Unused addresses in embedded memory block ? are they lost ?

You do have to be aware of the memory types in an FPGA to be able to allocate them optimally. The memories generally have true dual-ports, where you can read or write to one side, and read or write to the other in parallel, or simple dual-port, where you can write to one side and read from the other. FIFOs can be implemented using either type, with the latter being the most obvious choice. When you have dual-clocked FIFOs, the write-side and the read-side use different clocks.

If you configure the FIFO depth such that it does not use all the memory, then because the independent read and write ports are already used, you cannot implement another FIFO. This means that the unused memory cannot be accessed ... the only option is to make your FIFO deeper so that it can use the memory, or make it shallower, so that a different memory type is used.

Cheers,

Dave