sometimes you don't have to bother detecting whether your program is being debugged, you can just adopt behavior that would make it impossible to run your code

sometimes you don't have to bother detecting whether your program is being debugged, you can just adopt behavior that would make it impossible to run your code under a debugger. i got my start in programming by trying to break various copy protection schemes in MS-DOS games. i used a software debugger that would set int 3 breakpoints, as the article mentions. BUT, for the debugger to work, it has to rewrite the interrupt vector table so that int 3 instructions will cause the debugger's own code to be executed. really wily code would use the entry for int 3 in the interrupt vector table as a scratch variable to perform a bunch of its own calculations, perhaps thousands of them so you can't find and modify them all, which would ultimately wind up with an address to jump to, to continue execution of the program. this means that, when the debugger rewrites the int 3 vector, the program's internal calculations would be thwarted, effectively stopping the program at that point. this was really difficult to get past. there were other people doing this kind of thing who seemed to find ways around this technique, but i never did.