Memory Buying Guide

• How much do I need?
• What are the different types?
• SDRAM
• DDR SDRAM
• DDR2 SDRAM
• ECC memory
• Mixing memory
• Memory price and brands
• Performance factors
• Dual memory channel
• Buffered and unbuffered memory
• Latencies
• Integrated graphics controllers

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If you're buying a new PC, building your own or planning to upgrade an old one, getting the memory right is crucial. In everyday operations, memory is quite possibly the most important performance factor in your PC. Not having enough memory will slow your PC down far more than getting a processor that is a few hundred megahertz (MHz) off the pace.

Computer memory is commonly referred to as RAM - for Random Access Memory. You can think of RAM as your processor's "working space", a place where data and instructions can be stored temporarily for quick delivery to the processor (CPU). Unlike hard disk space, RAM is usually volatile storage, meaning that the RAM is emptied when the computer is powered off and any information held in RAM is lost. Your computer will not run without RAM, and it will not run well if you don't have enough RAM.

There's no quicker or cheaper way to give your PC a shot in the arm than to install more RAM.

How much do I need?

As a rule, the more RAM the better. Run a typical modern operating system (OS) such as Windows XP, plus a few applications, or a single game, and there's a very good chance you will exceed your standard system memory.

What happens then? Your OS calls on virtual memory. This enables you to keep working but, from a performance perspective, virtual memory is very bad news.

When your PC runs out of physical memory, the operating system starts to use the hard disk to store temporary data. The hard disk acts like a memory extender -- this is called virtual memory. The problem is that the hard disk has a lot slower access speed than real RAM, so when the processor wants to recall something from virtual memory, it takes a relatively long time. If you have ever switched between applications in Windows and found that it takes a few seconds and your hard disk churns violently before the application changes over, that's because it's accessing virtual memory. In fact, if you ever find you're waiting for your computer to complete an operation, there's a pretty good chance that the delay is because the computer is using virtual memory.

To minimise the chance of accessing virtual memory, you need to have a system with enough RAM to support the things you want your computer to do. There is no upper value limit on memory (the more the better) although, with a typical Windows XP system, you will start to see diminishing returns after about 1.5 gigabytes (GB). Here's a quick guide to the memory requirements of applications:

As we've said, a RAM upgrade is more likely to improve the overall performance of a system than a new processor. Memory is not expensive, and you'll really notice when you don't have enough of it.

The maximum amount of RAM you can have in your PC is largely determined by the number of RAM slots on your PC's motherboard. Single modules can be purchased with up to 1GB of RAM. Typical PC motherboards have 2-4 memory slots. Sometimes, you may have to swap out smaller modules if you want to upgrade with larger ones, simply because you no longer have any slots free.

You can freely mix and match modules of different sizes (having, for instance a 256MB module in one slot and 1GB in another). It is less advisable though, to mix and match memory brands and types.

What are the different types?

Buying memory can be a little confusing if you don't understand the terminology. Over the years, iterations of memory technology have appeared to go hand in hand with processor speed improvements. More RAM is always good, but faster RAM can also dramatically improve the speed of your PC.

A typical memory module, when you buy it, might be described something like: 512MB PC3200 DDR SDRAM DIMM. In this case, the "512MB" refers to the amount of memory the card has; "PC3200 DDR SDRAM" is the type and speed of the memory; and "DIMM" is the packaging.

We'll touch on the latter first. The packaging determines the type of slot the memory will fit into on your motherboard. It's generally not something you need to worry much about, since pretty much all modules now come in Dual Inline Memory Module (DIMM) format. Earlier memory often came in Single Inline Memory Module (SIMM) formats, and SIMM memory cards had to be installed in pairs.

The bigger concern is the type and speed of the memory. A given motherboard will only support RAM of a certain type and speed. You need to check your motherboard specifications to see what types of memory it supports. Here's a quick rundown of the types you'll commonly encounter:

SDRAM

SDRAM (Synchronous Dynamic Random Access Memory) is not found on new computers, but a significant installed base of computers still exists for which the only upgrade option is SDRAM.

SDRAM commonly comes in two speeds, denoted by a clock speed rating, in megahertz (MHz). You can purchase either 100MHz (PC100) or 133MHz (PC133) SDRAM. Much like processors, memory runs at a given number of cycles per second -- the more cycles, the faster the memory.

SDRAM synchronises with the clock speed of the computer's processor, and it's important to choose the right type of memory to go with your processor. It's often necessary to look up your processor documentation to find this out. Some processors require 100MHz memory, others need 133MHz. This is because CPUs run at a multiplier of the speed of the memory bus (ie, "front-side bus"). For instance, a 1.33GHz processor might run on a 133MHz memory bus (requiring PC133 memory) with a 10x multiplier.

