The New D RAM Interfaces: SDRAM, D RDRAM a nd Variants
Brian T. Davis, Bruce Jacob, Trevor Mudge · 2000
For the past two decades, developments in DRAM technology, the primary technology for the main of computers, have been direct- ed towards increasing density. As a result 256 M-bit chips are now commonplace, and we can expect to see systems shipping in volume with 1 G-bit chips within the next two years. Although densities of DRAMs have quadrupled every 3 years, access speed has improved much less dramatically. This is in contrast to developments in processor technology where speeds have doubled nearly every two years. The resulting memory has been widely commented on. The solution to this gap until recently has been to use caches. In the past several years, DRAM manufacturers have explored new DRAM structures that could help reduce this gap and reduce the reliance on complex multilevel caches. The new structures have not changed the basic storage array that forms the core of a DRAM; the key changes are in the interfaces. This paper presents an overview of these new DRAM structures. 1I ntroduction For the past two decades developments in DRAM technology, the primary technology for the main of computers, have been directed towards increasing density. As a result, 256 M-bit chips are now commonplace, and we can expect to see sys- tems shipping with 1 G-bit chips in volume within the next two years. Many of the volume applications for DRAM, particularly low cost PCs, will thus require only one or two chips for their primary memory. Ironically, then, the technical success of the commodity DRAM manufacturers is likely to reduce their future profits, because the fall in units shipped is unlikely to be made up by unit price increases. Although densities of DRAMs have quadrupled every 3 years, access speed has im- proved much less dramatically. This is in contrast to developments in processor tech- nology where speeds have doubled nearly every two years. The resulting memory has been widely commented on. The solution to this gap until recently has been to use