MEMORY
Memory is a subject that confuses a lot of people. Computer memory and hard drive storage space are both described using the same units: Megabytes or Gigabytes. Another factor of confusion is the fact that both types of storage are random access, therefore when someone asks a question about RAM (Random Access Memory), what exactly are they asking about?
The distinction is short-term versus long term storage. Memory, as we use the term while talking about computers, is describing the short term memory that the microprocessor uses to do work. Technically it is "volatile" memory - meaning it is only there when power is applied; when power is removed, everything disappears. I guess you could say that volatile memory has no memory. Every time you turn on your computer, your microprocessor has a blank field to work with. On the other hand, when you store something on your hard drive, that is a permanent record that will remain there until you change or erase it. No matter how many times your computer has been turned off, the record is still there next time you look for it. If you think of long term storage as being like a book, short term memory would be more like a mirror.
Properly, when people are talking about RAM, they're talking about short-term memory. Physically, RAM is mounted on the mainboard. The earliest computers had memory chips soldered to the mainboard - what you had was all you were ever going to have, usually, because it was so difficult to change. Soon the memory chips were soldered to their own little boards (called Memory Modules) which were then mounted on the mainboard using special slots. This made it much easier to change the RAM chips, so memory could be upgraded or easily replaced in case of failure.
The first really popular memory modules were 30 pin SIMMs (Single In-line Memory Module) - a memory module with 30 connector pads down one side. The first mainboards to use SIMMs usually had 4 of the 30 pin SIMM sockets soldered to the mainboard, and the first 30 pin SIMMs available were 256K modules, so it was possible to have 1 Megabyte of RAM on your mainboard. Of course those figures soon expanded. The standard number of SIMM-sockets soon became 8 and the modules themselves evolved to where each SIMM could hold as much as 16 megabytes of memory.
As the computer industry grew, the 30 pin SIMMs soon became obsolete. They were designed for a 16 bit bus and microprocessors soon needed 32 bit, and then 64 bit, busses. The next generation SIMM was the 72 pin SIMM, still widely in use today but rapidly being replaced with 168 pin DIMMs (Dual In-line Memory Modules).
The question is frequently asked, "How much memory do I need?" And the answer is (as is usually the case when talking about computers) "Well, that depends......". It depends on what kind of microprocessor you're using - the older processors need less memory, the newest ones more. It depends on what operating system you're running - different versions of Windows (or DOS) have different memory requirements. Usually it's like sex appeal or money - you can't have too much.
For most Pentium type computers running Windows 98, 16 Mb would be the minimum, 32 Mb would be adequate for most users most of the time and 64 Mb would be more than most folks would ever need. Double those figures for a Pentium II or III and halve those figures for a '486 machine running Windows 95. Those are just ballpark figures, but will probably hold up in most cases. If you are into action gaming or high-end graphics, your need for memory will skyrocket; all you can get will be just barely enough. If all you do is turn on your computer once a day and check your email and maybe play one of the games that came with Windows, you can get by with the minimum amount of memory. Most of us fit in between those two extremes.
It is difficult to talk about the memory in a particular computer - there are so many variations in the memory itself and so many variations in how it could be installed on the mainboard.For instance, let's say someone said they have 32 Mb of memory in their computer - what could you know from that? Only that the total is (probably) 32 Mb. You wouldn't have any idea whether their computer contains a single 32 Mb 168 pin DIMM, or a pair of 16 MB 72 pin SIMMs, or maybe 4 of the 8 MB 72 pin SIMMs, or....well, there are a bunch more possibilities that could total up to 32 MB. And until you know what's there, you can't know what kind of changes could be made. This question usually arises when someone is wondering about adding more memory; in most older computers, adding memory is the single best thing you can do to improve the speed and power of the computer. Notice I said most - not all. Each computer must be judged as it is. At any rate, the first thing in seeing what improvements can be made is learning what's there.
How much memory do you have? Usually when you first turn your computer on, one of the first things you see is the POST (Power On Self Test) checking your computer's memory. It will tell you what kind of processor you have (usually) and then count up your memory and come to a total. That figure is how much total memory your computer has installed. Another way to find out (while your computer is running) is to go to your Start Menu -> Programs -> Accessories -> System Tools -> System Information. That screen will tell how much usable memory you have, but not what kind. Sometimes your usable memory is not the same figure as the total memory installed. That could be because something installed in your computer (usually built-in) needs some memory and reserves it from the RAM. Anyway, you still need to know what kind of memory you have installed and the best way to find that out is to read the manual that came with your computer. Don't have one? Then you'll have to pull the cover off of the case and look.