It is possible to put PC100 memory in a PC133 system, but in order not to fry the memory, you'd have to lower the speed of the front-side bus (in the motherboard's BIOS settings, or with a jumper setting) to 100MHz - which would reduce, for instance, the aforementioned 1.33GHz processor to 1GHz.

DDR SDRAM

Following SDRAM, memory vendors introduced Double Data-Rate SDRAM (DDR SDRAM), which is the most common type of memory available. DDR SDRAM allowed data to be sent on both the rising and falling edge of a clock cycling, therefore doubling, as the name implies, the overall speed at which the memory can deliver data to the processor.

The memory vendors also changed the nomenclature, from a clock speed rating to an abstract numbering system. Thus, we have, in ascending order of speed: PC1600, PC2100, PC2700 and PC3200 DDR SDRAM. The motherboard specifications will detail what kind of memory your system can handle. Most new computers support PC3200 DDR SDRAM. It's always advisable to get the fastest memory your system can handle.

Here's a breakdown of the bus speeds supported by the different memory types:

• PC1600 memory runs at a 200MHz data rate (100MHz clock)
• PC2100 memory runs at a 266MHz data rate (133MHz clock)
• PC2700 memory runs at a 333MHz data rate (166MHz clock)
• PC3200 memory runs at a 400MHz data rate (200MHz clock)
• PC3500 memory runs at a 434MHz data rate (217MHz clock)
• PC3700 memory runs at a 466MHz data rate (233MHz clock)
• PC4000 memory runs at a 500MHz data rate (250MHz clock)
• PC4200 memory runs at a 525MHz data rate (262.5MHz clock)

As shown above, memory does extend above the PC3200 specifications, but currently no CPU has an front-side bus running that high. The very high-end modules are only really necessary for serious PC enthusiasts who intend to overclock their CPU, and wish to gain a corresponding increase in memory performance.

Much as with SDRAM, it is possible to run DDR SDRAM on slower buses. If your motherboard only supports PC2100, for instance, you can still put PC3200 memory in there and run it at 266MHz. It won't perform any better than PC2100, but if you ever upgraded your PC to one that supports PC3200 you could move the memory across readily.

DDR2 SDRAM

A new development, DDR2 SDRAM is just now becoming available at reasonable prices. Few motherboards support it yet (and, as of June 2005, only some Intel motherboards and processors have the potential for a faster front-side bus), but that is likely to change in the near future. AMD has yet to produce chips that support DDR2, but will undoubtedly do so this year.

The name might imply that DDR2 goes twice as fast as DDR, however that's not the case. DDR2 simply supports faster clock speeds than are supported by the DDR specification. DDR2 also has other tweaks and additions to make it work faster and more reliably than a DDR memory module, and it uses less power.

In terms of clock speeds, DDR2 picks up where DDR leaves off. Unlike DDR, most DDR2 memory is advertised by its data rate, in MHz.

At the present, there are three types of DDR2 available: • DDR2 400MHz • DDR2 533MHz • DDR2 667MHz

ECC memory

Error correcting code (ECC) memory is not a unique type of memory -- it's an added feature of some memory cards. ECC RAM has additional circuitry that tests and corrects the accuracy of data going to and from the memory.

ECC RAM is largely designed for mission critical applications, such as company servers, where data accuracy is paramount. It costs considerably more than regular DDR SDRAM -- sometimes more than four times as much - so it is used only rarely in consumer systems.

If you're building a personal PC, ECC memory is not required or recommended. In any case, DDR2 memory, with its reduction in signal noise, promises to deliver more reliable data.

Mixing memory

It's possible to mix and match memory types, as long as they're all supported by your motherboard. The caveat is that you're going to end up in a lowest-common denominator situation -- all the memory will run at the speed of the slowest memory module in your collection. Putting PC2700 DDR in with PC3200 DDR will result in both the modules running at PC2700 speed, for instance.

Memory price and brands

RAM is very often sold as generic, with no brand name given. This is largely the result of the perception that one PC3200 module is much the same as any other PC3200 module.

There are, in fact, important qualitative differences between memory brands, but these differences are often subtle and hard to identify with specifications alone.

One of the best indications of the quality of a given brand of memory is the warranty offered by the manufacturer. Premium RAM will often come with a lifetime guarantee; more cheaply manufactured modules often only have one year.

Other quality indicators include CAS and other latencies (which we'll touch on shortly), buffering, and the number of chips in a given module.

Overclockers, in particular, need to be cognisant of memory brands, since only quality memory will run at higher than its rated clock speed without frying.

