to your machine using USB or Firewire.
There are two things that distinguish hard drives from other forms of storage:
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A hard drive involves moving parts. The platters spin at speeds measured in thousands of revolutions per minute and the read/write head moves back and forth across the spinning platters.
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Because magnetic and not electronic components are used for storage, a hard drive retains its contents even when the power is removed.
Computers come with a hard disk on which the operating system and initial set of programs are installed. This is where you'll save your work. You can replace the hard disk with a larger one, taking care to move the data from the old hard disk to the new. You can also add additional hard disks either internally, if your computer has room, or externally, through USB and similar interfaces. Technically, there's no limit to how much hard disk space most computers can have, but practically, the sheer number of drives, connections and power required for more drives limits the amount of space you can add.
The machine that I'm working on now came with a 300-gigabyte drive. It has since been upgraded to include a one terabyte (one trillion bytes) drive, a 1.5 terabyte drive internally and a 500-gigabyte drive attached via a USB connection.
Memory
Memory is RAM. It's that simple.
Normally, the term memory refers only to RAM, not your hard disk.
It's this confusion that I see frequently. A statement like, "I just added 500 gigabytes of memory to my machine," is unlikely to be true. You might have added 500 gigabytes of hard disk space, but at this writing, 500 gigabytes of RAM is well beyond the reach of any consumer PCs, which tend to max out in the eight, 16 or maybe 32 gigabyte range.
Memory = RAM.
Hard disk space = hard disk space.
I can see why there's confusion. As hard disks are used to save and "remember" stuff, this certainly sounds like memory. But that's simply not how memory is used when it comes to computers. Memory means RAM.
Flash Confusion
A USB memory stick
Newer technologies blur the lines of which technology is used where.
Flash memory, for example, is used to make USB Thumb drives. Flash memory is a type of circuitry that's similar to RAM, except that it retains its contents when power is removed . Hence, it's most frequently used to create what we've come to call USB sticks, thumbdrives, jump drives, RAM sticks and a whole collection of other terminology.
Circuitry, sort of like RAM, is used to make storage that's sort of like a hard drive.
The important take-away here is that flash devices, even though they use flash memory, are not referred to as simply "memory". If anything, they're simply called drives, or flash drives. Occasionally, they might be called flash memory but it's the "flash" that distinguishes this from the plain old "memory", which is RAM.
Article C4727 - January 30, 2011 « »
Leo A. Notenboom has been playing with computers since he
was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed.
After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers
to common computer and technical questions. More about Leo.
February 1, 2011 1:34 PM
The analogy I use that seems to be easy for people to understand is to think of an office setting. You have a desk, and you have a couple file cabinets. The bigger the desk, the more papers, or "stuff" you can have open and laying out on your desk. This is memory - the more you have, the more "stuff" you can have out and working on. When it comes to putting everything away when you leave, you put all the papers back in the file cabinets (Hard drives, jump drives, etc.). When you turn off the lights as you walk out, everything left on your desk gets run into the shredder!
February 1, 2011 2:06 PM
hey SNERT: - yes, RAM really does stand for Random Access Memory, because any storage location in it can be accessed at random, without any regard to which locations have already been accessed. This is a very old term, which dates back many decades to the early computers which used all sorts of things to store working memory - mercury delay lines, CRT storage scopes, and more I can't remember. Human ingenuity knows no bounds - but both of the devices I mentioned are serial access devices - you have to work your way though all (or many) stored locations to access the one you want.
Random access (as in core memory) saves an enormous amount of time. It is possibly worth noting that bubble memory (and possibly flash) is a serial access technology - but it's so fast and arranged in such a way it's hardly noticed.
February 1, 2011 4:01 PM
Very clearly put and interesting. Ttwo thumbs up!
March 1, 2011 8:53 AM
2 things --
1) Random: meaning any memory location can be accessed at any time. Contrast this to an 8-hour VCR tape. When the tape is rewound to be beginning, you can't get to what's at the "hour 5" point without forwarding past the first 4 hours first. On a random-access device, you can jump directly to the "hour 5" point regardless to where you were when you started.
2) RAM vs Storage -- Here's the analogy I use:
First, imagine that the memory in your head is like the memory in your computer. For this case, pretend like the memory in your head is erased every time you go to sleep and is completely empty and fresh when you wake up. Now think of a book. Let's say the book is Steinbeck's "The Grapes of Wrath". This book equates to your hard drive. So now you open the book and read the first chapter. After that, all day, that first chapter is in your memory and you can recite it to anyone you want, all day long. But when you wake up tomorrow, it's gone because your memory is erased during sleep. You can always get another copy of this chapter back into your memory; all you have to do is read the book again, because the information is permanently stored in there. But it will only be retained in memory until the memory is turned off....
October 29, 2012 11:53 PM
I'm comparing desktops online right now since I'm considering getting a new one, and I keep seeing "Maximum memory" and "Total Hard drive capacity," where the numbers vary greatly.
I didn't know that memory referred to RAM, but now I know better. Thanks for clarifying the difference between RAM and Hard drive!
The flash bit on the USB was a bonus for me. I didn't know why people switched terms between USB and flash sometimes b/c they seemed like two different things.
30-Oct-2012