1. CS 345 Stalling’s Chapter # Project 1: Computer System Overview
2: Operating System Overview 4
P1: Shell
3: Process Description and Control
4: Threads
P2: Tasking
5: Concurrency: ME and Synchronization
6: Concurrency: Deadlock and Starvation 6
P3: Jurassic Park
7: Memory Management
8: Virtual memory
P4: Virtual Memory
9: Uniprocessor Scheduling
10: Multiprocessor and Real-Time Scheduling
P5: Scheduling
11: I/O Management and Disk Scheduling
12: File Management 8
P6: FAT
Student Presentations
BYU CS 345
OS Overview (Chapter 1)

2. 1. Compile and Validate
A task is a unit of execution (also referred to as a process).
A shell (Command Language Interpreter) is a task that functions as an interface between the user and an Operating System.
A shell interprets textual commands coming either from the user’s keyboard or from a script file and executes the commands either directly or creates a new child process to execute the command.
For Project 1:
Download all the project files from class website.
os345.c, os345interrupts.c, os345signals.c os345tasks.c, os345semaphores.c
os345.h, os345config.h, os345signals.h
os345p1.c, os345p2.c, os345p3.c, os345p4.c, os345p5.c, os345p6.c
os345park.c, os345park.h, os345lc3.c, os345lc3.h, os345mmu.c, os345fat.c, os345fat.h
Edit os345config.h (if necessary) to select host OS/IDE/ISA. (Only enable one of the following defines: DOS, GCC, MAC, or NET.)
Compile and execute your OS.
BYU CS 345
Project 1 - Shell

3. Why CS 345?
BYU CS 345
OS Overview (Chapter 1)

4. Operating Systems What is an operating system? How about…
Hard to define precisely, because operating systems arose historically as people needed to solve problems associated with using computers.
How about…
“Software that makes computing power available to users by controlling the hardware.”
“Software executes when nothing else is happening.”
“A collection of software modules including device drivers, libraries, and access routines.”
BYU CS 345
OS Overview (Chapter 1)

5. What Does a Modern OS Do? Provides Abstractions:
Hardware has low-level physical resources with complicated, idiosyncratic interfaces.
OS provides abstractions that present clean interfaces.
Goal: make computer easier to use.
Examples: Processes, Unbounded Memory, Files, Synchronization and Communication Mechanisms.
Provides Standard Interface:
Goal: portability.
Unix runs on many very different computer systems.
BYU CS 345
OS Overview (Chapter 1)

6. What Does a Modern OS Do? Mediates Resource Usage: Consumes Resources:
Goal: allow multiple users to share resources fairly, efficiently, safely and securely.
Examples:
Multiple processes share one processor. (preemptable resource)
Multiple programs share one physical memory (preemptable resource).
Multiple users and files share one disk. (non-preemptable resource)
Multiple programs share a given amount of disk and network bandwidth (preemptable resource).
Consumes Resources:
Solaris takes up about 8 Mbytes physical memory (or about $400).
BYU CS 345
OS Overview (Chapter 1)

7. The Future…
In the future, computers will continue to become physically smaller and more portable.
Operating systems have to deal with issues like disconnected operation and mobility.
Media rich information within the grasp of common people - information with psuedo-real time components like voice and video.
Operating systems will have to adjust to deliver acceptable performance for these new forms of data.
BYU CS 345
OS Overview (Chapter 1)

8. Finally
Operating systems are so large no one person understands whole system. Outlives any of its original builders.
The major problem facing computer science today is how to build large, reliable software systems.
Operating systems are one of very few examples of existing large software systems, and by studying operating systems we may learn lessons applicable to the construction of larger systems.
BYU CS 345
OS Overview (Chapter 1)

9. Chapter 1 – Computer Systems
Let’s figure out
what’s inside
this thing...
Alex Milenkovich

10. Learning Objectives
Describe the basic elements of a computer system and their interrelationship.
Explain the steps taken by a processor to execute an instruction.
Understand the concept of interrupts and how and why a processor uses interrupts
List and describe the levels of a typical computer memory hierarchy.
Explain the basic characteristics of multiprocessor and multicore organization.
Discuss the concept of locality and analyze the performance of a multilevel memory hierarchy.
Understand the operation of a stack and its use to support procedure call and return.
BYU CS 345
OS Overview (Chapter 1)

12. What is a computer system?
Objectives
Computer Systems
What is a computer system?
What are the basic elements of a computer system?
Registers
Interrupts
Caching
Input/Output
Protection
BYU CS 345
OS Overview (Chapter 1)

13. Registers Computer Systems Interrupts Caching Input/Output Protection
Objectives
Computer Systems
Registers
Interrupts
Caching
Input/Output
Protection
Summary
BYU CS 345
OS Overview (Chapter 1)

14. Registers
CPU
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

15. Processor Registers User-visible registers
May be referenced by machine language
Available to all programs - application programs and system programs
Data Registers – can be changed by user
Address Registers – could be separate from data register
Stack Registers – user / supervisor stacks
Condition Codes – results of operations
Control and status registers
May or may not be visible
Program Counter (PC) – address of next instruction
Instruction Register (IR) – most recently fetched instruction
MAR/MBR – memory reference registers
Program Status Word (PSW) – condition codes, interrupts, mode
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

