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xNeil Dearnley-Davison

Concise Guide Series

Understanding computer memory

Don't know the difference between SDRAM and backside bus?

A fool and his money may soon be parted, but even the savviest computer buyer has a hard time spotting a genuine supermodel in a shop full of wannabe clones. With PC specifications so frustratingly opaque, it's little wonder that magazines and superstores leave you with a sore head and, hopefully, a full wallet. I'm here to help you fight through the flannel.

How often have you seen something like this ..

Intel Pentium III 550, 64Mb 100MHz SDRAM, Joe Bloggs 100MHz Motherboard, etc., etc.

What does it mean, and is it a good deal? I'll help you find the answers.

Forget me not
There are two types of memory in a standard computer. The 'Hard Disk' is where your computer will store your data permanently, so any saved files or data can be retrieved at any time. But Random Access Memory, or RAM, is something quite different. RAM, often described as a working space, is strictly temporary. It stores whatever data happens to be in current use, but it keeps no record of its past activities. Once gone from RAM, forever forgotten.

RAM is a temporary storage place used in conjunction with the 'processor' to enable the system to move data it needs in lightning quick time. This means the computer doesn't waste time continuously trawling the hard drive for information. It's rated in megabytes (MB) .. a measure of capacity ... and the bottom line is you really can't have enough of the stuff (see
Catch 22). As programs and games become ever more powerful, many won't get out of bed unless the RAM is right.

Even your computer's operating system needs plenty of memory on tap just to tick over. When Windows 95 first appeared, it was commonly installed with just 16Mb of RAM, with a resulting slowdown in performance in every respect. 'So much for progress' went the cry, but we've gradually come to expect that fancy software demands appropriate hardware. These days Windows 98 needs at least 32Mb RAM while Windows 2000 (NT 5) looks for 64Mb minimum and will thank you for more, and the 'new out now' "Millennium Edition" .. 64MB+ ... Windows XP ... 128+ ... you get the picture.

Slam in the RAM
When all available RAM in a computer is committed, Windows has to relegate some bits of data to the hard disk to make room for more. This frequent swapping of data to and from between 'super fast' memory and the relative slow hard disk, in comparison, is known as thrashing. This sounds painful, and for your computer's hard drive it is. My friends at Helpdesk says "it's one of the biggest causes of hard disk failure". It's like a team of builders forever racing from one half-finished job to another depending on which client shouts the loudest, the end result is that everything gets done much more slowly - the moral - slam in the RAM.

Today, 128 Mb is an adequate starting point. Even in a budget system, it's a better bet to sacrifice, say, an inch or two of monitor screen size or perhaps a DVD ROM drive rather than compromising on memory. So thumb your nose at 32Mb, consider upgrading from 64Mb to 128Mb (see
Catch 22) for slicker performance and go for 256Mb in a 'Pentium III/IV' or 'Athlon/Duron' system if your pocket can stretch to it.

Techie bit on a Stick
RAM comes in skinny little circuit boards loaded with storage cells in a variety of sizes or capacity, commonly 8Mb, 16Mb, 32Mb, 64Mb and 128Mb (& now 512). The latest 'flavour' is known as SDRAM. My example spells this out in as many letters but you could worry if you see just RAM, or even memory, as you may be getting the older stuff.

Jargon Buster

A software program, such as Word and Excel.

A very fast processor made by a company called AMD.

A cut-down, slower version of the Intel Pentium II/III processor, used in budget PCs.

A group of microchips on the motherboard that provides various functions.

DTP/Desktop Publishing
A type of software that allows you to arrange text and graphics on a page.

Digital Versatile Disk, Read Only memory. This new medium can store huge amounts of data on one CD, including full-length movies, with excellent-quality sound and pictures.

Expansion slot
An electrical connector fitted to the motherboard that lets an expansion card, such as a graphics card, plug into a motherboard.

Hard Disk
A high-capacity storage device that a PC uses to store programs and data, measured in gigabytes (GB).

Megabyte (MB)
A measurement of storage space. 1MB roughly translates to a million characters of text, or 180,000 words, about the size of a novel.

Megahertz (MHz)
A measure of how fast the processor in your PC works. For example, a 500MHz processor carries out 500 million operations per second.

