We have used DDR (Double Data Rate) memory and they have served us well. However, it may be time to say goodbye to this technology - its mission has come to an end. DDR memory is too slow and has some operational problems. Some alternatives are being considered, including DDR2. We will study DDR2 to find out how it works. If you are looking to buy computer memory, these are valuable information for you.
Processors are faster and faster and hotter. Nowadays there are graphics hardware that consume up to 75W, with a medium shade. Memory is no exception. Computers are getting faster and faster, and power consumption and heat output are increasing as well.
The vast majority of computer systems sold so far have used DDR memory. DDR memory is twice as fast as its predecessors because it can read / write data at both the up and down cycles of the system clock, ie each clock cycle of the system clock. Two data samples can be manipulated. Therefore, DDR memories are usually labeled with their actual clock speed, which is twice the speed of the system clock. For example, a DDR memory running at a clock speed of 200 MHz is labeled as DDR400.
Another way of labeling is based on the actual data transfer rate of the memory. DDR400 memory, capable of transmitting 3.2 gigabytes of data per second, is also labeled PC320 (3200 Mbit / s). The speed at first seems pretty high, but it really does not meet current microprocessor standards. Pentium 4 (3.4 GHz) or Athlon 64 FX-53 (2.4 GHz) processors still have a lot of waiting time to read data from memory. The cache size of the processors has increased to minimize this waiting time.
There are a number of other issues related to DDR that affect the stability and performance of the computer. For example, memory termination (MT) is essential to minimize signal reflections that can reduce the stability. MT is not designed inside DDR memory but in fact it is controlled by a set of resistor packages in the motherboard. This design has a relatively high cost and further increases the likelihood of instability as the clock speed increases as the resistors control the memory termination away from the DRAM chips.
DDR2: Reduced power consumption and latency issues
The latest memory for mainstream PCs is DDR2. DDR2 fixes some potential problems with native DDR memory (also known as DDR1). For example, DDR2 has an on-die termination mechanism that improves signal integrity if the clock speed increases.
DDR2 also solves the problem of high power consumption and heat emission in a very effective way. The real clock rate of DDR2 memory is half the known rate of the system clock. Meanwhile, the DDR1 memory clock is kept at the same speed as the external I / O clock. For example, a DDR2 / 533 memory (266 MHz) has an actual internal clock frequency of 133 MHz. The input / output buffers are 266 MHz, but they are clock cycles reserved for the system. To solve this asynchronous clock cycle, DDR2 transfers up to 4 data samples per clock cycle. Because I / O buffers run twice as fast, it only processes two data samples per clock cycle I / O. Thus, in reality, the memory transfers data to quad data rate I / O but the system sees only two data samples per clock cycle I / O. .
But latency is a completely different thing. The latency of the DDR1 memory can be up to 2 clock cycles, for some modules in the original manufacturer's system, the latency can be up to 2.5 or 3 cycles. DDR write delay is a cycle, but even when the frequency of external hardware increases, this indicator is still quite low. Thus, DDR2 adopts a fairly simple algorithm approach, ensuring that the write delay is always less than the 1-cycle CAS latency. So, if the CAS latency is 4 (typical for current DDR2 modules), the write latency is always 3 cycles.
From a system-wide point of view, latency does not really matter much. The CAS 2 latency for DDR400 is about 15ms and the CAS 3 latency for DDR2.533 is similar. The overall bandwidth is still increasing, because this relatively low latency only occurs for the first read of a memory line. Then, the memory transfers the data to the system in a higher clock cycle. If the system is running with DDR2 / 400 memory, you will not see a significant difference in performance.
The problem of high power consumption is handled by lowering the voltage from 2.5V to 1.8V. As memory capacity increases, power consumption for memory increases as well. According to the calculations, 4GB DDR1 memory consumes 35-40W of power. The voltage drop helps power consumption for 4GB of memory is only 25-30W. Low voltage can also increase the frequency of the clock.
System bus and memory controllers
The LGA 774 Pentium 4 processors use a system bus (CPU bus and main memory on the motherboard) at 200 MHz (actual speed 800 MHz) but the actual speed of the DDR2 memory clock / 533 is 266 MHz. This difference reduces memory efficiency as well as affects flux. Ideally, make sure you want the clock of the system bus synchronized with the memory clock. It is likely that Intel will speed up the Pentium4 system bus by the end of this year.
AMD, on the other hand, is facing a slightly different problem. Athlon 64 memory controller is built into the CPU. The positive side of this design is that the memory controller latency is significantly reduced as they run at the same speed as the processor. But that also means AMD needs to revamp Athlon 64 to support DDR2. Whether they are making this improvement for today's CPUs or waiting for 90nm technology is still an open question. With excellent performance when running AMD's system with DDR400, this incompatibility has not had much impact on them in the short term.
The other memory
While DDR and DDR2 dominate the current memory market, other types of memory are vying for OEM OEM attention. These types of memory include Kentrol's quad-band memory and Rambus's new XDR memory. The support for QBM was very much announced by VIA last year. This Kentron product is not a completely new design that inherits from DDR and uses switching technology to double the throughput. It can be seen as a kind of dual-channel memory on a single module. Kentron's idea is to use a low-cost and improved memory to double the performance but not speed up the clock.
XDR is Rambus's latest attempt to convince PC makers and memory monitors to use its proprietary technology. Rambus's previous significant effort in the field of computer memory was the launch of RDRAM. This event has also had a significant impact on Intel's announcement of RDRAM in the Intel 820, 850 and 860 chipsets. The XDR DRAM is indeed a future-oriented technology. Rambus predicted that DDR2 / 667 would be the maximum DDR2 memory available, although some observers believe the top speed should be DDR2 / 800. Whatever the outcome, Rambus claims that the XDR DRAM will be the next step in the effort to improve memory performance. Of course, with the history of Rambus's struggles, it is not certain who will collaborate with them. XDR is based on Rambus's Yellowstone signaling technology, which does not have to contradict ownership but requires at least 18 months for it to occupy a significant position. There is a lot of change in the tech world around that time.
Choices when upgrading memory
Only two cases can happen: you decide to buy a new system or upgrade from the old system. If you are using DDR1, upgrading your memory may mean buying additional DDR1 modules. However, if you use an older system and plan to upgrade to a new processor, consider the memory requirements before implementing a system upgrade.
If you decide to buy a new system then you have to consider between two options: AMD or Intel. If you choose Intel, you need a socket T board that can support the new 900 series chipsets and DDR2 memory. In this case, you should spend more money and buy DDR2 / 533 memory. Some companies are also beginning to offer DDR2 / 667, but it is not yet clear whether DDR2 / 667 modules can actually work in DDR2 / 667 systems. Also, expensive price is another issue, so now DDR2 / 533 is the best option.
If you want to upgrade your PC to Athlon 64, your choice is simple: DDR400. With the advantages of an integrated memory controller in the Athlon 64's design, along with good quality, the low latency of the DDR400 also improves performance. But if you wait until the fall of this year, Athlon 64 can support both DDR2.
