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The 8n prefetch architecture is combined with an interface designed to transfer two data words per clock cycle at the I/O pins. A single read or write operation for the DDR4 SDRAM consists of a single 8 n -bit-wide 4-clock data transfer at the internal DRAM core and 8 corresponding n -bit-wide half-clock-cycle data transfers at the I/O pins.
This is the following word if an even address was specified, and the previous word if an odd address was specified. For the sequential burst mode, later words are accessed in increasing address order, wrapping back to the start of the block when the end is reached. So, for example, for a burst length of four, and a requested column address of ...
DDR2 and DDR3 increased this factor to 4× and 8×, respectively, delivering 4-word and 8-word bursts over 2 and 4 clock cycles, respectively. The internal access rate is mostly unchanged (200 million per second for DDR-400, DDR2-800 and DDR3-1600 memory), but each access transfers more data.
Unlike previous generations of DDR memory, prefetch has not been increased above the 8n used in DDR3; [9]: 16 the basic burst size is eight 64-bit words, and higher bandwidths are achieved by sending more read/write
The key difference between DDR2 and DDR SDRAM is the increase in prefetch length. In DDR SDRAM, the prefetch length was two bits for every bit in a word; whereas it is four bits in DDR2 SDRAM. During an access, four bits were read or written to or from a four-bit-deep prefetch queue.
Definition CAS latency: CL: The number of cycles between sending a column address to the memory and the beginning of the data in response. This is the number of cycles it takes to read the first bit of memory from a DRAM with the correct row already open.
After a memory word is fetched, the memory is typically inaccessible for an extended period of time while the sense amplifiers are charged for access of the next cell. By interleaving the memory (e.g. cells 0, 4, 8, etc. are stored together in one rank), sequential memory accesses can be performed more rapidly because sense amplifiers have 3 ...
Reading and writing to the DDRAM is done by setting the RS input high during bus transfers. The DDRAM must also be selected by using the Set DDRAM address command which selects the DDRAM for access and also sets the starting address for DDRAM access. Likewise reading and writing to the CGRAM is done by setting the RS input high during bus ...