Problem when downloading zImage to DE2-115 (cont.)

Yes, sorry, there's a SDRAM at 0x00000000 so it should be working. If it can't write to the memory and read the same content (verify) it means your memory is not working properly. Your design seems to be missing a clock to your SDRAM. You should add a PLL to generate a clock to your SDRAM. Please take a look at the projects found in the board CD-ROM (you can download it from Terasic site). You will find the right PLL configuration and other valuable information in these reference designs.