Unlike other types of DRAM, which use the asynchronous communication, in SDRAM of response to a control signal to the device is returned not immediately, but only when the next clock signal. Clock signals are used to organize the work of SDRAM executing incoming commands. In this case the incoming team can come in a continuous stream, without waiting for the execution of the previous instruction: immediately after the write command can do the following command without waiting for the data will be recorded. Receipt of read commands will lead to the fact that the output data will appear after a certain number of cycles - this time is called latency and is one of the important characteristics of this type of device.

Clock signals are used to organize the work of SDRAM executing incoming commands

Cycles updates are performed once for the whole line, in contrast to previous DRAM types, updated data on internal counters, using the update method for CAS team before RAS team.

Using the data bus in the SDRAM proved complicated latency 2 or 3 cycles between the reading and applying the appearance of the data signal on the data bus, whereas during recording delay should not be. It requires the development of a rather complex controller, which is not allowed to use the data bus for writing and reading in the same time.

For the same SDRAM memory uses two types of designations:

  • on the "effective frequency" - DDRxxx
  • on the theoretical bandwidth - PCxxxx

Designation "DDRxxx" historically evolved from a sequence of standard names - PC66-PC100-PC133, when the speed of the memory associated with the frequency (introduced new "DDR", to distinguish SDRAM SDR from the DDR SDRAM).

Almost simultaneously with DDR SDRAM memory appeared RDRAM (Rambus) memory, where marketers have decided to put not the frequency but bandwidth of PC800. The width of the data bus was 64 bits (8 bytes). Of course, from the name change, nothing has changed, and RS800 RDRAM - that same RS100 SDRAM, but in another case... This - the usual sales strategy. As a result, manufacturers are producing memory modules, began to indicate the theoretical capacity - RSxxxx. So there were RS1600, PC2100, and so on. At the same time DDR SDRAM effective frequency is twice, and therefore is greater the number in the notation.

Here is an example of matching symbols:

100 Hz: 1600 DDR SDRAM = DDR200 SDRAM = PC100 SDRAM = PC800 RDRAM

133 Hz: 2100 DDR SDRAM = DDR266 SDRAM = PC133 SDRAM = PC1066 RDRAM

166 Hz: 2700 DDR SDRAM = DDR333 SDRAM = PC166 SDRAM = PC1333 RDRAM

200 Hz: 3200 DDR SDRAM = DDR400 SDRAM = PC200 SDRAM = PC1600 RDRAM

250 Hz: 4000 DDR SDRAM = DDR500 SDRAM

When selecting the amount of memory required to consider in order to work with office programs and demanding games not sufficiently 128MB. Higher speed is obtained when the amount of memory to 256 MB. For some applications, such as video editing software, raster graphics editor, modern 3D-games are recommended to have 512 MB of RAM or more. As a rule, the recommendations can be found in the documentation for these programs. If you find that when you buy a computer, you "do not guess" to memory, you can always install additional memory.