Pentium IV
The latest and fastest member of the Intel Pentium family of processors.

A socket located at the back of the computer where you plug in items like the keyboard and printer.

This chip is the 'brain' of the computer. The faster the processor, the better a computer will perform.

Skip this section if you are allergic to 'geek speak'
The D, for dynamic, came first, replacing dull old static RAM. Dynamic means simply that the electronic charge within each memory cell must be refreshed constantly, so all those tiny binary1s and 0s are forever in a state of flux. S is for synchronous, which means the memory is synchronised to the processor's speed in order to transfer data at an optimal rate. While we're on this tortuous terminology, the random part of RAM means that the processor can access any particular piece of data directly rather than having to follow a sequence. This is just like skipping instantly to any track on a CD, whereas an audio cassette has to be fast-forwarded or rewound to the correct position.

So, now you know that SDRAM stands for Synchronous Dynamic Random Access Memory ... feel free to forget all about it.

Something the ads might not tell you is how the RAM sticks are physically installed in the system. This, however, is quite important. RAM clips onto the motherboard ... more on mum later ... using sockets called DIMMs (dual in-line memory modules). You should check both how many DIMM sockets the PC has in total (three would be normal, four a bonus, two too few) and how many are currently occupied. In our little example, the RAM could be fitted as a single 64Mb module in one DIMM socket, or as two 32Mb modules taking up a socket each, or even as one 16Mb and two 8Mb modules using up all three available slots. As a rule, look for two free DIMMs on a 128Mb machine and one free DIMM if 256Mb is fitted.

Missed the Bus?
My example memory spec has a third descriptive element: MHz or megahertz. But while 100MHz might look impressive what does it refer to, and why should we care?

For RAM to feed the processor with data, there has to be a shared gateway between them. This is known as frontside bus (I'm not making this up), or FSB (yep, there's a backside bus). The faster the FSB, the quicker data can move out of RAM and into the system. This prevents a powerful processor getting bored.

FSB is an element of the motherboard's 'chipset' rather than something physically built into RAM, but all memory is designed to shift data at one specific speed or another, generally 66MHz, 100MHz or 133MHz. Our spec is rated at 100MHz, which is in the middle of the range. This is normal at the moment and no less than I would expect in a new machine.

Incidentally, any reference you come across to PC100 or PC133 refers to the same FSB standards and don't settle for less than a ton.

Mum's the word
The motherboard (main board, system board or 'mobo') is central to a PC ... so central, in fact, that everything else plugs directly onto it. It's basically a big printed circuit board that hosts the processor, RAM, chipset, expansion slots and ports (see Jargon Buster). Take it away and you've got a load of loose wiring and a distinct lack of anything approaching a computer.

That said, some would argue that the motherboard is the component over which you need worry the least. I disagree, the big name manufacturers design each system from the motherboard up, including logistical considerations about how to get everything to fit into the box, and asking for an alternative motherboard is unlikely to meet with a favourable response.

If you were considering buying a motherboard off the shelf ... which would mean either that you were building a system from scratch or engaged in a serious upgrade ... you would do well to look for an established brand from a company renowned for reliability with a sound history of successful trading in the field. As most of us are more familiar with the likes of Microsoft and Intel than obscure motherboard manufacturers, we suggest that you do your homework first ... look on the net ... find a 'mobo' review site or two ...

In shape?
Just as important as who makes your motherboard is something called its 'form factor' ... or, as you would call it in the real world, shape. Motherboards come in a variety of designs but are governed by certain standards of physical dimension. Today's PCs generally come with an ATX (or sometimes a mini or micro ATX) motherboard, which is just fine. You wouldn't want an older AT motherboard these days. It's worth clarifying the form factor.

The chipset on a motherboard governs the flow of data between all the different components, and I covered this earlier. The other key element is the type of 'slot' or 'socket' on board. This is the part of the motherboard that the processor plugs into, and there are about a dozen specifications in existence. (Did I say this was easy?) In practice, only four designs are currently in widespread use. For what it's worth, a Pentium III will sit in a 'Slot 1', whereas an 'Athlon' processor is compatible with 'Slot A'. Like square pegs and round holes, one will not fit the other. The third is what's called a 'Supa 7' motherboard or 'super socket 7'. These motherboards, although 100MHz, are usually used by budget builders and take a 'socket 7' processor such as AMD's K6-2 or IBM's MII.

