1. CPS 110/EE 153 Course Intro

2. Operating Systems: The Big Picture
The operating system (OS) is the interface between user applications and the hardware.
An OS implements a sort of virtual machine that is easier to program than the raw hardware.
User Applications
Operating System
Architecture
virtual machine interface
physical machine interface
[McKinley]

3. The OS and the Hardware
The OS is the “permanent” software with the power to:
control/abstract/mediate access to the hardware
CPUs and memory
I/O devices
so user code can be:
simpler
device-independent
portable
even “transportable”
interrupts
Processor
Cache
Memory Bus
I/O Bridge
Main
Memory
I/O Bus
Disk
Controller
Graphics
Controller
Network
Interface
Disk
Disk
Graphics
Network

4. The OS and User Applications
The OS defines a framework for users and their programs to coexist, cooperate, and work together safely, supporting:
concurrent execution/interaction of multiple user programs
shared implementations of commonly needed facilities
“The system is all the code you didn’t write.”
mechanisms to share and combine software components
Extensibility: add new components on-the-fly as they are developed.
policies for safe and fair sharing of resources
physical resources (e.g., CPU time and storage space)
logical resources (e.g., data files, programs, mailboxes)

5. Overview of OS Services
Storage: primitives for files, virtual memory, etc.
control devices and provide for the “care and feeding” of the memory system hardware and peripherals
Protection and security
set boundaries that limit damage from faults and errors
establish user identities, priorities, and accountability
access control for logical and physical resources
Execution: primitives to create/execute programs
support an environment for developing and running applications
Communication: “glue” for programs to interact

6. The Four Faces of Your Operating System
service provider
The OS exports commonly needed facilities with standard interfaces, so that programs can be simple and portable.
executive/bureaucrat/juggler
The OS controls access to hardware, and allocates physical resources (memory, disk, CPU time) for the greatest good.
caretaker
The OS monitors the hardware and intervenes to resolve exceptional conditions that interrupt smooth operation.
cop/security guard
The OS mediates access to resources and grants/denies requests.

7. Other Useful Metaphors
1. Phone systems
Defines an infrastructure for users to call each other and talk.
- doesn’t dictate who you call; doesn’t dictate what you say
- supports services not imagined by creators, e.g., 900 numbers
2. Government
Sets rules and balances demands from a diverse community.
Users/subjects want high levels of service with low taxes.
3. Transportation
Wide range of choices for a wide range of user goals and needs.
- race car: simple interface to powerful technology…goes fast…crashes hard.
- cadillac: makes choices automatically…comfortable…no fun to drive.
- SUV: runs on any terrain…rolls over bumps…burns gas…but gas is cheap.
Race car: a thin layer between the driver and the engine.
Cadillac: dual air bags, power assist steering, kind of clunky.
Phcone system: digital communications, call waiting, caller ID, and other unanticipated uses….basic infrastructure, much the same everywhere.
SUV comes in lots of nice colors
timesharing: city bus…somebody else is driving…cheap, but takes a while to get where you’re going
The System as Taxi Cab
generic
goes where you want...fast enough
you only get it when its idle
you don’t need to know how to drive
you have little control over safety
(a network OS for a cluster of public workstations)

8. Studying Operating Systems
This course deals with “classical” operating systems issues:
the services and facilities that operating systems provide;
OS implementation on modern hardware;
(and architectural support for modern operating systems)
how hardware and software evolve together;
the techniques used to implement software systems that are:
large and complex,
long-lived and evolving,
concurrent,
performance-critical.

9. Server farms (clusters)
The World Today
desktop clients
Servers
database
file
web
...
Internet
Server farms (clusters)
mobile devices
Internet appliances
LAN/SAN
Network

10. The Big Questions
1. How to divide function/state/trust across components?
reason about flow of data and computation through the system
2. What abstractions/interfaces are sufficiently:
powerful to meet a wide range of needs?
efficient to implement and simple to use?
versatile to enable construction of large/complex systems?
3. How can we build:
reliable systems from unreliable components?
trusted systems from untrusted components?
unified systems from diverse components?
coherent systems from distributed components?

11. Classical View: The Questions
The basic issues/questions in this course are how to:
allocate memory and storage to multiple programs?
share the CPU among concurrently executing programs?
suspend and resume programs?
share data safely among concurrent activities?
protect one executing program’s storage from another?
protect the code that implements the protection, and mediates access to resources?
prevent rogue programs from taking over the machine?
allow programs to interact safely?

