Systems Architecture,
Sixth Edition
Chapter 11
Operating Systems
Chapter Objectives
•  In this chapter, you will learn to:
–  Describe the functions and layers of an operating
system
–  List the resources allocated by the operating
system and describe the allocation process
–  Explain how an operating system manages
processes and threads
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Chapter Objectives (continued)
–  Compare CPU scheduling methods
–  Explain how an operating system manages
memory
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FIGURE 11.1 Topics covered in this chapter
Courtesy of Course Technology/Cengage Learning
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Operating System Overview
•  Important part of all information systems
•  Manages all hardware resources and allocates
them to users and applications as needed
•  Accesses files and directories, creates and
moves windows, and accesses resources over a
network
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Operating System Functions
•  Functions are divided into five main groups
•  Resource allocation: ensure that overall system
objectives are achieved efficiently and effectively
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FIGURE 11.2 OS management functions
Courtesy of Course Technology/Cengage Learning
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Operating System Layers
•  Using layers makes the OS easier to maintain
•  Command layer (shell)
•  Command language or job control language
(JCL)
•  Service layer
•  Service call
•  Kernel
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User’s interface to OS
Contains set of
functions executed by
application programs
and command layer
Manages resources;
interacts directly with
computer hardware
FIGURE 11.3 OS layers
Courtesy of Course Technology/Cengage Learning
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FIGURE 11.4 Components of the kernel
Courtesy of Course Technology/Cengage Learning
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Resource Allocation
•  Single-tasking and multitasking
•  Resource allocation tasks
•  Real and virtual resources
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Single-Tasking Resource Allocation
•  Involves only two running programs
–  Application
–  OS (grants the application all unused hardware
resources)
•  Single-tasking operating systems are small and
efficient
•  There’s only one active program
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Multitasking Resource Allocation
•  Norm for modern general-purpose computers
•  Allows flexibility of application and system
software
•  Resource allocation goals
–  Meet resource needs of each program
–  Prevent programs from interfering with one
another
–  Efficiently use hardware and other resources
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Resource Allocation Tasks
•  Keep detailed records of available resources;
know which resources can satisfy which
requests
•  Schedule resources based on specific allocation
policies
•  Update records to reflect resource commitment
and release by programs and users
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Real and Virtual Resources
•  Real resources
–  Physical devices and associated system software
•  Virtual resources
–  Resources that are apparent to a process or user
–  Meet or exceed real resources by:
•  Rapidly shifting resources unused by one program
to other programs that need them
•  Substituting one type of resource for another
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Process Management
•  Process
–  Unit of executing software managed
independently by OS
–  Can request and receive hardware resources and
OS services
–  Can be stand-alone or part of a group that
cooperates to achieve a common purpose
–  Can communicate with other processes executing
on the same computer or on other computers
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Process Control Data Structures
•  Process control block (PCB)
–  Created, updated, and deleted by OS
–  Used by OS to perform many functions (e.g.,
resource allocation, secure resource access,
protecting active processes from interference with
other active processes)
–  Normally organized into a larger data structure
(called a linked list, process queue, or process
list)
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Process Control Data Structures
(continued)
•  Processes can spawn other processes and
communicate with them
–  Parent process
–  Child process
–  Sibling process
–  Process family
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Threads
•  Portion of a process that can be scheduled and
executed independently
•  Can execute concurrently on a single processor
or simultaneously on multiple processors
•  Share all resources allocated to parent process
•  Advantage: reduce OS overhead for resource
allocation and process management
•  Thread control block (TCB) and run queues
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CPU Allocation
•  OS makes rapid decisions about which threads
receive CPU control and for how long that
control is retained
•  Threads usually share CPUs (concurrent or
interleaved execution)
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FIGURE 11.7 Concurrent (interleaved) thread execution on a single CPU
Courtesy of Course Technology/Cengage Learning
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Thread States
•  Ready
–  Waiting for access to the CPU
•  Running
–  Retains control of CPU until the thread or its
parent process terminates normally or an interrupt
occurs
•  Blocked
–  Waiting for some event to occur (completion of
service request or correction of an error condition)
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FIGURE 11.8 Thread movement between states
Courtesy of Course Technology/Cengage Learning
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Interrupt Processing
•  Thread can be blocked waiting for resources
•  Thread is put into a wait state and its state
stored on the stack
•  Some interrupt handler will process the blocked
thread’s request
•  When the resource has been allocated, thread is
moved from block state to ready or running state
•  The thread remains in the blocked state until the
request is satisfied or a time out occurs
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Scheduling
•  Decision-making process used by OS to
determine which ready thread moves to the
running state
•  Typical methods
