Outline
• Announcements
• Midterm Review
Please turn in your homework #3 now
Announcements
• The midterm will be Oct. 23, 2003
– Please come to class as early as you can
– We will start when most of you have showed up
• If you cannot take the midterm this Thursday
– You need to have a legally valid reason
– You need to let me know as early as you can but
definitely before this Thursday so that we can
make arrangements
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Announcements – cont.
• The midterm will be close-book and closenote
– However, a letter-size sheet is allowed
– You may need a calculator to perform certain
calculations
• Recitation session tomorrow
– It is optional
– I will be answering questions mainly
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Announcements – cont.
• About Lab2
– The best type of semaphores for the lab is to use
sema_t
– The functions are sema_init, sema_wait,
sema_post, and sema_destroy (they are only
available on program)
– Some of you want to postpone the due date to
Nov. 4 instead of Oct. 30 due to midterms
• Let’s have a poll now
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System Overview
• A computer system consists of hardware and
software that are combined to provide a tool
to solve specific problems
– Hardware includes CPU, memory, disks, printers,
screen, keyboard, mouse ...
– Software includes
• System software
– A general environment to create specific applications
• Application software
– A tool to solve a specific problem
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System Overview
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What is an Operating System?
• The operating system is the part of the
system software that manages the use of the
hardware used by other system software and
all application software
– It is the system program that acts between the
hardware and the user programs
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What is an Operating System? - continued
• It provides services to user programs
– Through system calls / message passing
• File system services
• Memory services
• I/O services
• It hides hardware from user programs
– When your program shows a message on the monitor, it
does not need to know the details
– When your program generates a new file, it does not need
to where the free space is on your hard drive
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Differences between OS and System Software
• Major differences between OS and general
system software
– General system software relies on the
abstractions provided by OS
– OS abstracts the hardware directly
– OS provides the fundamental trusted mechanisms
for resource sharing
– A general purpose OS is domain-independent
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Operating System Functions
• Resource manager
– manage hardware and software resources
– Resource abstraction and sharing
• A nicer environment
– implement a virtual machine for processes to run in
• A program in execution is called a process
– a nicer environment than the bare hardware
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Resource Management Functions
• Transform physical resources to logical
resources
– Resource abstraction
• Make the hardware resources easier to use
• Multiplex one physical resource to several
logical resources
– Create multiple, logical copies of resources
• Schedule physical and logical resources
– Decide who gets to use the resources
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Resource Sharing
• Two types of sharing
– Time multiplexed sharing
• time-sharing
• schedule a serially-reusable resource among several
users
– Space multiplexed sharing
• space-sharing
• divide a multiple-use resource up among several users
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Time-multiplexing the Processor
- Called multiprogramming, resulting in concurrency
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Space-multiplexing Memory
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Time-multiplexing I/O Devices
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Multi-programmed Batch Systems
Several jobs are kept in main memory at the same time, and the
CPU is multiplexed among them.
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OS Features for Multi-programming
• I/O routine supplied by the system
• Memory management – the system must
allocate the memory to several jobs
• CPU scheduling – the system must choose
among several jobs ready to run
• Allocation of devices
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Time-sharing Systems
• The CPU is multiplexed among several jobs that are
kept in memory and on disk (the CPU is allocated to
a job only if the job is in memory).
• A job is swapped in and out of memory to the disk.
• On-line communication between the user and the
system is provided; when the operating system
finishes the execution of one command, it seeks the
next “control statement” not from a card reader, but
rather from the user’s keyboard.
• On-line system must be available for users to access
data and code.
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System Call Interface
• System call interface
– Operating system provides a set of operations
called system calls
– A programming interface
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How to Make a System Call
• For a programmer
– A system call is similar to a procedure/function
call in a traditional programming language
– System call interfaces are available in C/C++ as
library routines
– For example, fork to create a new process
• Commonly used system calls
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How to Make a System Call – cont.
• For the system
– through a trap instruction which causes an interrupt
• Hardware saves PC and current status information
• Hardware changes mode to system mode
• Hardware loads PC from system call interrupt vector
location.
• Execute the system call interrupt handler
• return from the handler, restores PC and other saved status
information
• User process continues.
