Memory

1. Take No Prisoners
2. I Remember When...
3. Memory Chips
4. Memory Speed Ratings

<PCMCIA : Table of Contents : Processor Upgrades>

Take No Prisoners
From Catalog 27, page 19

What will you do with 256K SIMMs when your application requires more than 2 meg of memory and all of your memory slots are full?

My answer is, "Why did you buy 256K parts in the first place?" That isn't entirely fair, but it certainly is relevant today. Two years ago, 1 meg parts were quite expensive when compared with the lower density parts, and 4 meg parts were only available in sample quantities.

Now that one 4 meg part costs 55% of the price of sixteen 256K parts, only rare situations warrant buying 256K parts.

Consider buying faster memory than required. Faster costs more initially, but can save in the long run if you later upgrade to a faster motherboard. If your computer requires 80ns parts, you can mix 60, 70 and 80ns parts without fear. As long as they are as fast or faster than required, you are safe.

Another thing, SIMMs with 3 chips have gotten a bad rap lately. Many 256K SIMMs have two 256K by 4 bit and one 256K by 1 bit chips. This is equivalent to nine 256K by 1 bit chips. When problems occur with three chip SIMMs, it is because the refresh requirements of the SIMM don't match those of your computer. Now that vendors know the difference, they no longer carry the wrong devices.

Don't feel bad if you got caught with unusable memory even after you read this. Despite my own efforts to avoid this situation, I still find myself "holding" low density memory after major upgrades.

The reality is that it is unavoidable unless you never upgrade.

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I Remember When...
From Catalog 15, page 83

When I got my first taste of microcomputers, 2048 bytes of memory was a fair amount of useful memory. Today, 2 kilobytes of memory won't begin to hold the operating system, much less a useful program.

In 1981 when the IBM PC was first announced, it used 4116 RAM chips. When nine of those chips were combined into an array, the computer had 16K of memory. The first IBM PC's had room for 64K on the motherboard.

Since then, we have seen 64K, 256K, 1 meg, and now 4 meg devices become popular. As each succeeding generation of chips came along, it started out quite expensive, but gradually reached a point where 4 devices from the older generation cost as much or more than one of the new devices. Designers and manufacturers call this "price parity" and plan for it in their card and board designs.* We have just reached that point for 4 megabyte SIMMs.

Since so many programs require copious amounts of memory, and 386/486 based computers can manage that memory, 4 megabyte devices are very competitively priced, in addition to being less power hungry. I suggest a careful analysis when purchasing both board and memory products.

Remember your history_the amount of memory you need to do your work today probably won't be enough two years from now. Look for flexibility and expansion capacity in your purchases.

I wonder how many old PCs with their 64K motherboards are sitting in attics, basements or have already found their way to the dump.

* Note: Actually, manufacturers use a 5-to-1 ratio when calculating price parity. They include the cost of making the board itself.

My first computer was a "Brown Box" Digital Group Z80. Based on the "Suding BUS" created by Dr. Robert Suding, it had 64K of memory, a 16 line by 40 character display, and an 8 inch floppy drive rescued from the dump.

My first assembly language program on the Z80 was a digital clock that inaccurately displayed the time of day. I was so "proud"!

(My wife thought I was an idiot. We had a perfectly good $10 clock that kept accurate time and didn't mess up the kitchen table.)

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Memory Chips
From Catalog 8, page 9

Here at JDR, we're constantly reminded how difficult it is to keep up with new developments. Four years ago, we assured our customers they needed nine chips at a time to expand their PC. When AT's became popular, we said nine for PCs and 18 for 80286-based machines. Later, we said that you added memory to an 80386 using 36 chips at a time.

When the available memory chips were designed so that one chip supported one bit on the data bus, it was easy to make rules like these. But the rules have changed, which can be very good as we shall shortly see. The number of chips is really determined by the width of the data bus AND the width of the dynamic RAM memory chip.

For example, if you used a 1 meg chip that is configured 256K by 4 bits wide, you could design an 80386 motherboard that is expandable in increments of 9 chips at a time (256K by 4*9=256K by 1*36). Obvious advantages are saving board space, which makes room for more memory, and the lower cost, since 1 meg chips cost less per byte.

Because the goal is to create products that are reliable, compatible, full featured and competitively priced, the new 1 meg chips in both 1 meg by 1 and 256K by 4 are very desirable. With that in mind, look for new products that take advantage of these more flexible memory options.

Oh, the folly of it all!

The purpose of this column was to educate our readers on the subject of memory selection. One of the most frequently asked questions on our Technical Support phone lines then was, "Why can't I add 512K to my 386 motherboard, I really don't need a whole meg?"

With the wholesale adoption of SIMM's to replace DIP's, and the not too distant conversion to 64 bit processors, the question will soon become "Why can't I add 16 meg to my Power PC motherboard, I really don't need another 32 meg?"

What I should have written was:

Don't ever throw away the manuals that come with your purchases. Someday you may want to expand your memory, and you will need the book to know what to order, how to install it, and how to reconfigure your PC to take advantage of it.

The above paragraph was, is, and will continue to be true no matter what changes take place in the computer field.

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Memory Speed Ratings
From Catalog 10, page 14

Memory speed ratings seem to cause a good deal more confusion than we would normally expect. This is probably because everyone wants as much speed as their hardware will permit. But many people are unsure of the maximum ratings of their equipment.

When you buy faster memory than your processor requires, you won't process data any faster than if you bought the slower memory. Similarly, buying a faster math co-processor than the board is specified to use will not make the arithmetic functions any faster either.

Many people do spend the extra 5-10% and buy memory faster than required. For some, the reason is "margin for error," and they want the extra security they feel using better than required. Others are looking into the future.

I am one of the "look to the future" crowd. I know that the next generation of processors is going to be faster and require quicker memory. I usually buy the second fastest memory even if I don't need it. The fastest memory usually has a premium price, and I won't pay that if it isn't required.

The list of Intel-based processors below has information that should determine which memory chip speed you require. This list is not exhaustive, and may differ from the specs which came with your computer. Please use the manufacturer's recommendations when in doubt.

Some memory chips are not available in the speeds required for very high speed computers. For this reason, those computers usually use interleaving and/or a memory cache. Memory caches use faster static RAM as opposed to the dynamic RAM used for most processor memory.

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