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Process Control
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Process Control Block (PCB)
Process State Information
Process Control Information
Functions of an Operating-System Kernel
Switch a Process
Change of Process State
Execution of the Operating System
Operating System Control
Structures
z Schedules and dispatches processed for
execution by the processor
z Allocates resources to processes
(Think about struct rlimt and getrlimit)
z Responds to requests by user programs
z (Think about getopt)
z Tables are constructed for each entity the
operating system manages
Memory Tables
z Allocation of main memory to processes
z Allocation of secondary memory to
processes
z Protection attributes for access to shared
memory regions
z Information needed to manage virtual
memory
I/O Tables
z I/O device is available or assigned
z Status of I/O operation
z Location in main memory being used as
the source or destination of the I/O
transfer
File Tables
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Existence of files
Location on secondary memory
Current Status
Attributes
Sometimes this information is maintained
by a file-management system
(Think about struct stat and stat( ) )
Process Table
z Process image consists of program, data,
stack, and attributes
z Attributes
y process control block
( Think about Process ID and getpid( ) )
(Think about Resource Limits and
getrlimit( ) )
Process State Transition Diagram
with Two Suspend States - SevenState Process Model
New
Admit
Admit
Suspend
Dispatch
Activate
Ready,
suspend
Ready
Suspend
Running
Time out
Event
Occurs
Event
Occurs
Activate
Blocked,
suspend
Blocked
Suspend
Event
Wait
Exit
UNIX Process State
Transition Diagram (1)
fork
Created
Preempted
return
to user
not enough memory
(swapping system only)
enough
memory
User
Running
preempt
return
system call,
interrupt
interrupt,
interrupt return
swap out
reschedule
process
Ready to Run
in Memory
Ready to Run
Swapped
swap in
Kernel
Running
sleep
wakeup
wakeup
exit
Zombie
Asleep in
Memory
swap out
Sleep,
Swapped
UNIX Process State Transition
Diagram (2)
z User running: Executing in user mode.
z Kernel running: Executing in kernel model.
z Ready to run, in memory: Ready to run as
soon as the kernel schedules it.
UNIX Process State
Transition Diagram (3)
z Asleep in memory: unable to execute until
an event occurs; process in main memory.
z Ready to run, swapped: process is ready
to run, but the the swapper must swap
the process into main memory before the
kernel can schedule it to execute.
z Sleeping, swapped: The process is
awaiting an event and has been swapped
to secondary storage.
UNIX Process State
Transition Diagram (4)
z Preempted: process is returning from
kernel to user mode, but the kernel
preempts it and does a process switch to
schedule another process.
z Created: process is newly created and not
yet ready to run.
z Zombie: process no longer exists, but it
leaves a record for its parent process to
collect.
Process Control Block
Process Identification
z Process Control Block
The collection of attributes is refereed to
as process control block.
z Unique numeric identifier
y may be an index into the primary process
table
UNIX Process Control
Table
z Process Identifiers
ID of this process and ID of parent
process.
z User Identifiers
real user ID, effective user ID
z Pointers
To U area and process memory (text,
data, stack)
z Process Size, Priority, Signal, Timers, ......
Processor State
Information
z Contents of processor registers
y User-visible registers (data/address register).
y Control and status registers (program
counter, instruction register.
y Stack pointers ( points to the top of the
stack).
z Program status word (PSW)
y contains status information - indicate the
mode of execution (user or kernel mode)
??????
Question 1
Process Control - Exercise/Home Work
z The VAX/VMS operating system makes use of
four processor access modes as follows:
z Kernel: Executes the kernel of the VMS operating
system, which includes memory management,
interrupt handing, and I/O operations.
z Executive: Executes many of the operating
system service calls, including file and disk
management routines.
z Supervisor: Executes other operating system
services, such as responses to user command.
z User: Execute user programs, plus utilities such
as compilers, editors, linkers, and debuggers.
Question 1
Process Control - Exercise/Home Work
z a) A number of operating systems have two
modes (UNIX), kernel and user. What are the
advantages and disadvantages of providing four
mode instead of two?
z b) Can you make a case for even more than four
modes?
Typical Functions of an
Operating-System Kernel (1)
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Memory Management
Allocation of address space to processes
Page and segment management
I/O Management
Allocation of I/O channels and devices to
processes
Support Functions
Interrupt handling and accounting
Typical Functions of an
Operating-System Kernel (2)
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Process Management
Process creation and termination
Process scheduling and dispatching
Process switching
Process synchronization and support for
inter-process communication
Receive Message from Child
Processes
* wait( ) System Call
* Status Information of wait( )
* Example: communication between
parent process and child processes
(jg73.cc and jg74.cc)
Process Control
Information
z Additional information needed by the
operating system to control and
coordinate the various active processes
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scheduling and state information
data structuring
interprocess communication
process privileges
resource ownership and utilization ??????
Question 2: Multiple Blocked Queues
Process Control - Exercise/Home Work
Ready Queue
Release
Dispatch
Admit
Processor
Time-out
Event 1 Wait
Event 1
Occurs
Event 1 Queue
Event 2 Wait
Event 2
Occurs
Event 2 Queue
Question 2
Process Control - Exercise/Home Work
z Figure “Multiple Blocked Queues”
suggests that a process can only be in
one Event queue at a time.
z a) Is it possible that you would to allow a
process to wait on more than one event at
the same time? Provide an example.
z b) In that case, how you modify the
queuing structure of the figure to support
this new feature?
When to Switch a Process
z Trap
y error occurred
y may cause process to be moved to Exit state
z Supervisor call
y such as file open
When to Switch a Process
z Memory fault
y memory address is in virtual memory so it
must be brought into main memory
z Interrupts ??????
y Clock
x process has executed for the maximum allowable
time slice
y I/O
Question 3
Process Control - Exercise/Home Work
z In a number of early computers, an
interrupt cased the register values to be
stored in fixed locations associated with
the given interrupt signal.
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Under what circumstances is this a
practical technique?
Explain why it is inconvenient in general.
Execution of the Operating
System
z Nonprocess Kernel
y execute kernel outside of any process
y operating system code is executed as a
separate entity that operates in privileged
mode
P1
P2
.....
Kernel
Pn
Execution of the Operating
System
z Execution Within User Processes
y operating system software within context of a
user process
y process executes in privileged mode when
executing operating system code
P1
P2
......
Pn
OS
OS
OS
Functions
Functions
Functions
Process Switching Functions
Execution of the Operating
System
z Process-Based Operating System
y major kernel functions are separate processes
y a process is invoked by the operating system
P1
P2 ...... Pn
OS1 OS2 ..... OSn
Process Switching Functions
Question 4
Process Control - Exercise/Home Work
UNIX is unsuitable for real-time
applications, because a process
executing in kernel mode may not be
preempted . Elaborate.
P1
P2
...…
Pn
OS
OS
OS
Functions
Functions
Functions
Process Switching Functions
Question 5
Process Control - Exercise/Home Work
z The UNIX kernel will dynamically grow a
process’s stack in virtual memory as
needed, but it will never try to shrink it.
z Explain why it would be possible to shrink
the stack, and
z Why the UNIX kernel does not shrink it.