The MT18VDDF12872DY is high-speed, CMOS dynamic random access 512MB and 1GB memory modules organized in a x72 configuration. These modules use 256Mb and 512Mb DDR SDRAM devices with four internal banks. DDR SDRAM modules use a double data rate architecture to achieve high-speed operation. The double data rate architecture is essentially a 2n-prefetch architecture with an interface designed to transfer two data words per clock cycle at the I/O pins. A single read or write access for DDR SDRAM modules effectively consists of a single 2n-bit-wide, one-clock-cycle data transfer at the internal DRAM core and two corresponding n-bit-wide, one-half-clock-cycle data transfers at the I/O pins. A bidirectional data strobe (DQS) is transmitted externally, along with data, for use in data capture at the receiver. DQS is a strobe transmitted by the DDR SDRAM during READs and by the memory controller during WRITEs. DQS is edge-aligned with data for READs and center-aligned with data for WRITEs. DDR SDRAM modules operate from differential clock inputs (CK and CK#); the crossing of CK going HIGH and CK# going LOW will be referred to as the positive edge of CK. Control, command, and address signals are registered at every positive edge of CK. Input data is registered on both edges of DQS, and output data is referenced to both edges of DQS, as well as to both edges of CK.