Operating-system-simulator headers.h contains clk functions, it should be included anywhere the clock functions are used. To get time call: getClk(); You can use Makefile to build and run your project To compile your project, use the command: make To run your project, use the command: make run If you added a file to your project add it to the build section in the Makefile Always start the line with a tab in Makefile, it is its syntax To clean .out files, .txt files: make clean To clean, build and run make all