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October 2013 |
We have been on DDR3 for several years as of 2013. During these years, we have seen a range of memory from DDR3-800 to DDR3-2000. What do these numbers mean? What can we expect in the future from memory technology? What do we really want from memory?
- Human has short term memory and long term memory. Computers are similar. Short term memory is the main memory. Long term memory is the hard disk. This training note focuses on computer main memory. Surprising, it is not just short term and it is volatile. The memory will disappear if the computer power is turned off. This type of memory is called Dynamic Random Access Memory (DRAM). DDR3 is DRAM.
- Fortunately USB memory sticks are not volatile so that we can keep data or video files on USB for several years perhaps. USB memory sticks use the same type of technology as SSD (static state disk) called NAND (Not AND). NAND is non-volatile but it allows ‘data written to it” for a finite number of times and the speed of writing and reading is much lower than DRAM.
- DDR3 stands for Double Data Rate Generation 3 and it succeeded DDR2, DDR, and SDR (Single) in reversed chronological order. All of them are DRAM. SDR used 1 clock signal to initiate one memory transfer. DDR used 1 clock signal for 2 transfers. DDR2 gets 4 transfers and DDR3 gets 8 transfers. DDR3-800 is therefore based on a 100MHz clock. These numbers are deceiving as the multipliers do not deliver correspondingly.
- Users care about the effective rate of data transfer and not the physical clock frequency or multipliers. One literature stated DDR3 as having 11GB/s. If a motherboard and its CPU are designed with 3 memory controllers, we can expect the data rate to increase to 33GB/s. Intel Core i7 with 2011pin is designed to have 4 memory controllers and it can hit 44GB/s in theory.
 The specifications for DDR4 have been completed by an international standard organisation called JEDEC. Preliminary tests claimed a data transfer rate of 24GB/s which is more than double of DDR3. This increase is not achieved with higher clock frequency along the DDR2 or DDR3 approaches, but is from a complete re-design. DDR2 and DDR3 use a parallel signal transmission scheme which requires the beginnings and ends of all signal transmission paths to be synchronized. DDR4 uses a packet signal transmission scheme which does not need to wait to get synchronized. DDR4 is not backward compatible with DDR3, and it will be available in computer systems on the market in year 2015.
Memory speed is important for the performance of computers.
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