If you just want to make a minor change, you can make your change in $TOP/fpga/bare_metal/examples/boot.c and then run make boot cfgmem-updated program-cfgmem-updated from current directory $TOP/fpga/board/nexys4_ddr which will burn the new bootloader into configuration memory (assuming you already followed the build from source instructions) You can just power cycle or press the reconfigure button at this point. If you want to completely replace the boot loader, proceed as follows: choose a template, e.g. hello.c cp $TOP/fpga/bare_metal/examples/{hello,foo}.c add foo wherever you see hello in $TOP/fpga/board/nexys4_ddr/Makefile and $TOP/fpga/bare_metal/examples/Makefile You can then make your changes to foo.c and run make foo cfgmem-updated program-cfgmem-updated as before On 21/02/18 02:49, Jawad Haj-Yahya wrote: