Karvell Li
Notes from Friday, 4/11/02
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The point of this lecture is
The CPU acts as the government to divide resources enabling multiple 
programs to run on the same machine at the same time.

There is one OS (government) and many address spaces for user programs 
(individuals) at any given time, but why is it this way?

	So programs can be tricked into thinking it is running in 
the same address space every time.

Historic reasons

Prior to 1960
A computer consists of
1. CPU			Prices Steady
2. Memory		Prices Steady
3. I/O devices		Prices Steady
	TTY (teletypewriter)
	Card Readers
	Glass Console

(for info on TTY, see
http://whatis.techtarget.com/definition/0,,sid9_gci213663,00.html)

Serial Access to the CPU is as efficient as parallel access. Since 
prices are steady and demand is low, there was no motivation to 
timeshare the CPU. 

Crash model
Only one program runs at a time, if it crashes, it will only mess up 
itself and won't affect other programs/users. There was no real need 
for an OS.


Year 1962
A computer consists of
1. CPU			Prices Steady or going up. Every new model 
				is different so is expensive.
2. Memory		Prices Going down. Memory is the same. Lots 
				of regularity = cheap.
3. I/O devices		Prices Going down.
	TTY	
	Card Readers
	Glass Console

I/O's are slow so it takes time to input programs. Having lots of 
them would keep the CPU busy. In order to run all these programs, 
more memory is needed (there is a synergy between I/O and memory). 
Since memory and I/O are relatively cheap, we can afford to have lots 
of them and improve the computer's performance by using parallel 
access (running multiple programs at the same time).

Since multiple programs are running at the same time, the crash model 
of pre 1960s no longer works.

New crash Model
  isolate one program from another
  isolate application from OS
  OS must be the government. Since there is a limited amount of 
  resources, the OS must decide how to share it among the different 
  programs.

Today
Things are changing, I/O becoming more expensive relative to the
CPU whose prices are constantly driven down by advances in technology 
and competition in the market.

____________________________________________________________________
Things that enable multi program/time share of CPU
 clock interrupt
 code in trap handler
 mechanism to context switch between address spaces

The essence of timesharing - slowly changing the states of many 
processes - more slowly than if only one process was running.

Scheduling - decides how fast a process runs (no CPU resource or full 
CPU resource, or more often, a point in between)

A Process is
	Program in execution
It consists of
	Address space
		code
		data
		empty space
	Registers
		needed to give instructions to CPU
		communicate w/ the CPU	

Address space and Registers are used to maintain the STATE of the 
program
A process also needs a MEANS to change the state

____________________________________________________________________
The OS has complete view of all address spaces and view the entire 
space as DATA.

Code is code because they are interpreted as code – that is the ONLY 
difference between code and data.

OS see Process as pile of data
Process sees itself as code and data (writing a program is to decide 
what data to use)

A sample program


int p()
{
    int x;
    x = foo(7);	//push 7 on stack, transfer to foo
		//assignment after return from foo
}

int foo(int y)
{
    return y + 2; //value of on stack (7), add 2, returns 7+2
}


Data structures are used to maintain flow of execution and to 
represent the state of execution.

OS only does state management between processes
 maintains queue to do this

The queues
Ready
Idle/waiting (like waiting for I/O devices)
Running (currently bound to processor)

Running processes has three options - keep running, go to idle, 
or go to ready.

>From OS's point of view, a process is just a process control 
block.

A process control block (PCB) consist of
Address space
Registers
Owner
TTY
Runtime
Priority

When OS switches process (context switch)
1. saves PCB of  currently running process into PCB old
2. restore states from PCB of new process