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Trap Instruction
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System Call Flow of Control
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von Neumann Architecture
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Central Processing Unit
• Datapath
– ALU – Arithmetic/Logic Unit
– Registers
• General-purpose registers
• Control registers
– Communication paths between them
• Control
– Controls the data flow and operations of ALU
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Instruction Execution
• Instruction fetch (IF)
MAR  PC; IR  M[MAR]
• Instruction Decode (ID)
A  Rs1; B  Rs2; PC  PC + 4
• Execution (EXE)
– Depends on the instruction
• Memory Access (MEM)
– Depends on the instruction
• Write-back (WB)
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Dual-mode Operation
• Ensure proper operation of a computer system
– Protect the operating system and all other programs and
their data from any malfunctioning program
– Protection is needed for any shared resource
• Dual-mode provides a means of doing that
– System operates in two modes
• User mode
• System mode
– Privileged instructions
• I/O instructions are privileged instructions
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Dual-mode Operation – cont.
• System mode
– all instructions are legal
– all addresses are absolute physical addresses
(base and bound are not used)
• User mode
– instructions that modify control registers are
illegal
– all addresses must be less than bound and have
base added to them
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Privileged Instructions
• Instructions that can only be executed in the
supervisor mode are called supervisor, privileged,
or protected instructions
• I/O instructions are privileged instructions
– A user program in user mode cannot perform its own
I/O
• Instruction to change the mode is a privileged
instruction
• Instruction to set the halt flag is a privileged
instruction
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Device Management Organization
Application
Process
System Interface
File
Manager
Device-Independent
Device-Dependent
Hardware Interface
Command
Status
Data
Device Controller
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Device Controllers
• A hardware component to control the detailed
operations of a device
– Interface between controllers and devices
– Interface between software and the controller
• Through controller’s registers
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Device Controllers – cont.
...
busy
Command
done
Error code
Status
...
busy done
0
0 idle
0
1 finished
1
0 working
1
1 (undefined)
Data 0
Data 1
Logic
Data n-1
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Memory-mapped I/O – cont.
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Device Drivers
• The part of OS that implements device
management is called collectively as device
drivers
– Device drivers can be complex
– However, the drivers for different devices
implement a similar interface so that a higherlevel component has a uniform interface for
different devices
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I/O Strategies
• Direct I/O or DMA for data transferring
• Polling or interrupt-driven
• I/O strategies
– Direct I/O with polling
– DMA I/O with polling
• Not supported in general
– Direct I/O with interrupts
– DMA I/O with interrupts
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Direct-Memory Access
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Direct I/O with Polling
read(device, …);
1
System Interface
Data
read function
5
write function
2
3
4
Hardware Interface
Command
Status
Data
Device Controller
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Interrupts
• An interrupt is an immediate transfer of control caused
by an event in the system to handle real-time events
and running-time errors
–
–
–
–
Interrupt can be either software or hardware
I/O device request (Hardware)
System call (software)
Signal (software)
• Page fault (software)
• Arithmetic overflow
• Memory-protection violation
– Power failure
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Interrupt Handling
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Interrupt-Driven I/O
read(device, …);
1
9
8b
Data
System Interface
Device Status Table
read driver
2
4
7
Device
Handler
write driver
6
3
Interrupt
Handler
8a
5
Hardware Interface
Command
Status
Data
Device Controller
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Interrupt-Driven Versus Polling
• In general, an interrupt-driven system will
have a higher CPU utilization than a pollingbased system
– The CPU time on polling by one process may be
utilized by another process to perform
computation
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I/O-CPU Overlap
Through interrupt-driven I/O
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I/O-bound and Compute-bound Processes
• I/O-bound process
– A process’s overall execution time is dominated
by the time to perform I/O operations
• Compute-bound process
– A process’s time on I/O is small compared to the
amount of time spent using the CPU
• Many processes have I/O-bound and
compute-bound phases
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Buffering
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Optimizing Access to Rotating Devices
• Access time has two major components
– Seek time – the time for the disk arm to move the
heads to the cylinder containing the desired sector
– Rotational latency – the additional time waiting for
the disk to rotate the desired sector to the disk head
• In a multiprogramming system with many
processes, there may be several pending
requests for a disk drive
– The average accessing time can be reduced by
scheduling the pending requests
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Randomly Accessed Devices
•Seek time dominates access time => minimize
seek time across the set
•Tracks 0:299; Head at track 76, requests for 124,
17, 269, 201, 29, 137, and 12
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Optimizing Access to Rotating Devices
-First-Come-First-Served
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Optimizing Access to Rotating Devices – cont.
