Chapter 2
Computer-System Structures
 Computer System Operation
 I/O Structure
 Storage Structure
 Storage Hierarchy
 Hardware Protection
 Network Structure
Operating System Concepts
2.1
Thanks to Silberschatz, Galvin and Gagne 2002
Computer-System Architecture
Operating System Concepts
2.2
Thanks to Silberschatz, Galvin and Gagne 2002
Computer-System Operation
 I/O devices and the CPU can execute concurrently.
 Each device controller
 is in charge of a particular device type.
 has a local buffer.
 CPU moves data from/to main memory to/from
local buffers
 I/O is from the device to local buffer of controller.
 Modern operating systems are interrupt driven.
 Device controller informs CPU that it has finished
its operation by causing an interrupt.
Operating System Concepts
2.3
Thanks to Silberschatz, Galvin and Gagne 2002
Common Functions of Interrupts
 Interrupt transfers control to the interrupt service
routine through the interrupt vector
 Interrupt vector contains the addresses of all the
service routines.
 Interrupt architecture must save the address of the
interrupted instruction.
 Incoming interrupts are disabled while another
interrupt is being processed to prevent a lost
interrupt.
 A trap is a software-generated interrupt caused
either by an error or a user request.
Operating System Concepts
2.4
Thanks to Silberschatz, Galvin and Gagne 2002
Interrupt Handling
 The operating system preserves the state of the
CPU by storing registers and the program counter.
 Also determines which type of interrupt has
occurred:
 polling
 vectored interrupt system
 Separate segments of code determine what action
should be taken for each type of interrupt
Operating System Concepts
2.5
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Interrupt Time Line For a Single Process
Doing Output
Operating System Concepts
2.6
Thanks to Silberschatz, Galvin and Gagne 2002
Chapter 2
Computer-System Structures
 Computer System Operation
 I/O Structure
 I/O Interrupts
 DMA Structure
 Storage Structure
 Storage Hierarchy
 Hardware Protection
 Network Structure
Operating System Concepts
2.7
Thanks to Silberschatz, Galvin and Gagne 2002
I/O Structure
 Two I/O methods, synchronous and asynchronous
 Synchronous I/O: After I/O starts, control returns
to user program only upon I/O completion.
 Wait instruction idles the CPU until the next
interrupt
 Wait loop (contention for memory access).
 At most one I/O request is outstanding at a time, no
simultaneous I/O processing.
Operating System Concepts
2.8
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I/O Structure
 Asynchronous I/O: After I/O starts, control
returns to user program without waiting for I/O
completion.
 System call – request to the operating system to
allow user to wait for I/O completion.
 Device-status table contains entry for each I/O
device indicating its type, address, and state.
 Operating system indexes into I/O device table to
determine device status and to modify table entry to
include interrupt.
Operating System Concepts
2.9
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Two I/O Methods
Synchronous
Operating System Concepts
Asynchronous
2.10
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Device-Status Table
Operating System Concepts
2.11
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Direct Memory Access Structure
 Used for high-speed I/O devices able to transmit
information at close to memory speeds.
 Device controller transfers blocks of data from
buffer storage directly to main memory without
CPU intervention.
 Only one interrupt is generated per block, rather
than the one interrupt per byte.
Operating System Concepts
2.12
Thanks to Silberschatz, Galvin and Gagne 2002
Chapter 2
Computer-System Structures
 Computer System Operation
 I/O Structure
 Storage Structure
 Main Memory
 Magnetic Disks
 Magnetic Tapes
 Storage Hierarchy
 Hardware Protection
 Network Structure
Operating System Concepts
2.13
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Storage Structure
 Main memory
 the only large storage media that the CPU can
access directly.
 To allow more convenient access to I/O devices,
memory mapped I/O is provided.
 Ranges of memory addresses are mapped to device
registers.
 Ideally we want the programs and data to reside
in main memory permanently, but it’s impossible.
 Main memory is usually too small.
 Main memory is a volatile storage device
 Typical solution: Secondary Storage
Operating System Concepts
2.14
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Storage Structure
 Secondary storage
 extension of main memory that provides large
nonvolatile storage capacity.
 Magnetic disks
 Rigid metal or glass platters covered with magnetic
recording material
 Disk surface is logically divided into tracks, which
are subdivided into sectors.
 The disk controller determines the logical interaction
between the device and the computer.
Operating System Concepts
2.15
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Storage Structure
 Magnetic Tapes
 Early secondary storage medium
 Relatively permanent and can hold large quantities
of data
 Disadvantages
 Slow access time
 Very slow random access
 Used mainly for backup, storage of infrequently
used data, and as a data transfer medium.
Operating System Concepts
2.16
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Moving-Head Disk Mechanism
Operating System Concepts
2.17
Thanks to Silberschatz, Galvin and Gagne 2002
Chapter 2
Computer-System Structures
 Computer System Operation
 I/O Structure
 Storage Structure
 Storage Hierarchy
 Caching
 Coherency and Consistency
 Hardware Protection
 Network Structure
Operating System Concepts
2.18
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Storage Hierarchy
 Storage systems organized in hierarchy.
