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Before You Buy More Memory

These days most computers use easy-to-install SIMM cards, and no special configuration is needed - you simply plug the SIMM into its slot and use it. That is, if you picked the correct type of memory:

The easiest part about choosing SIMM's is the physical size. There are only two to choose from - 72-pin and 30-pin. Some memory cards may not be designated as such, and only show the number of bits wide they are. If this is the case, remember that 30-pin SIMM's have 9 bits or less, and 72-pin SIMM's have 32 or 36 bits (more on this later).

Next, you need to know what memory sizes the computer is capable of recognizing. Every motherboard is different; some will let you mix different memory sizes in any combination, some will only work in one or two ways, and others are somewhere in between. Do SIMM's have to be installed in pairs? Do they all have to be the same size? What are the different amounts of memory you can put into each slot? Check the documentation for your computer to find out.

SIMM sizes are measured not by the number of bytes, but the number of bits. And it's not just a plain number either, but a dimension, which can be confusing to some people. Just remember your multiplication tables from grade school. For example, a 1Mx8 SIMM is 8 megabits. Then, remember to divide the product by 8 (because there are 8 bits in a byte) - thus, 8 megabits divided by 8 is 1 megabyte. Now try this on a 4Mx32 SIMM:

4 x 32 / 8 = 16

There's a glitch to the size: parity. Memory that uses parity will have 9 bits per byte instead of 8. So, a 1Mx36 SIMM is 1 x 36 / 9 = 4MB. The number after the 'x' tells you whether the SIMM uses parity: 9 and 36 have parity, 8 and 32 do not. So now you need to know, does the computer require parity or no parity, or does it care? Can you mix parity memory with non-parity memory?

Lastly, check the speed of your memory. Speed is measured much the same way as a hundred-yard dash - a lower finishing time means the memory is faster. The difference is that memory finishes in terms of nanoseconds (abbreviated ns). If the memory is too slow, the computer may end up slowing down until the memory is ready, or it may not read the memory at all. Faster memory is usually better, although it won't help. For example, if the computer reads memory after 80 nanoseconds but the memory is ready after only 70, those extra 10 nanoseconds are just wasted time. On the other hand, if memory is too fast, it may get tired of waiting for the computer and decide to go out to lunch.

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