–  Preemptive scheduling
–  Priority-based scheduling
–  Real-time scheduling
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Preemptive Scheduling
•  A thread can be removed involuntarily from the
running state
•  Functions of the supervisor (portion of OS that
receives control)
–  Calls appropriate interrupt handler
–  Transfers control to the scheduler
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Preemptive Scheduling (continued)
•  Functions of the scheduler
–  Updates status of any process or thread affected
by last interrupt
–  Decides which thread to dispatch to the CPU
–  Updates thread control information and the stack
to reflect the scheduling decision
–  Dispatches selected thread
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Preemptive Scheduling (continued)
•  Processing steps on
left occur after
Thread 1 makes an I/
O service call
•  Processing steps on
right occur after I/O
device completes I/O
operation
FIGURE 11.9 Interrupt processing
Courtesy of Course Technology/Cengage Learning
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Timer Interrupts
•  Generated at regular intervals by CPU to give
scheduler an opportunity to suspend currently
executing thread
•  Not a “real” interrupt
•  No interrupt handler to call
•  Supervisor passes control to the scheduler
•  Important CPU hardware feature for multitasking
OSs
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Priority-Based Scheduling
•  Determines which ready thread should be
dispatched to the CPU according to:
–  First come first served (FCFS)
–  Explicit priority
–  Shortest time remaining (STR)
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Real-Time Scheduling
•  Guarantees minimum amount of CPU time to a
thread if the thread makes an explicit real-time
scheduling request when it is created
•  Guarantees a thread enough resources to
complete its function within a specified time
•  Often used in transaction processing, data
acquisition, and automated process control
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Memory Allocation
•  The assignment of specific memory addresses
to system software, application programs, and
data
•  OS allocates memory
–  When threads are created; responds to requests
for additional memory during a thread’s lifetime
–  To itself and for other needs (buffers and caches)
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Physical Memory Organization
•  Main memory can be regarded as a sequence of
contiguous, or adjacent, memory cells
•  Most significant byte
•  Least significant byte
•  Addressable memory
•  Physical memory
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Single-Tasking Memory Allocation
•  Bulk of OS normally occupies lower memory
addresses
–  Application program is loaded immediately above
it
•  Contiguous memory allocation
•  Address resolution
–  Process of determining physical memory address
that corresponds to memory reference
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FIGURE 11.13 Contiguous memory allocation in a single-tasking OS
Courtesy of Course Technology/Cengage Learning
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Multitasking Memory Allocation
•  The operating system:
–  Finds free memory regions in which to load new
processes and threads
–  Reclaims memory when processes or threads
terminate
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Multitasking Memory Allocation
(continued)
•  Goals of multitasking memory allocation
–  Allow as many active processes as possible
–  Respond quickly to changing memory demands
of processes
–  Prevent unauthorized changes to a process’s
memory region(s)
–  Perform memory allocation and addressing as
efficiently as possible
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FIGURE 11.14 Dividing main memory into fixed-size partitions
Courtesy of Course Technology/Cengage Learning
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FIGURE 11.15 Several processes and the OS loaded into 250 MB
fixed-size memory partitions
Courtesy of Course Technology/Cengage Learning
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TABLE 11.2 Fixed-size memory partitions maintained by the OS
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Memory Fragmentation
•  Occurs when memory partitions allocated to a
single process or purpose are scattered
throughout physical memory
•  To address the problem
–  Compaction (large overhead)
–  Noncontiguous memory allocation
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FIGURE 11.16 Changes in memory allocation as processes execute
and terminate
Courtesy of Course Technology/Cengage Learning
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FIGURE 11.17 Compaction combines multiple free space fragments (a) into
a single contiguous partition (b)
Courtesy of Course Technology/Cengage Learning
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Noncontiguous Memory Allocation
•  Portions of a process can be allocated to free
partitions anywhere in memory
•  Uses small fixed-sized partitions
•  More flexible than contiguous memory allocation,
but requires more complex partition tables and
address calculations
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FIGURE 11.18 Process 6 is allocated to available free
memory partitions
Courtesy of Course Technology/Cengage Learning
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Virtual Memory Management
•  Allocates portions of processes (pages) to small
memory partitions (page frames)
•  Swaps pages between memory and secondary
storage as needed
•  Page hits, page faults, page tables
•  Page files and victims
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TABLE 11.3 Portion of a process’s page table
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Memory Protection
•  Refers to protecting memory allocated to one
program from unauthorized access by another
program
•  Prevents errors in one program from generating
errors in another
•  Adds overhead to each write operation
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Memory Management Hardware
•  Complex memory management procedures incur
substantial overhead
•  Modern CPUs incorporate advanced memory
allocation and address resolution functions in
hardware (e.g., Intel Pentium)
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Summary
•
•
•
•
•
Operating system overview
Resource allocation
Process management
CPU allocation
Memory allocation
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