 Speed
 Cost
 Volatility
 Going up in the pyramid, increases speed, cost
and volatility.
Operating System Concepts
2.19
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Storage-Device Hierarchy
Operating System Concepts
2.20
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Caching
 Caching
 Copying information into faster storage system
 Main memory can be viewed as a last cache for
secondary storage.
 Use of high-speed memory to hold recently-
accessed data.
 Requires a cache management policy.
 Caching introduces another level in storage
hierarchy. This requires data that is
simultaneously stored in more than one level to
be consistent.
Operating System Concepts
2.21
Thanks to Silberschatz, Galvin and Gagne 2002
Coherency and Consistency
 Regarding the hierarchical storage structure, the
same data may appear in different levels
 Requires mechanisms to provide Coherency and
Consistency
 Coherency: In a multiprocessor environment,
every variable value update in one cache must
immediately be reflected in all other caches where
the variable resides.
 Consistency: In a distributed environment, when a
replica is updated all other replicas must be
updated as soon as possible.
Operating System Concepts
2.22
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Migration of A From Disk to
Register
Operating System Concepts
2.23
Thanks to Silberschatz, Galvin and Gagne 2002
Chapter 2
Computer-System Structures
 Computer System Operation
 I/O Structure
 Storage Structure
 Storage Hierarchy
 Hardware Protection
 Dual-Mode Operation
 I/O Protection
 Memory Protection
 CPU Protection
 Network Structure
Operating System Concepts
2.24
Thanks to Silberschatz, Galvin and Gagne 2002
Dual-Mode Operation
 Sharing system resources requires operating
system to ensure that an incorrect program
cannot cause other programs to execute
incorrectly.
 Provide hardware support to differentiate between
at least two modes of operations.
1. User mode
Execution done on behalf of a user.
2. Monitor mode (also kernel mode or system mode)
Execution done on behalf of operating system.
Operating System Concepts
2.25
Thanks to Silberschatz, Galvin and Gagne 2002
Dual-Mode Operation (Cont.)
 Mode bit added to computer hardware to indicate
the current mode: monitor (0) or user (1).
 When an interrupt or fault occurs, hardware
switches to monitor mode.
Interrupt/fault
monitor
user
set user mode
Privileged instructions can be issued only in monitor mode.
Operating System Concepts
2.26
Thanks to Silberschatz, Galvin and Gagne 2002
I/O Protection
 All I/O instructions are privileged instructions.
 Must ensure that a user program could never gain
control of the computer in monitor mode (i.e., a
user program that, as part of its execution, stores
a new address in the interrupt vector).
Operating System Concepts
2.27
Thanks to Silberschatz, Galvin and Gagne 2002
Use of A System Call to Perform
I/O
Operating System Concepts
2.28
Thanks to Silberschatz, Galvin and Gagne 2002
Memory Protection
 Must provide memory protection at least for the
interrupt vector and the interrupt service routines.
 In order to have memory protection, add two
registers that determine the range of legal
addresses a program may access:
 Base register
 Holds the smallest legal physical memory
address.
 Limit register
 Contains the size of the range
 Memory outside the defined range is protected.
Operating System Concepts
2.29
Thanks to Silberschatz, Galvin and Gagne 2002
Use of A Base and Limit
Register
Operating System Concepts
2.30
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Hardware Address Protection
Operating System Concepts
2.31
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Memory Protection
 When executing in monitor mode, the operating
system has unrestricted access to both monitor
and user’s memory.
 The load instructions for the base and limit
registers are privileged instructions.
Operating System Concepts
2.32
Thanks to Silberschatz, Galvin and Gagne 2002
CPU Protection
 Protecting CPU from getting stuck by a user
program
 User program may fall in an infinite loop and never
return control to the OS.
 Timer – interrupts computer after specified period
to ensure operating system maintains control.
 Timer is decremented every clock tick.
 When timer reaches the value 0, an interrupt
occurs.
 Timer commonly used to implement time sharing.
 Timer also used to compute the current time.
 Load-timer is a privileged instruction.
Operating System Concepts
2.33
Thanks to Silberschatz, Galvin and Gagne 2002
Chapter 2
Computer-System Structures
 Computer System Operation
 I/O Structure
 Storage Structure
 Storage Hierarchy
 Hardware Protection
 Network Structure
 Local Area Networks (LAN)
 Wide Area Networks (WAN)
Operating System Concepts
2.34
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Local Area Network Structure
Operating System Concepts
2.35
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Wide Area Network Structure
Operating System Concepts
2.36
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Exercises
 Answer these exercises
 1, 3, 5, 6, 7, 8, 11
 Send your answers in two weeks to the teacher
assistant.
 Late Policy
 You will lose half the score in the case of less
than 3 days latency.
 You will lose all the score after that.
Operating System Concepts
2.37
Thanks to Silberschatz, Galvin and Gagne 2002