Last, but not least, the Celeron processor, by Intel, also used in budget machines and in two flavours of 66MHz or 100MHz. Confused? Read on ...

Catch 22
Technically, Windows 95 / 98 and Windows 98 Second Edition can address up to 4Gb (gigabytes) of memory. But there's an important and very serious limitation to this impressive-sounding capacity ... your system may not be able to store much of the addressable memory in its secondary cache (another bit of memory, built-in on budget motherboards) and this shortcoming can severely impede your performance. This is usually so on budget machines using 'socket 7' motherboards. It's a hardware limitation .. ask ...

2000 & 10?
It stands to reason that anyone buying a new computer should look for a system that suits their needs. A £499 budget model with a tiny monitor won't cut the Desktop Publishing (DTP) butter, while conversely there's no need to splash out two grand just for internet access and email. The trouble is that most newcomers have little clear idea of what they'll do with a PC until they've got one ... or, rather, until they develop new interests and computer-based hobbies. The smart money, therefore, is spend on future-proofing. This means making sure today's purchase can be upgraded easily tomorrow to meet your changing requirements.

As we've already mentioned, make sure that you have a couple of spare DIMM slots so that more RAM can be added later. If there's only one DIMM free, it's worth checking with your supplier that the motherboard supports 128Mb RAM modules. Most will, but some only accept 64Mb modules, in which case I'd suggest your upgrade options are too limited and you should look elsewhere.

It's also worth checking your motherboard has an AGP (Advanced Graphics Port) port. This is essential for high performance graphics, particular the kind of 3D modelling used in games.

Although you may plan to work mainly with text and figures, I've seen too many serious-minded individuals succumb to PC gaming to recommend that you rule out a graphics card upgrade. Besides, even some of the funkier websites these days need AGP functionality to be seen in their true colours.

As for slots and sockets, a Celeron (the slot 1 type) processor can generally be upgraded to a Pentium III, but the next generation of processors will inevitably come with a completely new breed of connections to render every previous design obsolete.

You can and should future proof as much as possible, but don't be surprised if the goal posts are suddenly moved.

The Future
Motherboards link all the bits and pieces that make up a computer, and they'll doubtless continue to quietly evolve in line with changing standards and new technologies. RAM, by contrast, is absolutely crucial to a PC's performance and the race is always on to find sleeker, faster, more efficient forms of memory.

While SDRAM has been the benchmark for some time, there's no shortage of contenders nipping at its heels. Intel, makers of all things Pentium, is thoroughly in bed with a company called Rambus that makes an entirely different type of memory. Called RDRAM (Rambus Dynamic RAM), this is a completely new design of memory that transfers data at a mighty 800MHz. It is, however, prohibitively expensive to manufacture so it hasn't yet hit the mainstream (it is actually emerging now). Nor is it as quick as you might think. RDRAM modules are connected in series rather than, as with SDRAM, parallel, which means data from one module may have to pass through one or two others before it reaches the processor. This delay ... known as latency .. can hamper overall performance.

Keep an eye open for SLDRAM (the SL stands for SyncLink ... you can guess the rest). This is an alternative proprietary design, which is also fast but less pricey. SyncLink would like to steal Rambus' thunder, but which standard, if either, comes to dominate is dependent upon industry support. This is currently anybody's guess.

A fairly safe short-term bet is the widespread introduction of Double Data Rate memory, or DDR. This is really a natural evolution from SDRAM, only twice as fast. It transfers data at 266MHz compared with the previous best of 133Mhz. Expect to see DDR in high-end PCs by the end of the summer.

Unfortunately, none of these newcomers to the memory market are backward-compatible with SDRAM, so upgrading an existing system won't be a realistic option.

Nobody said this game was an easy one to win ..

Special thanks to
... Kyle MacRae ...
... and my friends at Helpdesk ...

Neil Dearnley-Davison
(aka Muckshifter)

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