Techvision srilanka

PC Memory

Introduction

In planning for our monthly upcoming series on the Best Memory for the Money, we decided to take a look only at three kinds of PC memory, and to skip the SODIMMs (small outline dual in-line memory modules) so commonly used in notebook PCs nowadays. These will probably make their first appearance later, but for this first iteration, we plan to cover only three types of RAM used in desktop PCs:

* DDR or Dual Data Rate memory is by no means the oldest type of RAM available for sale nowadays, but its heyday has already come and gone, and it's the oldest type of RAM that we will cover. You must actively seek out older motherboards and processors to put a new system together that uses this kind of RAM, though plenty of existing systems use DDR RAM. DDR gets its moniker from its ability to access two individual memory locations (data bits) during each clock cycle (one on the leading edge of the cycle on the incoming transition, the other on the trailing edge of the cycle on the outgoing transition). DDR operates at 2.5 volts (more when overclocked, usually) and is the heaviest power consumer of the three types of memory covered here.
* DDR2 or Double Data Rate 2 memory is the most common type of memory used in current, modern PCs. It's neither the oldest nor the newest type of RAM around, but it appears in most ready-made PCs and notebooks in today's marketplace (while the format differs, DDR2's speed and capabilities in laptops are the same as those for conventional desktop DDR2 flavors). DDR2 generally runs faster than DDR, which is why it offers faster data rates than its older counterpart (the slowest DDR2 is as fast as the fastest DDR). DDR2 operates at 1.8 volts and, like DDR, usually at higher voltages when overclocked. DDR2 falls in the middle of the power consumption range for the three types of memory covered here.
* DDR3 or Double Data Rate 3 memory is the putatively faster new kid on the memory block. Once again, DDR3 ups the speed ante over DDR2, so that its slowest speed (which is generally not commercially available) is as fast as DDR2's fastest speed (800 megatransfers per second). DDR3 is the least power hungry of the three types covered here, while operation is at 1.5 volts and more when it is overclocked.

Memory Specifications Tell Only Part Of The RAM Story

JEDEC is a standards body for the global semiconductor industry. Originally the acronym stood for Joint Electron Device Engineering Council, but now it is decoded as the JEDEC Solid State Technology Association.

From the standpoint of PC memory devices, JEDEC sets the basic specifications to which all RAM must adhere at a minimum. As you look at the values in Table 1 and compare them to information that appears in Tables 2 through 4, however, you'll see that overclocking parts push beyond the JEDEC limits, to the point where there is usually overlap between neighboring types of memory, as with DDR-400 or PC-3200, and DDR2-400 and PC2-3200.




DDR


DDR2


DDR3

Rated Speed


100-400


400-800


800-1600

Voltages


2.5v +/- 0.1v


1.8V +/- 0.1V


1.5V +/- 0.075V

Internal Banks


4


4


8

Termination


Limited


Limited


All DQ signals

Topology


TSOP


TSOP or Fly-by


Fly-by

Driver Control


None


OCD Calibration


Self Calibration with ZQ

Thermal Sensor


No


No


Yes (Optional)
Table 1: JEDEC Memory Specification Values

Notes: Fly-by is also sometimes called FBGA (fine-pitch ball grid array); TSOP stands for "thin small-outline package;" OCD stands for off-chip driver circuitry; and ZQ refers to a connection with a built-in, extremely precise 1% 240? RZQ resistor against which the chip calibrates itself.
A Matter Of Timing

JEDEC standards also dictate standard timings for memory modules as well as the other information covered in Table 1. Memory costs hinge on two different key values, while improvements to these values increase the per-unit price.

The first key value is memory latency, which has to do with the delay between initiating a request for memory access (usually by memory location or address) and the time until the data stored at that location is retrieved. Latency is a fundamental measure of memory speed, whereby lower latency means faster memory access.

The other key value also involves latency and is generally referred to as memory timing. Memory timing hinges on four performance metrics, listed as follows in order of importance:

* CAS, usually expanded as column address strobe (or sometimes as column address select), which refers to the column for some physical memory location in an array composed of columns and rows of capacitors used in dynamic random access memory (DRAM) modules (of which all three types of RAM in this guide are sub-types). CAS latency generally appears first in timing sequences for RAM and indicates the number of clock cycles that elapse between when the memory controller instructs the memory module to access a particular column in its current row, and when such access produces the data that resides there.
* Trcd or tRCD, usually expanded as RAS to CAS delay, where RAS is expanded as row address strobe, where R refers to the row for a physical memory location in an array composed of columns and rows of capacitors used for DRAM modules. This value specifies the number of clock cycles between a Row Address Strobe (RAS) and a CAS, and represents the row address to column address delay for a memory module.
* Trp or tRP, usually expanded as RAS precharge, which represents the number of clock cycles required to end access to the current row of memory, and commence access to the next row of memory, so that tRP = time for row precharge.
* tRAS or Tras, usually expanded as RAS access time measured by the number of clock cycles needed to access a certain row of data in DRAM between the initial data request and the precharge command required to commence the next memory access. By definition, tRAS must be greater than or equal to the CAS plus the tRCD, plus an additional two cycles, to leave time for accesses to complete, as they read or write multiple bits of memory, which DDR (2 bits), DDR2 (4 bits), and DDR3 (8 bits) all do in lesser or greater numbers.

RAM memory timings usually appear as sequences of four numbers separated by dashes, as in 5-5-5-15. This indicates that the CAS, tRCD and tRP values are all equal to five clock cycles, and that the tRAS value is equal to 15 clock cycles. The smaller the numbers that appear in these sequences, the tighter memory timings are said to be. Likewise, larger numbers are said to indicate looser timings. Simply put, lower latency costs more, tighter timings cost more and the combination of the two costs the most where memory is concerned.
RAM Value Versus Performance

As we already explained, JEDEC memory specifications set the basis for what kinds of performance and capabilities RAM must deliver. In general, memory that's designed to meet the JEDEC specs but not to exceed them costs the least to design and implement. That's why it's usually the cheapest and is so often used in budget and commercial systems, for which component prices are usually more important than their performance.

Memory vendors usually call this kind of memory "value line memory," where low cost trumps other considerations. This explains why value RAM is drab looking, and generally devoid of heat spreaders, fancy logos and even, advertising coverage. If cost is more important than performance, you should buy value RAM every time. If you plan to add memory modules to your PC, buy value RAM if that's what's already installed, otherwise, match performance as closely as possible if you can't buy exactly the same kind as is already in use.

On the other hand, performance memory is memory that exceeds JEDEC specifications by deliberate design. As any price check will clearly illustrate, performance memory costs more than value memory - sometimes, indeed, a great deal more than value memory of the same type and speed. Most performance memory not only exceeds JEDEC specifications for timings (by making them tighter); it also exceeds JEDEC specifications for speed (by making them run faster than the maximum speeds defined in those documents). Beyond the maximum JEDEC speed for some type of memory, all faster memory is performance memory by definition. In fact, there are those who might be inclined to argue that all DDR3 memory is performance memory because of its relatively high cost. Technically speaking, however, only DDR3-1800 (and the soon-to-be-released DDR3-2000) memory exceeds the defined DDR3 speed range.
DDR And A Trip Down Memory Lane

Dual Data Rate or DDR memory has been around for some time now and represents the trailing edge of memory technology. You have to work at buying new components that support DDR these days, but it's still in demand for upgrades or extensions to existing systems. Just for the record, we define a memory upgrade as a complete swap-out of older, slower RAM for newer, faster RAM, generally with lower latency or tighter timings, if not both. We define a memory extension as adding more memory modules to an existing installation without getting rid of the older modules.

Label


Bus speed


Data rate


Typical timings


Remarks

PC2100


133


266


2.5-3-3-7


Older PCs, notebooks

PC2700


166


333


2.5-3-3-7


Older PCs, notebooks

PC3200


200


400


2.5-3-3-8


Last of the standard DDR timings

PC3500


217


433


2.5-3-3-7


Overclockers only for the rest of table

PC3700


233


466


2.5-3-3-7




PC4000


250


500


2.5-3-3-7




PC4400


275


550


2.5.-3-3-7




PC4800


300


600


2.5-4-4-10




Table 2: Common DDR Memory Speeds and Nomenclature

Note: Bus speed refers to the communications rate (in MHz) between the memory controller and the memory modules. Data rate refers to the number of memory transfers per second as a mega number (multiply by 1.048,576 or 210). Timings are in numbers of clock cycles. PC3700 and PC4800 are hard to find, though numerous forms of PC3500 and PC4400 remain available on today's market.

Entries in bold in Table 2, represent memory types that exceed JEDEC specifications. These are designed to appeal to overclockers. For DDR memory, PC3200/DDR400 is by far the most popular type still being purchased, according to all the memory vendors we interviewed for this story. All entries with labels that are not in bold are value products, with overclockable versions missing entirely or being extremely scarce.
DDR2: Memory Technology's Sweet Spot

Dual Data Rate 2 or DDR2 memory is relatively new, and follows the introduction of Intel's LGA 775 socket and accompanying chipset architectures, as well as subsequent support for DDR2 in the AMD AM2 socket architecture and its accompanying chipsets. DDR2 has been in circulation for over three years as we write this article, and is the prevailing type of memory used in new computing equipment currently on sale in retail channels.

