The 256Mb DDR SDRAM is a high-speed COMS, dynamic random-access memory containing 268,435,456 bits. It is internally configured as a quad-bank DRAM. The 256Mb DDR SDRAM uses 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 the 256Mb DDR SDRAM 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 an intermittent 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. The 256Mb DDR SDRAM operates from a differential clock (CLK and CLK#; the crossing of CLK going HIGH and CLK# going LOW will be referred to as the postive edge of CLK). Commands (address and control signals) are registered at verey positive edge of CLK. 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 CLK.