These are two dual-port memory structures, 18K bit each, which can be used to build large buffers and storage. Spartan-3 has something unique in that it has a 16-bit distributed memory structure. This means it is a logically used 4-input (16-bits memory each) lookup table. This is distributed throughout the fabric and can be cascaded. Old lookup tables, as shown, can be used as a 16 X 4-bit memory structure. A pair can also be cascaded for a 32 X 2-bit memory structure or all of them can be cascaded for a 64 X 1-bit memory. This versatility is unique to the Spartan-3 architecture and is very useful when building things, such as small FIFOs and buffers.