The SRL16E has real life implications when it comes to reducing costs. This shows an 8-channel J.83 modulator, which is used in cable modem head-end application. This is a very register-intensive design. If you did not use the SRL16E feature you would run out of registers faster than you would run out of lookup tables in any other FPGA fabric. The bottom table shows what other resources are required to implement this design with and without using the SRL feature. In both cases, we look at the lowest cost Spartan-3 device needed to implement this design. If SRL16E feature is not used, a XC3S4000 device is required just to accommodate the number of registers that you have. By using the SRL16E feature, you are able to balance the number of lookup tables and the numbers of registers. By doing that, you can fit the same design in a significantly smaller Spartan-3 device, the XC3S1200E.