Download Computer system structure overview

Computer System Structures
 Objectives
To provide coverage of basic computer system
organization
 How understanding the basic organization will
help us in understanding the potential
vulnerabilities in OS

Lecture 2
1-1
What is a Computer System?
 Computer system divided into four components:
Lecture 2
1-2
Computer System Organization
 Computer-system organization
 One or more CPUs, device controllers connect through
common bus providing access to shared memory
 Concurrent execution of CPUs and devices competing for
memory cycles
Lecture 2
1-3
Computer Startup
 bootstrap program is loaded at power-up
or reboot
Typically stored in ROM or EPROM, generally
known as firmware
 Initializes all aspects of system
 Loads operating system kernel and starts
execution
 OS starts executing the first process “init” and
waits for some events

Lecture 2
1-4
Computer-System Operation
 After computer starts…
 I/O devices and the CPU can execute concurrently
 Each device controller is in charge of a particular
device type
 Each device controller has a local buffer
 CPU moves data from/to main memory to/from local
buffers
 But how would CPU know when the devices are ready?
 The occurrence of an event is signaled by an
“interrupt” from either the hardware or software
Lecture 2
1-5
Common Functions of Interrupts
 Modern Operating systems are interrupt-driven
 Occurrence of event from hardware side

Interrupt (signal) sent to CPU through system bus
 Occurrence of event from software side


Interrupt sent by executing a special operation (system call/monitor call)
“Trap” – mostly software-generated interrupt caused by an error
Lecture 2
1-6
Interrupt Architecture
 What happens when an interrupt is signaled…
 Interrupt architecture must save the address of the
interrupted instruction
 CPU stops its “operation”
 Moves to the interrupt service routine address and executes
 On completion, CPU resumes the earlier operation
 Also remember: Incoming interrupts are disabled while
another interrupt is being processed to prevent a lost
interrupt
 Example: Operating system as different as MS-DOS and
UNIX both follow the same interrupt principles
Lecture 2
1-7
Interrupt Timeline - example
Device controller informs
CPU by triggering an
interrupt
e.g., CPU makes a
read request
I/O device controller starts
the transfer from the
device to local buffer
Lecture 2
1-8
I/O interrupts
 There are two ways that the I/O operations are handled
by OS
Synchronous I/O
 After I/O starts, control returns to user program only
upon I/O completion


Special “Wait” instruction idles the CPU until the next
interrupt
Wait loop (contention for memory access)
• Loop: jmp Loop

At most one I/O request is outstanding at a time, no
simultaneous I/O processing
Lecture 2
1-9
I/O interrupts
Asynchronous I/O
 After I/O starts, control returns to user program without
waiting for I/O completion

System call is then needed to request to the operating system to
allow user to wait for I/O completion
 Asynchronous I/O is more complex
 Need to be able to keep track of many I/O requests
 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
Lecture 2
1-10
Device Status Table
Device: keyboard
Status: idle
Device: laser printer
Status: busy
Request for
laser printer
address: 38546
Device: mouse
Status: idle
Device: disk unit 1
Status: idle
Device: disk unit 2
Status: busy
Request for
disk unit 2
Request for
disk unit 2
file: abc
operation: read
Address: xyz
file: def
operation: write
Address: zyx
Lecture 2
1-11
I/O interrupts
 Adv./disadv. of synchronous I/O and asynchronous I/O?
Lecture 2
1-12
I/O Structure (contd.)
 Asynchronous I/O works well with slow I/O structure



E.g., in a typical slow I/O, one character takes approx. 1 ms. (1000
micro sec.)
Typical interrupt service routine takes 2 micro sec. per character
to input character into a buffer
Leaving 998 micro sec. out of 1000 micro sec. to do other CPU
computations
 With high-speed I/O device, the above assumption not
valid anymore


I/O transfer at close to memory speed
Asynchronous I/O will not be efficient any more…
 To solve this problem, Direct memory Access (DMA)
is used for high speed I/O devices
Lecture 2
1-13
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
 DMA still has “problem” of “stealing” memory cycles from the CPU
Lecture 2
1-14
Storage Structure
 Main memory – only large storage media that the CPU can access
directly
 Computer programs must be in main memory (RAM)
 The disk controller determines the logical interaction between the
device and the computer
 Secondary storage – extension of main memory that provides large
nonvolatile storage capacity
Lecture 2
1-15
Storage Structure – Main Memory

An example of Machine instructions with operators and
operands for Motorola M68HC11
ORG $2000 Reserves space in memory for storing
 Main memory implemented in a
semiconductor technology
called dynamic random access
memory (DRAM)
 Volatile storage device
MEMSTO FCB $00
STRING
FCC 'The final number is: ‘
MEMSTO1 FCB $00
MEMSTO2 FCB $00
STRING1
FCC ' in Hexadecimal.‘
FCB $04
START
 Interaction with main memory
ORG $2030 Reserves memory for the program
LDAA #10 Loads ten in decimal to accumulator A
STAA MEMSTO
ADDA MEMSTO
STAA MEMSTO
is achieved through series of
load and store instructions
Lecture 2
1-16
Secondary Storage
 Secondary storage – extension of main memory that provides large
nonvolatile storage capacity
 Magnetic disks – most popular



rigid metal or glass platters covered with magnetic recording material
Disk surface is logically divided into circular tracks, which are
subdivided into sectors
Set of tracks in one arm position forms a cylinder
Lecture 2
1-17
Magnetic Disks
 When the disk is in use, a drive motor spins it at high speed
(typical, 60 – 200 times per second)
 Disk “time” (operation on the disk) has two parts