The price of memory is, of course, determined by supply and demand, and high supply in the last five years has resulted in very low prices. If you're going with the lowest-cost option, you can readily pick up 512MB of PC3200 DDR memory for around $70. DDR2 running at 533MHz can be purchased for about $100 for 512MB. There is considerable variation in memory costs across brands, especially for DDR2 memory.

Performance factors

Ultimately, memory performance is about how fast the RAM can deliver data and instructions to the processor. The biggest determinant of that is the RAM's clock speed.

It's difficult to quantify the effect of faster RAM on overall system performance, since much depends on the particular application in use. Applications that use a lot of memory (imaging and video applications for instance) will see the greatest benefit from faster memory. Typically, there is not a huge price premium on higher performance memory. We recommend getting the fastest memory your system can support.

In addition to the clock speed of the memory, there are a number of other, rather more subtle, indicators of a given RAM module's speed. These details are often hard to find, however, especially with memory marketed as "generic". If you're keen on squeezing maximum performance from your PC components, you might want to look into some of the following details.

Dual memory channel

This is actually quite a large determinant of performance, although it has more to do with your motherboard than your RAM.

Many new motherboard chipsets offer a feature called dual memory channel. On a dual memory channel motherboard, RAM slots are given independent data pathways ("buses") to the processor. That is, each memory slot has its own set of wires leading to the processor, which means that it doesn't have to share the wires with other memory slots (we haven't seen motherboards with more than two memory channels; if there are more than two slots, then at least two memory modules will have to share the bandwidth of one channel).

With independent buses, the total amount of data that can be transported from RAM to the processor can be increased vastly, increasing the overall performance of the system. It's like having two traffic lanes instead of one.

This assumes that each memory channel is being used, which requires each memory channel to contain at least one RAM module. On a dual memory channel motherboard, it's better to fill the slots than to leave them open for future expansion. Say, for instance, you have a choice between two 512MB modules and one 1GB module. If you have a dual memory channel motherboard, it's better to get the two 512MB DIMMs -- one for each memory channel.

Dual memory channel motherboards with more than two DIMM slots can get quite complicated, and will often require reference to the motherboard manual to figure out which memory slots to put your modules in. For example, if you have two memory modules and a motherboard with three slots, you may need to fill slots one and three, leaving slot two empty, to make the most of dual-channelling.

Buffered and unbuffered memory

Buffered modules have internal circuitry to help them deal with the electrical load required by servers with huge amounts of memory. Because of the buffer, they're actually slower (but more expensive) than unbuffered memory.

If you're building a regular PC, you'll almost certainly want to get unbuffered memory, which is faster and cheaper. Unless the product description says memory is buffered, you can assume that any commodity memory you find in stores is unbuffered. Some unbuffered modules do have what is called a register, which ensures reliability in data handling, but again, this is not really necessary for standard PC systems.

Latencies

A key differentiator, especially for premium memory brands, is the speed at which the module can locate data. This is not the raw transport speed, which is usually measured in GB per sec, but the amount of time, in clock cycles, it takes to start transmitting the requested information.

When a memory system receives a request for data, it has to figure out which module contains the desired data. Then it has to switch to that module, and that module has to figure out which of the chips on its board contains the data, then which bank of memory it is in. It then has to call the memory forward before it can be put on the bus for transmission to the CPU. The time it takes to locate and draw on the information before the data even starts to get sent, is called the memory latency.

Memory latency is measured in clock cycles, and has many components. Advanced researchers will encounter tRAS, tRP, tRCD, and command rate as factors in latency, but the most common, and most important, latency factor is known as the CAS (Column Access Strobe) latency.

In DDR SDRAM, CAS latency (sometimes called CL) can be 1.5, 2 or 2.5 clock cycles. In DDR2, it can be 3, 4 or 5. The CAS latency tells us how many clock cycles it takes to access a column of data in the memory banks -- the lower the CAS latency, the better. If you can get DDR memory with a CAS latency of 1.5, it's going to be roughly 5-10 per cent faster overall than one with a CL of 2.5. Remember however, that this is relative: DDR2 has higher CAS latencies than DDR, but makes up for it by having faster clock cycles.

Integrated graphics controllers

Motherboards that have integrated graphics controllers have become quite common, and if you have purchased one of these, you need to be aware of one important point: they usually use system memory for graphics operations. You can usually assign a fixed amount of memory to the graphics controller in the system BIOS. This amount is taken out of your usable system memory. Say you have a 512MB system, and assign 32MB to graphics -- your system will then tell you it has only 480MB available.

Note, also, that common system memory is often slower than the memory you find on dedicated graphics cards, which have specialised GDDR (graphics DDR) or eDRAM (embedded DRAM) to meet the heavy memory requirements of 3D graphics. Embedded chips using system memory will often perform poorly, especially in 3D.

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