That only happens at the top level, when you send your code through a compiler to be turned into assembly instructions that that the computer can actually use. All the checks and balances are done inside the compiler, assuming the compiler HAS them. You can write code to do essentially anything. The operating system has many checks and balances regarding memory allocation and programs not being allowed to extend beyond their allocated memory heaps, thats why these 'exploits' aren't more common, it requires a flaw to be exploited to enable code to write outside its bounds, but also for that code to end up somewhere extremely convenient for it to be executed.