FPGAs referenced in this article have complex dedicated I/O circuitries that are primarily designed to support EMIF. The ALTMEMPHY megafunction is designed to support the most common memory standards, such as the DDR , DDR2 SDRAM, and QDR II+/QDR II SRAM (in a burst length of 4) interfaces. Other external memory standards such as Mobile DDR, QDR II+/QDR II SRAM (in burst length of 2), or customized DDR and DDR 2 SDRAM external memory standards are not supported. Instead, the ALTDLL and ALTDQ_DQS megafunctions are used to access the FPGA architecture and build a custom EMIF.