We don't have operating systems that limit possible side effects at the discretion of the user at run time. This is the root cause of computer insecurity in the

We don't have operating systems that limit possible side effects at the discretion of the user at run time. This is the root cause of computer insecurity in the US, and the rest of the world. --- Background --- If I owe you $5.00, I can hand you an $5.00 bill, and that's the maximum I can lose. It's all down to my discretion at the time of the transaction. If I owe you $5.00, and can't use cash, then I have to put a bank account at risk, to make a transaction. There are rules about it, but the risks are far less constrained. The bank enforces my choices, most of the time. If you run a DOS program on an IBM XT with only 2 floppy disks, the only thing it can alter is the contents of any non-write-protected disks that happen to be mounted. It's possible to completely protect the operating system, and run almost any software you want, because the side effects are limited at the discretion of the user.