-Shortest-Seek-First-First
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Optimizing Access to Rotating Devices – cont.
-Scan/Look
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Optimizing Access to Rotating Devices – cont.
- Circular Scan/
Circular Look
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Process Manager
• Process manager consists of
– Basic algorithms and data structures to
implement the process and resource abstractions
– Managing resources used by processes/threads
– Tools to create/destroy/manipulate processes and
threads
– Scheduling
– Process synchronization
– A deadlock strategy
– Protection strategy
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Implementing the Process Abstraction
Pi CPU
Pj CPU
Pi Executable
Memory
Pj Executable
Memory
Pk CPU
…
Pk Executable
Memory
OS Address
Space
Pi Address
Space
CPU
ALU
Pk Address
Space
…
Control
Unit
Pj Address
Space
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OS interface
52
Modern Processes and Threads
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Processes
• A process is a sequential program in
execution
–
–
–
–
The object program to be executed
The data on which the program will execute
Resources required by the program
The threads in the process
• A host process environment
• The status of each thread’s execution
– Thread-specific data, such as a stack, a PC, and set of
register values
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The Address Space
Address
Space
Address
Binding
Executable
Memory
Process
Files
Other objects
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Tracing the Hardware Process
Hardware process progress
Machine is
Powered up
Bootstrap
Process Interrupt
Loader Manager Handler P1
Load the kernel
Initialization
Execute a thread
Schedule
P,2
Pn
…
Service an interrupt
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Context Switching
Old Thread
Descriptor
CPU
New Thread Descriptor
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Process Descriptors
• OS creates/manages process abstraction
• Descriptor is data structure for each process
–
–
–
–
–
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Type & location of resources it holds
List of resources it needs
List of threads
List of child processes
Security keys
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Threads
• A thread shares with its peer threads its:
–
–
–
–
code section
data section
operating-system resources
collectively known as a task
• A traditional or heavyweight process is equal
to a task with one thread
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Threads – cont.
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Thread Abstraction
• The major tasks in managing a thread include
– Create/destroy a thread
– Allocate thread-specific resources
– Manage thread context switching
• The thread descriptor is the data structure
where the OS keeps all information to
manage that thread
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Diagram of Process State
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UNIX State Transition Diagram
Request
Wait by
parent
zombie
Sleeping
Done
Running
Schedule
Request
I/O Request
Start
Allocate
Runnable
I/O Complete
Traced or Stopped
Uninterruptible
Sleep
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Process Scheduler Organization
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Scheduling Criteria
• CPU utilization – keep the CPU as busy as possible
• Throughput – # of processes that complete their
execution per time unit
• Turnaround time – amount of time to execute a
particular process
• Response time – amount of time it takes from when
a request was submitted until the first response is
produced, not output (for time-sharing
environment)
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First-Come-First-Served
• Gantt chart of FCFS Scheduler
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Shortest-Job-Next Scheduling
• Associate with each process the length of its
next CPU burst.
– Use these lengths to schedule the process with
the shortest time.
• SJN is optimal
– gives minimum average waiting time /
turnaround time for a given set of processes.
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Shortest-Job-Next Scheduling – cont.
• Two schemes:
– non-preemptive – once CPU given to the process
it cannot be preempted until completes its CPU
burst.
– Preemptive – if a new process arrives with CPU
burst length less than remaining time of current
executing process, preempt. This scheme is know
as the Shortest-Remaining-Time-Next (SRTN).
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Shortest-Job-Next Scheduling – cont.
• Gannt chart of preemptive SJN
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Round Robin (RR)
• Each process gets a small unit of CPU time
(time quantum), usually 10-100 milliseconds.
After this time has elapsed, the process is
preempted and added to the end of the ready
queue.
• If there are n processes in the ready queue
and the time quantum is q, then each process
gets 1/n of the CPU time in chunks of at most
q time units at once. No process waits more
than (n-1)q time units.
