DDR1 Double Data Rate interface was developed to enable more bandwidth. This technology accepts commands once per cycle but reads or writes two words of data per clock cycle. DDR made this possible by reading and writing data on both the rising and falling edges of the clock signal.
Some other changes were made to the SDR timing and lowering of the supply voltage from 3.3 volts to 2.5 volts. These adjustments were a drawback and caused DDR SDRAM not to be backward compatible with SDR SDRAM.
Some typical clock rates of DDR SDRAM were 133, 166 and 200 MHz generally referred to as DDR-266, DDR-333 and DDR-400.
IN PC terms, these 184-pin DIMMS (Dual Inline Memory Modules)were referred to as PC-2100, PC-2700 and PC-3200.
DDR2 SDRAM is almost like DDR1 but doubles the minimum read or write to 4 words per clock cycle.
The clock rates of DDR2 are 200, 266, 333 and 400 MHz and are referred to as DDR2-400, DDR2-533, DDR2-667 and DDR2-800. These were 240-pin DIMMS and are referred to as PC2-3200, PC2-4200, PC2-5300 and PC2-6400.
DDR3 SDRAM follows suit by doubling the DDR2 minimum read or write from four words to eight words. This enables doubling the bandwidth and external bus without changing the internal clock rate.
Some clock rates in this category are 400, 533, 667 and 800 MHz respectively. These are referred to as DDR3-800, DDR3-1066, DDR3-1333 and DDR3-1600.
In PC terms, these are PC3-6400, PC3-8500, PC3-10600 and PC3-12800.
There are also DDR3-2200 modules available.
DDR4 was released in 2012 and is quite successful. This module is still in production and, hopefully, DDR 5 should emerge in 2019.
How much computer memory do you need for your computer?
You might have experienced what it is like to use a computer that does not have enough memory. You would see the 'hour glass' appearing on your computer monitor screen often. The amount of memory that you need depends on:
Your type of system
The tasks that you are performing
Your software applications that you are using
The amount of memory that you will require for a desktop computer will be different from what is required for a server.
Memory requirements for a desktop computer
Your memory requirements for a desktop will depend on:
Your operating system.
Your application software.
2GB is now considered the minimum memory for light tasks such as word processing, email and web surfing.
In addition to the above tasks, if you are multi-tasking such as doing spreadsheets and some graphics simultaneously, you should consider upgrading to 4GB.
If there is a shortage of memory (RAM), the system will revert to a memory management technique called 'swapping' or virtual memory.
While using virtual memory, information is broken up into small bits of data called pages. If more memory is required, the operating system (OS) determines which pages are the least likely to be required next and writes them to the hard disk. If these pages are required again, the system re-loads them into memory and displaces other pages of information.
Since a hard drive is slower than RAM, using virtual memory a lot can negatively affect your system's performance. Therefore, if you do a lot of memory-intensive work which always uses up large amounts of virtual memory, you should consider adding more RAM.
Computer Memory Requirements for a Server
A server needs more memory especially if there is disk swapping.
Typical server memory configuration varies from approximately 32GB up to 64GB. Extensive server systems can use in exess of 256GB. To decide on the amount of memory a server needs will depend on what it is used for, its operating system, the number of users accessing it at one time and the number and type of processors installed on the server.
Do not mix different types of memory modules. For example, do not mix SDRAM with DDR SDRAM, etc. Modules of a higher group are not backward compatible. This feature of differentiating memory types is secured by assigning different pin configurations to the different memory types.