1. Process Description and Control

2. Operating System Control Structures
Information the OS needs to keep around:
Memory tables
I/O tables
File tables
Process tables
Tables are constructed for each entity the operating system manages
The OS has to manage and protect these tables

3. Process Control Structures
Process Image
Collection of program, data, stack, and attributes

4. Process Control Block Process identification
Processor State Information
User-Visible Registers
Control and Status Registers
Stack Pointers
Process Control Information
Scheduling and State Information
Process state
Priority
Scheduling-related information
Event
Data Structuring
Interprocess Communication
Process Privileges
Memory Management
Resource Ownership and Utilization

5. Process Control Block (continued…)
Process identification
Numeric identifiers that may be stored with the process control block and include
Identifier of this process
Identifier of creator process (parent process)
User identifier
Processor State Information
User-Visible Registers
Control and Status Registers
Stack Pointers

6. Process Control Block (continued…)
Processor State Information
User-Visible Registers
A register is one that may be referenced by means of the machine language that the processor executes. (Typically, 8 to 32)
Control and Status Registers
•Program counter: Address of the next instruction to be fetched
•Condition codes: Results of the most recent arithmetic or logical operation (e.g., sign, zero, carry, equal, overflow)
•Status information: Interrupt enabled/disabled flags, execution mode
Stack Pointers
Each process has one or more last-in-first-out (LIFO) system stacks associated with it. A stack is used to store parameters and calling addresses for procedure and system calls. The stack pointer points to the top of the stack.

7. Process Control Block (continued…)
Process Control Information
Scheduling and State Information
This is information that is needed by the operating system to perform its scheduling function. Typical items of information:
•Process state: defines the readiness of the process to be scheduled for execution (e.g., running, ready, waiting, halted).
•Priority: One or more fields may be used to describe the scheduling priority of the process. In some systems, several values are required (e.g., default, current, highest-allowable)
•Scheduling-related information: This will depend on the scheduling algorithm used. Examples are the amount of time that the process has been waiting and the amount of time that the process executed the last time it was running.
•Event: Identity of event the process is awaiting before it can be resumed

8. Process Control Block (continued…)
Process Control Information
Data Structuring
A process may be linked to other process in a queue, ring, or some other structure. For example, all processes in a waiting state for a particular priority level may be linked in a queue. A process may exhibit a parent-child (creator-created) relationship with another process. The process control block may contain pointers to other processes to support these structures.

9. Process Control Block (continued…)
Process Control Information
Interprocess Communication
Various flags, signals, and messages may be associated with communication between two independent processes. Some or all of this information may be maintained in the process control block.
Process Privileges
Processes are granted privileges in terms of the memory that may be accessed and the types of instructions that may be executed. In addition, privileges may apply to the use of system utilities and services.

10. Process Control Block (continued…)
Process Control Information
Memory Management
This section may include pointers to segment and/or page tables that describe the virtual memory assigned to this process.
Resource Ownership and Utilization
Resources controlled by the process may be indicated, such as opened files. A history of utilization of the processor or other resources may also be included; this information may be needed by the scheduler.

11. User Processes in Virtual Memory

12. When a program ‘forks()’
We already know how new processes get created in the UNIX/Linux environment (i.e., via the ‘fork()’ function or system-call)
A child-process gets constructed using the same code and data as its parent-process
Yet each task’s memory-regions are kept ‘private’ (i.e., accessible to it alone)
So how exactly is this feat achieved?

13. Similar memory-mappings
virtual memory
virtual memory
physical memory
kernel
space
kernel
space
stack
stack
user space
stack
user space
data
data
stack
text
data
data
text
text
parent-process
child-process

14. Processor State Information
Contents of processor registers
User-visible registers
Control and status registers
Stack pointers
Program status word (PSW)
contains status information
Examples:
EFLAGS register on Pentium machines
MSR on Power PC’s

15. Pentium II EFLAGS Register

16. Motorola Power PC MSR POW = Power management enable SE =
Single-step trace enable
TGPR =
Temporary GPR remapping
BE =
Branch trace enable
ILE =
Exception little-endian mode
FE1 =
Floating-point exception mode 1
EE =
External interrupt enable
CE =
Critical interrupt exception enable
PR =
Privilege level
IP =
Exception prefix
FP =
Floating-point available
IR =
Instruction address translation
ME =
Machine check enable
DR =
Data address translation
FE0 =
Floating-point exception mode 0
RI =
Recoverable exception
LE =
Little-endian mode enable

17. Modes of Execution User mode System mode, control mode, or kernel mode
Less-privileged mode
User programs typically execute in this mode
System mode, control mode, or kernel mode
More-privileged mode
Kernel of the operating system

18. Process Creation Assign a unique process identifier
Allocate space for the process
Initialize process control block
Set up appropriate linkages
Ex: add new process to linked list used for scheduling queue
Create or expand other data structures
Ex: maintain an accounting file

19. When to Switch a Process
Clock interrupt
process has executed for the maximum allowable time slice
I/O interrupt
Memory fault
memory address is in virtual memory so it must be brought into main memory
Trap
error occurred
may cause process to be moved to Exit state
Supervisor call
such as file open

20. Context Switch
Saving the state of one process and loading another process onto the CPU
Pure Overhead
length of time varies from machine to machine
somewhere between 1 and 1000 microseconds
hardware support makes it faster
Time to perform a context switch can determine how long a process is active.

21. CPU Switch From Process to Process

22. Change of Process State
Save context of processor including program counter and other registers
Update the process control block of the process that is currently running
Move process control block to appropriate queue - ready, blocked
Select another process for execution
Update the process control block of the process selected
Update memory-management data structures
Restore context of the selected process

23. Execution of the Operating System
Non-process Kernel
execute kernel outside of any process
operating system code is executed as a separate entity that operates in privileged mode
Execution Within User Processes
operating system software within context of a user process
process executes in privileged mode when executing operating system code
Process-Based Operating System
major kernel functions are separate processes
Useful in multi-processor or multi-computer environment

24. Where does the OS execute?P1 P2 Pn
OS func-
tions …
Process switching functions
Kernel P1 P2 Pn … P1 P2 Pn …
OS1
OSk
Process switching functions

25. UNIX SVR4 Process Management
Most of the operating system executes within the environment of a user process P1 OS
func-
tions P2 OS
func-
tions P3 OS
func-
tions Pn OS
func-
tions
Process Switching Functions

26. UNIX Process States

27. Process Scheduling Queues
Job queue – set of all processes in the system.
Ready queue – set of all processes residing in main memory, ready and waiting to execute.
Device queues – set of processes waiting for an I/O device.
Process migration between the various queues.

28. Ready Queue And Various I/O Device Queues

29. Representation of Process Scheduling

30. Schedulers
Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue.
Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU.

31. Addition of Medium Term Scheduling

32. Schedulers (Cont.)
Short-term scheduler is invoked very frequently (milliseconds)  (must be fast).
Long-term scheduler is invoked very infrequently (seconds, minutes)  (may be slow).
The long-term scheduler controls the degree of multiprogramming.
Processes can be described as either:
I/O-bound process – spends more time doing I/O than computations, many short CPU bursts.
CPU-bound process – spends more time doing computations; few very long CPU bursts.