CPSC 3220 - DAY 2
AUGUST 29, 2017

================================================================================

Kernel
------

	Lowest level of software running on the system

	Purpose is to implement protection

	Kernel is fully trusted and has access to all the hardware.

Untrusted Code
--------------

	We should restrict privileges of untrusted code

	Should not have access to all the hardware

	Should not have ability to modify the kernel or other applications

Early Operating Systems
-----------------------

	One application at a time.
	
	OS was runtime library

	Users would stand in line to use the computer

	Batch systems - keep CPU busy by having a queue of jobs

Time-Sharing Operating Systems: Computers and People are expensive
------------------------------------------------------------------

	Multiprogramming: running multiple programs at the same time.

	Interactive performance: try to complete everyone's tasks quickly

	As computers became cheaper, more important to optimize for user time,
	not computer time.

Challenge: Protection
---------------------

How do we execute code with restricted privileges?

	Either because the code is buggy or if it might be malicious

Some examples:

	A script running in a web browser

	A program you just downloaded off the internet

	A program you just wrote that you haven't tested yet

Stages of a Program
-------------------

Source file on disk

Executable file on disk

	After compiling and linking

	Set of machine instructions (with a specified entry point) and
	initialized data

Memory image

	After loading

	Stack, heap and unitialized data areas added to provide full execution
	environment