It seems like most of the tutorials I've seen on the internet ask you to read the information that came with your computer to find out what size and type RAM modules you have and what to buy to upgrade your computer. I guess that works pretty often, but it seems to me that in the real world a lot of folks have second hand, or hand-me-down, computers and they don't have (or have lost) the manual. So it still boils down to a real person looking inside the computer and figuring out what's what.
Once you've taken the cover off the computer,-(you *HAVE* followed all electrical safety procedures, haven't you?)-lay the case so that the mainboard is lying flat and just study what's inside before you do anything else. Get familiar with what you have. Identify what you are looking at. The memory modules will usually be at the right rear of the mainboard, close to where the power leads plug into the board - often with the wide flat gray cables that run to your drives plugged into the mainboard between the memory modules and the cards that are plugged into the expansion slots. These flat cables may obscure your view of the memory modules and, on some boards, may even have to be unplugged before you can access the RAM.Now that you have found your RAM, what do you have? Usually the 72 pin SIMM sockets will be light colored - white or a light gray - and the 168 pin DIMM sockets are black, with white ears at each end. Often you'll have both type of sockets on the mainboard but usually only one type will have memory plugged into them. An older mainboard, one with a '386 or '486 microprocessor, will probably have 30 pin SIMM sockets or a combination of 30 pin and 72 pin sockets. I've never seen a board that had all three types of sockets mounted.
Once you've found your memory and identified what type of memory modules are there, you can identify what they are. For example, say you know your computer has 16 Mb of memory, you look inside and see 4 white looking sockets that contain a total of two memory modules. You can be pretty sure that your computer contains two 72 pin 8 Mb SIMMs. Or, with the same example, suppose you see 4 memory modules in the 4 sockets. The answer then is probably dependent on what type of microprocessor you have: if your processor is a '486, you probably have 4 30-pin SIMMs of 4 MB each; if it is a Pentium the odds are that you have 4 72-pin SIMMs of 4 Mb each.
Another thing that helps a little bit is the way microprocessors access memory. That usually requires that SIMMs be installed in a certain pattern. Usually if a computer has 30 pin SIMM sockets, it is a requirement that 4 identical modules be installed at the same time. You can't, for instance, install just 3, or 2 modules - the computer won't work. Or you can't install 2 4-Mb modules and 2 16-MB modules - that won't work either. With 72 pin SIMM sockets, it's two identical modules at the same time.
Memory is installed in "Banks", and a bank of 30-pin modules is 4, a bank of 72-pin modules is 2, and a single 168-pin DIMM is a bank. So if you look inside your computer and see a single memory module (and it's in a black socket) it's most probably a 168 pin DIMM. If you see a whole bunch of them (like 8, and they're all in light colored sockets) the odds are pretty good you're looking at 30 pin SIMMs.
The way the memory modules are installed is standard. Both 30 pin and 72 pin SIMMs have a notch at one end of the module (on the edge that contains the pins, or pads) which fits onto a tiny little shoulder built into the SIMM socket. That's so they can only fit one way. The modules are inserted into the socket (notch to shoulder) at about a 30 degree angle and, when they are completely seated (pushed all the way to the bottom of the socket), are rocked up to the vertical position. A little catch (metal on most 72 pin, plastic on some 30 pin sockets) will lock onto the SIMM keeping it in a vertical position, locked in place. To remove the SIMMs, you must first release the little catches at the ends of the socket. That will allow the SIMMs to lean forward and be released from the socket. DIMMs are a little different. They are inserted and removed in a vertical position. Downward pressure causes the little plastic levers at the ends of the DIMM socket to lock onto the DIMM. You push down on the the little plastic levers to release the DIMM - it will pop out of the socket.
That should give you enough information to upgrade your computer's memory. Or, at the very least, you now know that an average person can do it - it doesn't require a technical degree or black magic. There is still much you can learn about memory: I've not talked about different basic types of memory (video memory is usually built differently, for instance) or the speed of memory (it needs to be matched to the speed of the microprocessor) - but common sense goes a long way if questions arise about these things. The speed thing for instance. If you're buying memory to upgrade your computer, and the salesperson asks you about speed, tell them what processor and speed your computer uses and ask THEM what speed you need. With Pentium microprocessors, it's usually a choice between 60 Ns (nanosecond) and 70 Ns memory - you can't go wrong with the faster (60 Ns) memory, if you're not sure. A rule to remember is that fast memory works with slow processors but the reverse is not true. If you're working with older (30 pin) memory, you need to know more. Things wern't as standardized with the 30 pin SIMMs and you may run into some different schemes of parity or error checking. All the SIMMs you use in a bank on a mainboard must be identical.
Additional Information:
Bits and Bytes
Mainboards
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