Label


Bus speed


Data rate


Typical timings


Remarks

PC2-3200


200


400


3-3-3-12


Seldom encountered, not covered

PC2-4200


267


533


4-4-4-12


Popular, esp. on older DDR2 mobos

PC2-5300


333


667


5-5-5-15


Most popular DDR2 speed

PC2-6400


400


800


5-5-5-15


Widely used, growing support

PC2-8000


500


1000


5-5-5-15


Overclockers only here on down

PC2-8500


533


1066


5-5-5-15




PC2-8888


556


1111


5-5-5-15




PC2-9136


571


1142


5-5-5-15




PC2-10000


625


1250


5-5-5-18




Table 3: Common DDR2 Memory Speeds and Nomenclature

Though PC2-3200 falls within JEDEC specifications, you hardly find it in actual use on desktop PCs (perhaps because it's the same speed and data rate as DDR-400/PC-3200). Our standard coverage concentrates on PC2-4200, -5300 and -6400, because those are the most commonly purchased varieties (even PC2-4200 memory can be hard to find). According to our vendor sample, in fact, PC2-5300/DDR-667 is the most-popular type of memory in the marketplace today. As in the preceding section, we cover performance DDR2 memory at the higher echelons of the price spectrum.
DDR3 Sets Cost-and-Performance Limits

DDR3 is a relative newcomer to the memory scene, where memory modules of this type have only been available for about one year. That said, its performance ceiling continues to expand, with the JEDEC limit just met and exceeded within the past five months. Today DDR3-1600 (the JEDEC top speed) is pretty widely available, and an increasing number of vendors also offer DDR3-1800 (an overclocking variety by definition). Prototypes of DDR3-2000 have been shown at recent trade events, and products with this label are expected to be commercially available by year's end or early next year. While the JEDEC specification calls for PC3-6400/DDR3-800, we couldn't find any such memory for sale, and therefore didn't include it in the table.

Label


Bus speed


Data rate


Typical timings


Remarks

PC3-8500


533


1066


7-7-7-20


Usually called DDR3-1066

PC3-10666


667


1333


7-7-7-20


Usually called DDR3-1333
Kingston calls it PC3-11100 @ 1375

PC3-12800


800


1600


9-9-9-24


Usually called DDR3-1600

PC3-14400


900


1800


9-9-9-24


Usually called DDR3-1800

PC3-16000


1000


2000


TBD


Usually called DDR3-2000
Note yet commercially available

Table 4: Common DDR3 Memory Speeds and Nomenclature

The percentage of market penetration for DDR3, according to the memory vendors we spoke to while researching this story, still falls in the 1%-2% range, which means that DDR3 has a long way to go to match DDR sales (still in the 12%-16% range) let alone approach DDR2 sales (somewhere in the 25%-35% range in terms of current manufacturing output). Nevertheless, as the current leading-edge memory technology, this is where we expect to see the most innovation over the next year or two, particularly as motherboard chipsets and BIOS designs begin to make more room for the faster bus speeds that DDR3 demands.
How To Shop For Value Or Performance

When it comes to value, very often the low price leads the pack. However, the best buy may be fleeting and you may want to consider giving your business to vendors whose products delivered good results in the past. Particularly in the value category, in fact, price differences will often be negligible, so other factors such as availability, speed of delivery or pick-up may play a surprisingly important role. Even so, it still pays to shop around, and to check the latest prices Tom's stores offer or at other price-comparison sites before you buy.

When it comes to performance, things can get more complicated. A performance-RAM buying decision requires you to determine the real value that higher performance or better overclocking potential really offers. You must then purchase on the basis of how far (and fast) you can push your RAM, as well as its rated abilities. This is where reading reviews of the RAM device that interests you can be helpful, and where Tom's reviews can be particularly illuminating. If you can buy a lot of extra performance for just a little extra cost, that's something of a no-brainer to decide. But deciding if it's worth an extra hundred dollars to boost performance by 10%, 20%, or even 30% is something between you and your account balances (and perhaps also, the chancellor of the exchequer, when family budgets come into play).
Summary And Conclusions

In the future, we plan to report on specific memory modules that represent good buys, both in the value lines and overclocker or performance-memory categories. Anybody who can make a case for lower costs for value memory, or lower costs on a price-performance basis for performance/overclocking memory is welcome to chime in on our discussion boards. We will adjust our coverage and methods over the months ahead, as we turn this new adventure into an ongoing routine, and all input and suggestions are entirely welcome. We hope you find value in the information presented here, as we hope you'll help us to improve the value and information content in our forthcoming charts.