Operating Systems 2014F Lecture 6

Library - code you did not write but it is sitting in your address space. The routines in there can be called, but to a first look, it is opaque. I am going to call it to do something, and it should do whatever I ask. On older systems, that's what the operating system does, they are just routines, you might call them directly, or you might call a dispatcher. Let's say you don't like what this operating system does, you just write your own routine to do it.

What happens if the operating system changes? Your code becomes tied to the exact version of the operating system.

When you call a library you call an API. If accessing private functions / data structures, you can do it, it will work, but when the api changes your code will break.

When you used an Apple 2 or commodore 64, you wouldn't normally have a hard disk with everything you are running, you would load off of a floppy. There might be multiple programs on that disk. It had the operating system on the disk as well. It would first load the OS and then the program would load off the disk. Every bootable floppy would have a copy of the OS. Stick in the first floppy of the program, and at some point the program would request you to put in another disk.

What would you do for copy protection? You would modify the operating system on the disk. The operating system had such low level control. Take one, create one from scratch or modify one. On these systems, the OS is just a library. A library that can do I/O stuff.

If you don't like it, you want to mess with it, you can. You can overwrite the OS. It's just routines to help you. This is good when your program essentially controls the machine. Lets' say I want to run more than 1 program. Instead of code and data..