16. Instruction Execution
Registers
Instruction Execution
Processor executes instructions in a program
Instructions are fetched from memory on at a time
Fetch Cycle
Execute Cycle
Fetch Next
Instruction
Execute
Instruction
START
HALT
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

17. Registers
Lots of Registers…
BYU CS 345
OS Overview (Chapter 1)

18. Interrupts Computer Systems Registers Caching Input/Output Protection
Summary
BYU CS 345
OS Overview (Chapter 1)

19. Interrupts
Interrupts
The interrupt was the principle tool available to system programmers in developing multi-tasking systems!
Improves processing efficiency by allowing the processor to execute other instructions while an I/O operation is in progress
A suspension of a process caused by an event external to that process and performed in such a way that the process can be resumed
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

20. Processing of Interrupts
Classes of Interrupts
Program
arithmetic overflow
division by zero
execute illegal instruction
reference outside user’s memory space
Timer
I/O
Hardware failure
An interrupt handler determines nature of the interrupt and performs whatever actions are needed
Control is transferred to this program
Generally part of the operating system
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

21. Interrupt Cycle Processor checks for interrupts
If no interrupts fetch the next instruction for the current program
If an interrupt is pending, suspend execution of the current program, and execute the interrupt handler
HALT
Fetch Next
Instruction
Execute
Check for
Interrupt:
Process Interrupt
Fetch Cycle
Execute Cycle
Interrupt Cycle
Interrupts
Disabled
Enabled
START
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

22. Simple Interrupt Processing
Interrupts
Simple Interrupt Processing
Device controller or
other system hardware
issues an interrupt
Save remainder of
process state
information
Processor finishes
execution of current
instruction
Processor signals
acknowledgment
of interrupt
Process interrupt
Processor pushes PSW
and PC onto control
stack
Restore process state
information
Processor loads new
PC value based on
interrupt
Restore old PSW
and PC
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

23. Multiple Interrupts 2 Choices Disable Interrupts Allow Interrupts
Disable upon entering an interrupt handler
Enable upon exiting
Allow Interrupts
Allow an interrupt handler to be interrupted
Priorities?
BYU CS 345
OS Overview (Chapter 1)

24. Multiple Interrupts - Sequential Order
Disable interrupts so processor can complete task
Interrupts remain pending until the processor enables interrupts
After interrupt handler routine completes, the processor checks for additional interrupts
What happens to the interrupts that occur when disabled?
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

25. Multiple Interrupts - Priorities
Higher priority interrupts cause lower-priority interrupts to wait
Causes a lower-priority interrupt handler to be interrupted
Example when input arrives from communication line, it needs to be absorbed quickly to make room for more input
How does this change interrupt handlers?
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

26. Caching Computer Systems Registers Interrupts Input/Output Protection
Summary
BYU CS 345
OS Overview (Chapter 1)

27. Memory Hierarchy More Expensive Faster & Smaller Bigger Slower
Caching
Memory Hierarchy
More Expensive
Faster & Smaller
Bigger
Slower
Registers
Cache
Main Memory
Disk Cache
Magnetic Disk
Magnetic Tape
Optical Disk
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

28. Memory Cache Given: Cache: Processor first checks cache
Caching
Memory Cache
Given:
Processor speed is faster than memory speed
Execution/data localizes
Cache:
Contains a portion of main memory
Invisible to operating system
Used similar to virtual memory
Increases the speed of memory
Processor first checks cache
If not found in cache, the block of memory containing the needed information is moved to the cache
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

29. Cache Design Mapping function Replacement algorithm Write policy
Caching
Cache Design
Cache size
small caches have a significant impact on performance
Block size
the unit of data exchanged between cache and main memory
hit means the information was found in the cache
larger block size more hits until probability of using newly fetched data becomes less than the probability of reusing data that has been moved out of cache
Mapping function
Determines which cache location the block will occupy
Replacement algorithm
Determines which block to replace
Least-Recently-Used (LRU) algorithm
Write policy
write a block of cache back to main memory
main memory must be current for direct memory access by I/O modules and multiple processors
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

30. Cache Disk Cache I/O Cache
Caching
Cache
Disk Cache
A portion of main memory used as a buffer to temporarily to hold data for the disk
Disk writes are clustered
Some data written out may be referenced again. The data are retrieved rapidly from the software cache instead of slowly from disk
I/O Cache
Circular buffers
Lists
Streams
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

31. Input/Output Computer Systems Registers Interrupts Caching Protection
Summary
BYU CS 345
OS Overview (Chapter 1)

32. Programmed I/O I/O module performs the action, not the processor
Input / Output
Programmed I/O
I/O module performs the action, not the processor
Sets appropriate bits in the I/O status register
No interrupts occur
Processor is kept busy checking status
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

33. Input / Output
Interrupt-Driven I/O
Processor is interrupted when I/O module ready to exchange data
Processor is free to do other work
No needless waiting
Consumes a lot of processor time because every word read or written passes through the processor
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

