As mentioned earlier, the data used by the CPU comes from the memory stick. Whether it is a software program or data, it must be read into the CPU in the memory before it can be used.
The main component of personal computer memory is Dynamic Random Access Memory (DRAM). DRAM is divided into several generations according to technical updates. There are two widely used SDRAM and DDR SDRAM. The difference between them is in addition to the pin position and working voltage. DDR is a double data transfer rate (Double Data Rate), which can transfer data twice in one working cycle.
DRAM memory has undergone several technological developments in the past 20 years, from SDRAM to the current DDR4, but some things have not changed, one of which is the operating frequency of memory.
At the beginning, it developed from SDR to DDR. The biggest improvement is to change the number of times the memory transmits data from 1 to 2 times in one clock cycle, making full use of the transmission time of the memory: SDR can only be raised from a low level to a high level. The transmission occurs when the level is flat, and the DDR can also transmit data when the high level falls back to the low level. In fact, a cycle is actually exactly one low to high and one high to low.
In fact, a more important reason for adopting this technology at that time was that it was impossible to simply increase the operating frequency of SDR memory, because the heating of the memory particles became uncontrollable because the operating frequency was slightly increased. So the earliest SDR stopped when it reached 200MHz.
After the development of DDR, the heating of the chip will not improve (the working principle and process of the chip have not improved), but because data is transmitted twice in one cycle, DDR memory has twice the transmission capacity of SDR. Since then a concept has arisen: Equivalent Operating Frequency. As the name suggests, the maximum operating frequency of DDR memory is also 200MHz, but because the performance is twice that of SDR, the equivalent operating frequency of DDR memory is 400MHz.
Since then, all the performance expressions of the memory are no longer described by the actual operating frequency of the memory, but instead use the equivalent operating frequency. Including DDR 400Mhz, DDR2 800MHz, DDR3 1600MHz, etc., it is said that it is equivalent to the operating frequency of SDR.
After that, upgrading from DDR memory to DDR2 memory is similar to all subsequent upgrades. Compared with DDR, DDR2 transmits twice the amount of data in each transmission process, but the operating frequency does not change. DDR3 is twice as fast as DDR2, and DDR4 is twice as fast as DDR3.
Another problem is memory latency. It can be seen from the above that the actual frequency of DDR2 800MHz memory is 200MHz, and the actual frequency of DDR3 800MHz memory is 100MHz. Therefore, under the condition of the same equivalent operating frequency, the previous generation memory will have lower latency than the descendant memory.
In addition to frequency/bandwidth and model to consider, capacity is also important. Because all data must pass through the memory stick to be read by the CPU, if the memory stick is not large enough, some large-capacity data will not be able to be loaded completely. Data must be freed before it can be loaded until the amount of available memory is greater than the new data.
Therefore, large memory often means a faster system, because the system does not need to release some data inside the memory often.
Dual-channel design: increase the width of the busbar, one memory stick is 64 bits, and two channels are 128 bits.
The size of the memory is the same as the model, and the data is written/read out of the pair of memory sticks synchronously to improve the overall bandwidth.
The color of the memory slots on the motherboard is the same, and they form dual channels instead of two adjacent channels of different colors.
Some servers also support three or four channels. For example, the Intel CPU with E5-2650 v3 launched in 2014 has a maximum acceptable channel count of four-channel DDR4.
The following table lists the relationship between the frequency and bandwidth of some memory models:
