a few bits of "bank address" which accompany each command, a second bank can be activated and begin reading data while a read from the first bank is in progress. By alternating banks, an SDRAM device can keep the data bus continuously busy, in a way that asynchronous DRAM cannot. Single data rate synchronous DRAM Single data rate SDRAM (SDR SDRAM or SDR) is the original generation of SDRAM; it made a single transfer of data per clock cycle. Double data rate synchronous DRAM Double data rate SDRAM (DDR SDRAM or DDR) was a later development of SDRAM, used in PC memory beginning in 2000. Subsequent versions are numbered sequentially (DDR2, DDR3, etc.). DDR SDRAM internally performs double-width accesses at the clock rate, and uses a double data rate interface to transfer one half on each clock edge. 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. Direct Rambus DRAM Direct RAMBUS DRAM (DRDRAM) was developed by Rambus. First supported on motherboards in 1999, it was intended to become an industry standard, but was outcompeted by DDR SDRAM, making it technically obsolete by 2003. Reduced Latency DRAM Reduced Latency DRAM (RLDRAM) is a high performance double data rate (DDR) SDRAM that combines fast, random access with high bandwidth, mainly intended for networking and caching applications. Graphics RAM Graphics RAMs are asynchronous and synchronous DRAMs designed for graphics-related tasks such as texture memory and framebuffers, found on video cards. Video DRAM Video DRAM (VRAM) is a dual-ported variant of DRAM that was once commonly used to store the frame-buffer in some graphics adaptors. Window DRAM Window DRAM (WRAM) is a variant of VRAM that was once used in graphics adaptors such as the Matrox Millennium and ATI 3D Rage Pro. WRAM was designed to perform better and cost less than VRAM. WRAM offered up to 25% greater bandwidth than VRAM and accelerated commonly used graphical operations such as text drawing and block fills. Multibank DRAM Multibank DRAM (MDRAM) is a type of specialized DRAM developed by MoSys. It is constructed from small memory banks of , which are operated in an interleaved fashion, providing bandwidths suitable for graphics cards at a lower cost to