TDC 311
OS Process Description and Control
OS States
 An operating system (as any program) is always in a current
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state
It moves from one state to the next as it performs operations
It is very important that you don’t move into a state that is a
“black hole”
Likewise, “miracle” states should not exist
Let’s consider a very simple two-state model for an OS (on
the whiteboard)
Reasons for Process Creation
 New batch job
 Interactive log on
 Created by OS to provide a service
 Spawned by an existing process (for purposes of modularity
or to exploit parallelism)
Reasons for Process Termination
 Normal completion
 Time limit exceeded
 Memory unavailable
 Bounds violation
 Protection error
 Arithmetic error
 Time overrun (process has waited too long for an event to
happen)
 I/O failure
Reasons for Process Termination
 Invalid instruction
 Privileged instruction
 Data misuse
 Operator or OS intervention
 Parent termination
 Parent request
Five-state Model
 What if the process has to wait for something to finish (like
I/O)?
 Let’s consider a five-state model (on the whiteboard)
Five-state Model with Suspend
 Let’s add suspend states to our previous five-state model
 Use a suspend state when all processes are blocked
 When moved to Suspend, process is swapped out to
secondary storage, so a new process can be brought in
 Suspend Ready – has been suspended and event has occurred
 Suspend Blocked – blocked and suspended
 Let’s examine this model on the whiteboard
UNIX Process State Transition Diagram
 Let’s see how UNIX models its states (on the whiteboard)
Process Description
 What is a process made of?
 Control structures
 Memory tables – keep track of what memory is assigned to
what process
 I/O tables
 File tables
 Process tables – keeps a list of pointers to process images
Process Description
 A process is also described by its process control block (PCB)
 What goes into a PCB? Process attributes
 Identifier of this process
 Identifier of the process that created this process
 User identifier
 User-visible registers
 Stack pointers
 Process state indicator
 Scheduler priority indicator
 Scheduling-related information
Process Description
 What else goes into a PCB? (some more process attributes)
 Event (that this process is currently waiting for, if any)
 Data structuring (this process may be linked to other processes
in a queue, ring, or some other structure)
 Various flags, signals, and messages that may be associated with
communication between two independent processes
 Process privileges (does this process have access to certain
system utilities and services?)
 Memory management (pointers to segment/page tables)
 Resource ownership and utilization (any opened files?)
Process Control
 Or, what kinds of “things” can a process do?
 But first, a process can be in either user mode or supervisor mode.
This determines what a process can or can’t do.
 Process can “create” or “spawn” another process
 If a process can spawn another process, OS probably does the
following:
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Assigns a unique PID to new process
Entry added to process table
Allocate space for process image
PCM is initialized
Appropriate linkages are set
Miscellaneous data structures created, accounting updated
Process Control
 What else can a process do?
 Process can “terminate” another process
 Process can “switch” execution to another process
 When does a process switch execution? When an interrupt
occurs. Currently executing process is switched out, and an
interrupt handler or supervisor process is switched in.
Process Switching
 What happens when one process switches to another?
 Save the context of the current process, including program counter
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and other registers
Update the PCB of the current process to denote Blocked Ready or
whatever
Move the PCB of this process to the appropriate queue
Select the next process for execution
Update the PCB of the selected process
Update memory management structures
Restore the context of the processor to that which existed at the time
the selected process was last switched out of the Running state by
loading in the previous values of the program counter and other
registers
Context Switching
 Sometimes it is not necessary to perform a process switch
when an interrupt occurs
 What if you just want to change the settings on a flag or two?
 Don’t do a complete process switch – just do a context
switch
 Some OSes don’t differentiate between process switching
and context switching
Threads
 Like lite beer, a thread is a lite process
 Each thread may contain:
 Thread ID
 Program counter
 Stack
 Register set
 Child threads
 State
 How can it be so lite?
Threads
 A thread shares with other threads belonging to the same
process
 For example, a thread may share:
 Code
 Data
 Other OS resources such as open files and signals
 Thus, a group of peer threads share code, address space, and
OS resources
 One process may have multiple threads
 Why a thread? A thread takes far less time to create and
move, and much less space
Threads
 If threads share things, is security a problem?
 No, because the threads do not belong to unrelated
competing processes, but to the same process
 Threads also make inter-communication easier since multiple
threads share the same files and storage
 Thread operate pretty much like processes – they have a
state, they can get blocked, and they can spawn new threads
 There are kernel threads and there are user threads
Common Thread Examples
 Database server on a LAN – as each new query arrives, a
new thread is spawned to service the request (one process,
multiple threads). When one thread gets blocked requesting
disk I/O, another thread can start up almost immediately and
begin its operation
 Web browser – might have one thread display images or text
while another thread retrieves data from the network
 Word processor – one thread for displaying graphics, another
for reading keystrokes from the user, and a third for
performing spelling and grammar checking in the
background
The Process in VAX/VMS
 Let’s consider a more complete example from a real OS
(from VAX/VMS OS Concepts by Miller)
 “Only way a process can be created is by the action of
another process.”
 Always some processes running on a system
 These processes can be in one of the following states:
 SUS – suspended (can be outswapped)
 COM – computable (waiting in ready queue)(can be outswapped)
 HIB – hibernating (can be outswapped)
 LEF – waiting for a local event flag
 CUR – currently running; and there are others
The Process in VAX/VMS
 Each process has a priority assigned to it
 0 – 31
 31 is the highest
 4 is typical interactive user
 Let’s examine what happens when a user logs in
User Login Procedure
Terminal device reports via interrupt that a key has been
pressed
2. Terminal driver attempts to associate the keystroke to an
existing process using the translation table
3. Since no association exists (new LOGIN),
JOB_CONTROL is awakened
4. JOB_CONTROL calls function $CREPRC which creates a
process image which will eventually become the user’s.
For now, the new process is just like JOB_CONTROL and
has the following information:
1.
ACB (AST Control Block)
Process ID
PCB Vector
Forward Link
Backward Link
Housekeeping
ACB Links
Physical PCB Ptr
Quota Limits
Local Event Flags
EF Cluster Ptrs
PID
PHD Ptr
JIB Ptr
Privileges
ACL Links
State
Priority
Name
PCB
Asynchronous System
Traps
PHD (Process Header)
Accounting
Physical PCB
Memory Info
(for the code)
Job Info Block
Access Rights
Event Flags, etc.
JIB
ACL
User LOGIN Procedure continued
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All quotas, privileges, priorities are temporarily the same
as JOB_CONTROL
Image of LOGINOUT is placed into its PHD Memory Info
$CREPRC marks new process as computable outswapped
(COMO) and informs VMS to insert it into the appropriate
queue
$CREPRC returns to JOB_CONTROL
JOB_CONTROL hibernates
Eventually the new process gets a chance to execute, which
runs LOGINOUT procedure
User LOGIN Procedure continued
LOGINOUT prompts for Username and Password using
$QIOW routine
10. LOGINOUT takes Username and Password and searches
file SYSUAF.DAT for valid account
9.
User LOGIN Procedure continued
11. If account if valid, LOGINOUT extracts following info
from user account info:
Process name (usually user name)
b) Default disk drive and directory info
c) User privileges
d) User quotas
e) Run-time restrictions
f) Process priority
g) Default CLI (command line interpreter, or shell)
and updates the PCB accordingly
a)
User LOGIN Procedure continued
12. LOGINOUT initializes the CLI, which is usually DCL
program (the standard VAX shell)
13. DCL eliminates LOGINOUT
14. DCL executes LOGIN.COM file
15. DCL puts $ prompt on screen via $QIOW