1. Lecture Topics: 11/1 General Operating System Concepts Processes
processes vs. programs
Process Management
process control blocks
Address Spaces
Threads

2. HW5 Turnin Turn in your shell.exe via the web
courses/410/CurrentQtr/hw/turnin_hw5.html
Link to it from the class web
Limited to 2MB file (turn in release version if it’s too big)

3. Review: What is an O.S?
Operating system implements a virtual machine that is easier to program than the raw hardware
Application
Virtual Machine Interface
Operating System
Physical Machine Interface
Hardware

4. Review: What does an O.S. do?
Coordinator—allow multiple apps/users to work together fairly and efficiently
security (I can’t trash your memory)
communication (we can work together)
resource management (CPU, memory, disk)
Provider of standard services—most programs have basically the same needs
standard libraries, window system, file system
What if there was no O.S.?
Source code  compiler  executable  hardware
How does the executable get onto the hardware?
How do you get the output of your program?

5. Review: O.S. History
Originally, HW was expensive and humans were cheap
OS was optimized for the HW
Now, HW is cheap and humans are expensive
OS optimized for humans

6. What does an O.S. look like?
Hardware (CPU, devices)
Application Interface (API)
Hardware Abstraction Layer
File Systems
Memory Manager
Process Manager
Network Support
Device Drivers
Interrupt Handlers
Boot & Init
PowerPoint
Solitaire IE
Operating System
Portable

7. Dual Mode
Operating system can do things that a user program cannot. Examples?
Two modes of execution
user mode (applications)
supervisor mode (operating system)
Privileged instructions can only be called in supervisor mode (e.g. halt)
Hardware implements this with a “mode bit” in the CPU
mode bit = 0, no privileged instructions
mode bit = 1, can do anything

8. Getting control back
How does the OS get control back from a running process?
The process could explicitly return control to the OS (in many real-time systems)
We can’t trust the process to do this
OS sets a timer on the CPU (privileged instruction) and starts a user process
When the timer expires control is passed to the OS
timer expires 100–1000 times per second

9. Processes vs. Programs A program is passive A process is active
just a file on the disk with code that is potentially runnable
A process is active
one instance of a program in execution
also called job, task, sequential process
At any given time, there may be many processes running the same program
even though they run the same program, they are separate and independent
What state defines a process?

10. What’s in a Process? A process consists of at least:
the code for the running program
the data for the running program
an execution stack tracing the state of procedure calls made
the Program Counter, indicating the next instruction
a set of general-purpose registers with current values
a list of open files and open network connections

11. Process State
Each process has an execution state that indicates what it is currently doing:
ready: waiting to be assigned to the CPU
running: executing instructions on the CPU
waiting: waiting for an event, e.g., I/O completion
As a program executes, it moves from state to state

12. Process State Changing
new
running
terminated
ready
waiting
Processes move from state to state as a result of
actions they perform (e.g., system calls), OS actions
(rescheduling) and external actions (interrupts)

13. Process Data Structures
At any time, there are many processes active in a system
The OS must have data structures representing each process
this data structure is called the PCB (Process Control Block)
The PCB contains all of the info about a process, when the process is not running

14. PCB PCB contains lots of information, e.g.: process state
process number
program counter
stack pointer
32 general-purpose registers
memory management info
username of owner
queue pointers for state queues
scheduling info (priority, etc.)
accounting info

15. PCBs and Hardware State
When a process runs, its PC, SP, and registers, are loaded on the CPU
When the OS switches to a new process, it
saves the current process’s register values to its PCB
loads the next process’s register values from its PCB
This is called a context switch. It occurs times per second
why so often?
why not more often?

16. State Queues
OS maintains a collection of PCB queues for each possible state
PCB Word
PCB Tetris
PCB MSVC
Ready Queue Header
head ptr
tail ptr
Wait Queue Header
head ptr
PCB Defrag
PCB Telnet
tail ptr
Many wait queues—one for disk, one for user input, etc.

17. PCBs and State Queues PCBs are data structures in OS memory
A PCB is created for a process when it starts and put on the ready queue
While the process is active, PCB is on one of the state queues
When the process is terminated, its PCB is deallocated (after a little while)

18. Address Spaces
An address space is the range of addresses that a process may use
The legal address space is the range of addresses that a process may use right now
Reserved
Text
Static data
Heap and stack

19. Address Spaces How can multiple programs use the same addresses?
base register (value added to every memory reference)
How do you protect one program against the others?
bounds register (no memory reference allowed beyond here)
Now computers use virtual memory to solve these problems
Memory
Word
Solitaire NT
base reg =0x200000
bounds reg =0x500000
base reg =0x600000
bounds reg =0x700000

20. Processes and Address Spaces
Each process has its own address space
Process A’s memory references (loads and stores) are interpreted within process A’s address space
Can Process A load a word from Process B’s address space?
Can Process A load a word from kernel memory?

21. Threads An address space defines a process
A thread is a sequential execution stream
Multiple threads share the same address space
What state does a thread share with other threads in the process?
What state does a thread not share with other threads in the process?