1. Processes A process is a running program.
The cpu can be used by one process
while another is waiting on I/O.
Multiprogramming – can switch from
one program to another (interleave
them). To user seems like more than
one program running at a time on
a single cpu. With a multiprocessor
can run more than one at a time.

2. Processes A process is an executing program
with values for program counter,
registers and variables. Also owns
resources.
Example: 3 processes A, B, C C B A
Time

3. Processes Must have a scheduling algorithm to
decide when to stop a running
process and how to pick another one
to run.
Processes have different states:
New, Running, Ready, Sleeping (wait)
Zombie State
Child that exited
Parent has not called wait() to get exit status
Resources have been deallocated
Still has entry in process table
Can be killed with SIGCHLD (kill -18 ppid)
Blocked Process → waiting on resource (I/0 waitpid etc)
Ready
Running
New
Exit
Sleeping

4. Process Switching During a process switch the state of
the current process must be saved.
Then the new process can be given
time on the cpu – context switch.
The process that was taken off the cpu
can be loaded back on some time
later. Must save program counter,
contents of registers, etc. to do so.
Takes some amount of time.

5. Processes OS Must have a way to create and
terminate processes. Start off with
initial process (init, pid 1). Then create
more processes to form a tree.
How to create a process
- allocate resources
- create process table entry
fork(2), ps command, pid, parent pid

6. Processes Processes have three sections. Text section (code) can
be shared
between
processes
Stack
Data
Text
Memory

7. Process Hierarchy - SVR4
0 - sched
Use ps and
ptree commands
1 - init
inetd
sshd
tcsh

8. Process Termination Normal voluntary exit Error voluntary exit
involuntary fatal error
killed by another process
free up resources
remove process table entry
How many processes can a system
support?

9. Process Table Array of structures about active processes
- process state
- program counter
- stack pointer
- memory allocation
- open files
+ other stuff
Interrupt handling – suspend current
process, save registers, etc, run handler
then process scheduler.