34. Input / Output
Direct Memory Access
Transfers a block of data directly to or from memory
An interrupt is sent when the task is complete
The processor is only involved at the beginning and end of the transfer
What does this mean with respect to a paged system?
BYU CS 345
OS Overview (Chapter 1)
Alex Milenkovich

35. Protection Computer Systems Registers Interrupts Caching Input/Output
Summary
BYU CS 345
OS Overview (Chapter 1)

36. Hardware Protection Why protect hardware? From what? How?
Shared hardware resources – memory, disk, …
Errant programs
How?
CPU provides 2 modes of operation
User Mode (non-privileged)
Supervisor mode (privileged)
Privileged instructions can only be executed in monitor mode
All I/O instructions are privileged
BYU CS 345
OS Overview (Chapter 1)

37. Memory Protection OS Job 1 Job 2 Job 4 Job 3 256000 300040 420940
256000
300040
420940
880000
We must not allow a program access to memory outside of its space.
How? Add two registers
Base
Limit
Use the registers to check every reference
300040
120900
Base
Limit
BYU CS 345
OS Overview (Chapter 1)

38. CPU Protection What is there to protect? Timer Configuration
CPU as a resource
Timer
Task Switching uses the timer
When process is loaded timer is set
Timer is decremented each cycle
When time is zero, an interrupt is generated
Process is switched
Load-timer is a privileged instruction
BYU CS 345
OS Overview (Chapter 1)

39. Protection
Protect at all cost?
If all I/O instructions are protected, how do you perform input or output?
System calls
Often the call is a trap to a ISR
Control is passed to the ISR which sets the mode bit to monitor mode
ISR verifies that parameters are correct
ISR executes request, resets mode
Control is returned to user program
BYU CS 345
OS Overview (Chapter 1)

40. Summary…

41. Summary
What is an O.S.?
Not always a clear definition as to what constitutes an O.S. and what is an application
CD-Rom Driver
Scandisk
Internet Explorer
Intermediary between the hardware and the users
Allocate resources (CPU, Memory, disk space, etc.) between programs and users efficiently
Allow the user to conveniently access data and programs
Protect the system from incorrect or malicious programs and users
BYU CS 345
OS Overview (Chapter 1)

42. Hardware Review Elements of a system: Processor
Summary
Hardware Review
Elements of a system:
Processor
Registers (address, data, control)
Instruction cycle (fetch, decode, execute)
Interrupts
Usually includes hardware and special instructions to help the O.S. manage memory, devices, etc.
Memory
Different levels (cache, main memory, disk)
Operating system will generally manage memory (both RAM and disk), and move data back and forth as required
I/O
Usually use Interrupts, DMA
Operating system usually controls use of I/O devices
BYU CS 345
OS Overview (Chapter 1)

43. Registers Used for frequently accessed items
Summary
Registers
Used for frequently accessed items
User-Visible registers – Available to the programmer and compiler
Data Registers
Address Registers (Index, Segment, Stack Pointer)
Condition code/flags
Control and Status registers – Used to control the processor
Program Counter/Instruction Pointer
Memory address/data
Processor Status Word
Debugging registers
Temp registers
Memory Management registers
BYU CS 345
OS Overview (Chapter 1)

44. Interrupts Interrupts I/O techniques
Summary
Interrupts
Interrupts
Allow I/O devices to get the CPUs attention at regular intervals (Program, Timer, I/O, Hardware failure)
Helps the O.S. by reducing the time spent monitoring I/O devices
CPU checks for interrupts after each instruction, starts the handler if needed
May allow nested interrupts
I/O techniques
Programmed I/O
Interrupt-Driven I/O
Direct Memory Access
BYU CS 345
OS Overview (Chapter 1)

45. Interrupts and I/O Handling and Interrupts: Figure 1.10 (pg 23)
Summary
Interrupts and I/O
Handling and Interrupts:
Figure 1.10 (pg 23)
Device sends interrupt request to CPU
CPU finishes current instruction
CPU acknowledges request
CPU saves PC and PSW
CPU loads PC with the address of the first instruction in the interrupt handler (may get help from interrupt request)
Interrupt handler starts, often saves other CPU registers and key values
Interrupt handler responds to the device
Interrupt handler restores CPU registers and key values
CPU restores PC and PSW and resumes previous program
BYU CS 345
OS Overview (Chapter 1)

46. Memory Varying types of memory
Summary
Memory
Varying types of memory
Registers, Cache, RAM, Disk, CD
Vary in speed, size, cost
CPU and O.S. try to keep frequently used data in faster memory
Cache – Use a small high-speed memory to improve the apparent speed of a larger low-speed memory
Keep track of what is currently being used, load into high-speed memory
Replacement Algorithm – What do we get rid of when we run out of memory?
Write Policy – How do we respond to modifications?
BYU CS 345
OS Overview (Chapter 1)

47. BYU CS 345
OS Overview (Chapter 1)

48. Topics to Cover… OS Objectives OS Services Resource Manager Evolution
Achievements
Processes
Memory management
Information protection and security
Scheduling and resource management
System architecture
BYU CS 345
OS Overview (Chapter 1)