The block RAM in Spartan-6 FPGAs stores up to 18K bits of data and
can be configured as either two independent 9 Kb RAMs, or one 18 Kb
RAM. Each RAM can be addressed through two ports, but can also be
configured as a single-port RAM. The block RAM
resources include output registers to increase pipeline
performance. Block RAMs are placed in columns. The total number of
block RAMs depends on the size of the Spartan-6 device.