12. Memory and the CPU OS data OS code CPU Data Data registers
OS code
CPU
OS data
Program A
Data
data R0 x
Program B Rn
Data PC x
registers
code library 2n
main memory

13. A First Look at Some Key Concepts
kernel
The software component that controls the hardware directly, and implements the core privileged OS functions.
Modern hardware has features that allow the OS kernel to protect itself from untrusted user code.
thread
An executing stream of instructions and its CPU register context.
virtual address space
An execution context for thread(s) that provides an independent name space for addressing some or all of physical memory.
process
An execution of a program, consisting of a virtual address space, one or more threads, and some OS kernel state.

14. The Kernel The kernel program resides in a well-known executable file.
The “machine” automatically loads the kernel into memory (boots) on power-on or reset.
The kernel is (mostly) a library of service procedures shared by all user programs, but the kernel is protected:
User code cannot access internal kernel data structures directly.
User code can invoke the the kernel only at well-defined entry points (system calls).
Kernel code is like user code, but the kernel is privileged:
Kernel has direct access to all hardware functions, and defines the machine entry points for interrupts and exceptions.

15. A Protected Kernel OS data OS code CPU
OS code
CPU
Mode register bit indicates whether the CPU is running in a user program or in the protected kernel.
Some instructions or data accesses are only legal when the CPU is executing in kernel mode.
OS data
Program A
data
Data
mode R0 x
Program B Rn
Data PC x
registers
code library 2n
main memory

16. Threads A thread is a schedulable stream of control.
defined by CPU register values (PC, SP)
suspend: save register values in memory
resume: restore registers from memory
Multiple threads can execute independently:
They can run in parallel on multiple CPUs...
- physical concurrency
…or arbitrarily interleaved on a single CPU.
- logical concurrency
Each thread must have its own stack.

17. The Problem of Concurrency

18. Making It Concrete
It is best to learn the general concepts of systems from a careful study of particulars.
Microsoft Windows and NT (Windows 2000)?
Unix or Sun Microsystem’s Solaris?
MacOS?
Linux or *BSD?
Java?
Nope: we will use “Nachos”.

19. Course Materials for CPS 110
Course viewgraphs on the web site
The Nachos instructional OS
Nachos Project Guide
Nutt: Operating Systems: A Modern Perspective (1998)
Other readings on the course web site
Birrell: An Introduction to Programming with Threads.
etc.?

20. Nachos Projects Labs 1-3: concurrency and synchronization
race conditions with processes and threads
implementing/using synchronization for safe concurrent code
Lab 4: protected kernel with multiprogramming
OS kernel with system calls, memory allocation, virtual address translation, protection
Lab 5: I/O and inter-process communication
Labs 6-7: virtual memory
page faults and demand loading
page replacement and page cache management

21. We’re Not in Kansas Anymore
Be careful out there: CPS 110 is For Mature Audiences Only.
Any OS is a complex beast with lots of moving parts.
Concurrency adds an unfamiliar and difficult element.
Virtual Machines are difficult to think about and debug.
For Nachos you will extend a base of Someone Else’s Code.
Working in teams is a double-edged sword.
These labs by design leave more opportunity for creative interpretation than is common in introductory classes.
The Unix/C++ development environment is a powerful tool offering many opportunities to “shoot yourself in the foot”.

22. Secrets of the Nachos Labs
We skip the hand holding; you skip the hand wringing over picky details of what you are “supposed to do”.
You are free to resolve ambiguity as you see fit, and you must justify your choices.
It’s the thought that counts.
Think before you design it.
Think before you code it.
Think before you run it.
Think before you debug it.
The time needed to conceive and write the code is moderate, but debugging time is potentially unbounded.

23. Secrets of CPS 110 1. If you work hard, we work hard to help you.
Slackers are persona non grata (you know who you are).
2. Be ready to work concurrency problems on the exams.
Drill it and use it in the labs.
3. Pay careful attention to team management for the labs.
New rule this semester: teams may eject slackers by unanimous vote of the remaining members.
4. Pay attention to the newsgroup, and post if you need help.
cc: chase
5. Have confidence in the grading.

24. Overview of Nachos Labs 1-3
In the thread assignments, you build and test the kernel’s internal primitives for processes and synchronization.
Think of your program as a “real” kernel doing “real basic” things.
boot and initialize
run main(), parse arguments, initialize machine state
run a few tests
create multiple threads to execute some kernel code
shut down
...runs native, directly on the host machine.
Kernel only; no user programs (until Lab 4)

25. What To Do Next 1. Form teams of four.
(2-5)
2. Look at the course web and the Nachos Project Guide.
On the Web, and I will hand out hard-copy on Tuesday.
3. Install and build the Nachos release.
Determine where your source code will reside.
Set up ACLs so your team and my TAs can access your code.
Optional: set up version control (e.g., CVS).
Report any problems.