Transfer time
Positioning time
• Seek time (move the disk arm to the desired cylinder)
• Rotational latency (time for the desired sector to rotate to the disk head)
 Disk protection and head crash
Lecture 2
1-18
Storage Hierarchy
 There are wide variety of storage systems in a computer
system


Main memory, magnetic disk, magnetic tapes and many more…
Depending on three criteria, the storage systems are organized
in hierarchy
 “The Factors”
1.
2.
3.
Speed
Cost
Volatility
Lecture 2
1-19
Storage-Device Hierarchy
increasing
speed
cost
decreasing
volatility
speed
Lecture 2
cost
volatility
1-20
Caching
 Important principle, performed at many levels in a computer (in
hardware, operating system, software)
 Information in use copied from slower to faster storage
 Faster storage (cache) checked first to determine if
information is there


If it is, information used directly from the cache (fast)
If not, data copied to cache and used there
 Cache smaller than storage being cached


Cache management important design problem
Cache size and replacement policy
Lecture 2
1-21
Performance of Various Levels of Storage
Lecture 2
1-22
Coherency and Consistency Problem
 First major issue in designing a secure and protected OS
 Multitasking environments must be careful to use most
recent value, no matter where it is stored in the storage
hierarchy
 Multiprocessor environment must provide cache coherency in
hardware such that all CPUs have the most recent value in
their cache
 Distributed environment situation even more complex

Several copies of a datum can exist
 Lot of research conducted and various solutions achieved
Lecture 2
1-23
Open-Source Operating Systems
 Operating systems made available in source-code format
rather than just binary closed-source
 Counter to the copy protection and Digital Rights
Management (DRM) movement
 Started by Free Software Foundation (FSF), which has
“copyleft” GNU Public License (GPL)
 Examples include GNU/Linux, BSD UNIX (including core of
Mac OS X), and Sun Solaris
 However, with increase in open-source OS popularity,
increase in vulnerabilities
Lecture 2
1-24
Networking increased the vulnerabilities
even more
 1960s


Advanced Research Project Agency (ARPA) began to examine
feasibility of redundant networked communications
Larry Roberts developed ARPANET from its inception
 1970s and 1980s



ARPANET grew in popularity as did its potential for misuse
No safety procedures for dial-up connections to ARPANET
Nonexistent user identification and authorization to system
 Late 1970s and 1980s


Information security began with Rand Report R-609 (paper that
started the study of computer security)
Scope of computer security grew from physical security to include:
• Safety of data
• Limiting unauthorized access to data
• Involvement of distributed systems from multiple levels of organizations
Lecture 2
1-25
MULTICS
 Early focus of computer security research was a system called
Multiplexed Information and Computing Service (MULTICS)
 Initial planning started in 1964!
 First operating system created with security as its primary goal
 Mainframe, time-sharing OS developed in mid-1960s by General
Electric (GE), Bell Labs, and Massachusetts Institute of
Technology (MIT)
 Several MULTICS key players created UNIX
 Primary purpose of UNIX was text processing!
Lecture 2
26
Protection and Security
 While MULTICS was commercially not a success it was able to
teach us the correct lesson…
 Importance of protection and security
 When a program written by one user may be used by another
user, misuse and unexpected behavior would occur
 Protection – any mechanism for controlling access of processes
or users to resources defined by the OS
 Security – defense of the system against internal and external
attacks

Huge range including: denial-of-service, worms, viruses, identity
theft, theft of service
Lecture 2
1-27
Malware
 Trojan horse
 Hidden part of some otherwise useful software
 E.g., a text-editor program written by a user may include
hidden code to search the file for certain keywords
 Another example may be a key-stroke logger
 Trojan horse often may open a “backdoor” and start
a covert channel

Covert channel is not a virus and thus not detected by
antivirus
Lecture 2
1-28
What is Covert Channel?
 A covert channel is a “parasitic communication channel” that is
neither designed nor intended to transfer information at all
[Lampson 1973]
 A covert channel refers to the mechanism of stealth information
transfer using a legitimate communication channel visible to the
rest of the world
 The main focus is to hide secret, valuable information through the
usage of some other “normal, harmless” information
Lecture 2
1-29
A simple illustration: “Harmless” Communication
Hello
Bob
I
am
Adam
Adam
(Transmitter)
Are
Ed
you(Eavesdropper)
Bob
(Receiver)
There
Listening?
Lecture 2
1-30
Covert Channel
Inter-arrival
time Covert Bit
Sequence
Hello
Adam
(Transmitter)
Bob
1s
0
I
2s
1
am
2s
1
Adam
2s
1
1s
Bob
(Receiver) 1s
0
There
2s
1
Listening?
2s
1
Are
Ed
you(Eavesdropper)
Lecture 2
0
1-31
Other System Threats
 Worm:




A process that uses spawn mechanism
The processes “eat” system resources
self- replicating: propagates to other hosts, users
Do not even have to execute them to get started
 Virus


infection by receiving object (e.g., e-mail attachment),
actively executing
Unlike worms, virus is a fragment of code
Lecture 2
1-32
Denial of Service Attacks
 Denial of service (DoS): attackers make resources
(CPU resources, bandwidth) unavailable to legitimate
traffic by overwhelming resource with bogus traffic
Lecture 2
1-33
Protection and Security
 Systems generally first distinguish among users, to determine
who can do what




User identities (user IDs, security IDs) include name and associated
number, one per user
User ID then associated with all files, processes of that user to
determine access control
Group identifier (group ID) allows set of users to be defined and
controls managed, then also associated with each process, file
Privilege escalation allows user to change to effective ID with more
rights
Lecture 2
1-34