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Round Robin – cont.
• Gannt chart for RR (q=4)
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Multilevel Queue
• Ready queue is partitioned into separate
queues
– foreground (interactive)
– background (batch)
• Each queue has its own scheduling algorithm
– foreground – RR
– background – FCFS
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Multilevel Feedback Queue
• A process can move between the various queues
• Multilevel-feedback-queue scheduler defined by the
following parameters:
–
–
–
–
–
number of queues
scheduling algorithms for each queue
method used to determine when to upgrade a process
method used to determine when to demote a process
method used to determine which queue a process will
enter when that process needs service
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Interacting Processes
• Independent process
– Cannot affect or be affected by the other
processes in the system
– Does not share any data with other processes
• Interacting process
– Can affect or be affected by the other processes
– Shares data with other processes
– We focus on interacting processes through
physically or logically shared memory
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Race Condition
• Race condition
– When several processes access and manipulate
the same data concurrently, there is a race among
the processes
– The outcome of the execution depends on the
particular order in which the access takes place
– This is called a race condition
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Bounded-Buffer
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Bounded-Buffer - cont.
• Suppose we have one producer and one
consumer, the variable counter is 5
– Producer: counter = counter +1
P1: load counter, r1
P2: add r1, #1, r2
P3: store r2, counter
– Consumer: counter = counter - 1
C1: load counter, r1
C2: add r1, #-1, r2
C3: store r2, counter
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Bounded-Buffer - cont.
• A particular execution sequence
– P1: load counter, r1
– P2: add r1, #1, r2
--- Context switch ---– C1: load counter, r1
– C2: add r1, #-1, r2
– C3: store r2, counter
--- Context switch ---– P3: store r2, counter
– What is the value of counter?
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The Critical-Section Problem
• n processes all competing to use some shared
data
• Each process has a code segment, called
critical section, in which the shared data is
accessed.
• Problem – ensure that when one process is
executing in its critical section, no other
process is allowed to execute in its critical
section, called mutual exclusion
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The Critical-Section Problem – cont.
• Structure of process Pi
repeat
entry section
critical section
exit section
reminder section
until false;
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Requirements for Critical-Section Solutions
• Mutual Exclusion.
– If process Pi is executing in its critical section, then no
other processes can be executing in their critical sections.
• Progress
– If no process is executing in its critical section and there
exist some processes that wish to enter their critical
section, then the selection of the processes that will enter
the critical section next cannot be postponed indefinitely.
• Bounded Waiting
– A bound must exist on the number of times that other
processes are allowed to enter their critical sections after
a process has made a request to enter its critical section
and before that request is granted.
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Algorithm 1
• Shared variables:
– var turn: (0..1);
initially turn = 0
– turn - i  Pi can enter its critical section
• Process Pi
repeat
while turn  i do no-op;
critical section
turn := j;
reminder section
until false;
• Satisfies mutual exclusion, but not progress
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Algorithm 2
• Shared variables
– var flag: array [0..1] of boolean;
initially flag [0] = flag [1] = false.
– flag [i] = true  Pi ready to enter its critical section
• Process Pi
repeat
flag[i] := true;
while flag[j] do no-op;
critical section
flag [i] := false;
remainder section
until false;
• Satisfies mutual exclusion, but not progress requirement.
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Proposed Algorithm 3
• Shared variables:
– boolean lock;
initially lock = FALSE
– lock is FALSE  Pi can enter its critical section
• Process P1
do
Process P2
do
while (lock) do no-op;
lock = TRUE;
critical section
lock = FALSE;
reminder section
while(TRUE);
while (lock) do no-op;
lock = TRUE;
critical section
lock = FALSE
reminder section
while(TRUE);
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Solution through Disabling Interrupts – cont.
• Process Pi
repeat
disableInterrupts(); //entry section
critical section
enableInterrupts(); //exit section
reminder section
until false;
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Solution through Disabling Interrupts – cont.
• This solution may affect the behavior of the I/O
system
– Interrupts can be disabled for an arbitrarily long time
• The interrupts can be disabled permanently if the program
contains an infinite loop in its critical section
– User programs are not allowed to enable and disable
interrupts directly
• However, if the operating system can provide system calls,
a user can use the system calls for synchronization
– Called semaphores and related system calls
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Synchronization Hardware
• Test and modify the content of a word atomically
(indivisibly)
– The procedure cannot be interrupted until it has completed the
routine
– Implemented as a test-and-set instruction
boolean Test-and-Set (boolean target)
{ boolean tmp
tmp = target;
target = true;
return tmp;
}
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Semaphores
• Semaphore S – non-negative integer variable
– Binary semaphore – can only be either 0 or 1
– Counting semaphore – can be any positive number
• It can only be changed or tested via two
indivisible (atomic) operations
– P(S) or wait(S)
P(S): while S 0 do no-op;
S := S – 1;
– V(S) or signal(S)
P(S): S := S + 1;
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Example:Mutual Exclusion using Semaphores
• Shared variables
– semaphore mutex;
• Process Pi
repeat
P(mutex);
critical section
P(mutex);
remainder section
until false;
How shall we initialize the semaphore mutex in order
to achieve mutual exclusion?
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Bounded-Buffer using Semaphores – cont.
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The Readers-Writers Problem
• A resource is shared among readers and
writers
– A reader process can share the resource with any
other reader process but not with any writer
process
– A writer process requires exclusive access to the
resource whenever it acquires any access to the
resource
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First and Second Policy
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Readers-writers with active readers – cont.
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Writer Precedence
reader() {
writer() {
while(TRUE) {
while(TRUE) {
<other computing>;
<other computing>;
P(writePending);
P(mutex2);
P(readBlock);
writeCount++;
P(mutex1);
if(writeCount == 1)
readCount++;
P(readBlock);
if(readCount == 1)
V(mutex2);
P(writeBlock);
P(writeBlock);
V(mutex1);
access(resource);
V(readBlock);
V(writeBlock);
V(writePending);
P(mutex2)
access(resource);
writeCount--;
P(mutex1);
if(writeCount == 0)
readCount--;
V(readBlock);
if(readCount == 0)
V(mutex2);
V(writeBlock);
}
V(mutex1);
}
}
}
int readCount = 0, writeCount = 0;
semaphore mutex = 1, mutex2 = 1;
semaphore readBlock = 1, writeBlock = 1, writePending = 1;
fork(reader, 0);
fork(writer, 0);
Issues Using Semaphores
• Deadlock
– two or more processes are waiting indefinitely for
an event that can be caused by only one of the
waiting processes.
• Let S and Q be two semaphores initialized to 1
P0
P(S);
P(Q);

V(S);
V(Q)
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P1
P(Q);
P(S);

V(Q);
V(S);
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Monitors
• High-level synchronization construct that
allows the safe sharing of an abstract data
type among concurrent processes.
class monitor {
variable declarations
semaphore mutex = 1;
public P1 :(…)
{ P(mutex);
........
V(mutex);
};
........
}
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Monitors – cont.
• To allow a process to wait within the monitor, a condition
variable must be declared, as
condition x, y;
• Condition variable can only be used with the operations wait
and signal.
– The operation
x.wait;
means that the process invoking this operation is suspended until
another process invokes
x.signal;
– The x.signal operation resumes exactly one suspended process. If no
process is suspended, then the signal operation has no effect.
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Monitors – cont.
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Inter-Process Communication
• Inter-Process Communication (IPC)
– Allow running processes to communicate with
each other
• IPC – special cases
– Parent and child
• Parent passes arguments to child
• Parent collect returned status from child using wait
– Processes with common ancestors
• pipe
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Communication through Messages – cont.
• IPC facility provides two operations:
– send(message) – message size fixed or variable
– receive(message)
• If P and Q wish to communicate, they need to
– establish a communication link between them
– exchange messages via send/receive
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Simultaneous Semaphores
• The orders of P operations on semaphores are
critical
– Otherwise deadlocks are possible
• Simultaneous semaphores
– Psimultaneous(S1, ...., Sn)
– The process gets all the semaphores or none of
them
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UNIX Signal
• A signal in UNIX is a mechanism by which
the OS can inform a process of the
occurrence of some event
– A signal can be raised by one process using kill
system call, thus causing another process to be
interrupted and to optionally catch the signal
– Signals can also be used among user processes
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