Transcript What is an Operating System?

Chapter 1: Introduction
• What is an operating system?
• Simple Batch Systems
• Multiprogramming Batched Systems
• Time-Sharing Systems
• Personal-Computer Systems
• Parallel Systems
• Distributed Systems
• Real -Time Systems
Operating System Concepts
1.1
What is an Operating System?
• A program that acts as an intermediary between a
user of a computer and the computer hardware.
• Operating system goals:
– Execute user programs and make solving user
problems easier.
– Make the computer system convenient to use.
• Use the computer hardware in an efficient manner.
Operating System Concepts
1.2
Operating System Definitions
• Resource allocator – manages and allocates
resources.
• Control program – controls the execution of user
programs and operations of I/O devices .
• Kernel – the one program running at all times (all
else being application programs).
Operating System Concepts
1.3
Memory Layout for a Simple Batch System
Operating System Concepts
1.4
Multiprogramming Batch Systems
Multiprogramming: several jobs are kept in main memory
at the same time, and the CPU is multiplexed among them
which requires memory management and protection.
Switching
between jobs
Job Pool
CPU
Disk
CPU
Scheduling
Job
Scheduling
Operating System Concepts
1.5
Multiprogramming Batch Systems
• The O.S. picks and begins to execute one job from memory.
Once this job needs an I/O operation the O.S. switches to
another job (CPU or O.S. always busy).
• The number of jobs in memory is less than the number of jobs
in disk (Job Pool).
• If several jobs are ready to be brought into memory and there
is not enough room for all of them, then the system choose
jobs among them (Job Scheduling).
• If several jobs are ready to run at the same time, the system
must choose among them (CPU Scheduling).
• Having several programs in memory at the same time requires
memory management.
• In non-multiprogrammed system, CPU sit idle.
• In multiprogramming system, CPU will never be idle.
Operating System Concepts
1.6
OS Features Needed for Multiprogramming
• I/O routine supplied by the system.
• Memory management – the system must allocate the memory to
several jobs.
• Job scheduling – the system must choose among several jobs
ready to run from Disk.
• CPU scheduling – the system must choose among several jobs
ready to run in memory.
• Allocation of devices.
___________________________________________________
• Two main disadvantages of Multiprogrammed Batched Systems:
– Users cannot interact with their jobs, while executing.
– A programmer cannot modify a program as it executes to
study its behavior.
Operating System Concepts
1.7
Time-Sharing Systems – Interactive Computing
A time-sharing system uses CPU scheduling and
multiprogramming to provide each user with a small portion of
a time-shared computer.
Main Memory
I/O
operations
O.S.
Interactive
CPU
User1
Scheduling
Job1
User2
Disk
Job2
Job3
User3
Multiprogramming
Memory Management
Operating System Concepts
1.8
CPU
Time-Sharing Systems–Interactive Computing
• The CPU is multiplexed among several jobs that are kept in
memory and on disk (the CPU is allocated to a job only if the
job is in memory).
• When a job needs an I/O operation, the CPU switches between
jobs. Therefore, the CPU is always busy.
• A job is swapped in and out of memory to the disk which serves
as a back up for main memory.
• On-line communication between the user and the system is
provided; when the operating system finishes the execution of
one command, it seeks the next “control statement” not from a
card reader, but rather from the user’s keyboard.
• On-line system must be available for users to access data and
code.
Operating System Concepts
1.9
Time-Sharing Systems (Cont.)
• Time-Sharing Systems provide the following:
– On-Line file system, where the files are on a
collection of disks. Therefore, disk
management must be provided.
– A mechanism for concurrent execution, which
requires CPU scheduling schemes.
– Mechanisms for job synchronization and
communication to ensure orderly execution.
– A mechanism to avoid deadlock - a job waiting
for another job forever.
Operating System Concepts
1.10
Personal-Computer Systems
• Personal computers – computer system dedicated to a
single user.
• I/O devices – keyboards, mice, display screens, small
printers.
• PC operating systems were neither multi-user nor
multi-tasking.
• The goal of PC operating systems were to maximize
user convenience and responsiveness instead of
maximizing CPU and I/O utilization.
• Examples: Microsoft Windows and Apple Macintosh
Operating System Concepts
1.11
Parallel Systems
• Multiprocessor systems with more than one CPU in close
communication.
• Tightly coupled system – processors share memory and a
clock; communication usually takes place through the
shared memory.
Processor
Storage
Processor
Shared Memory
I/O
Operating System Concepts
Single O.S.
1.12
I/O
Parallel Systems (Cont.)
• Advantages of parallel system (multiprocessor systems):
– Increased throughput – number of processes that are
completed per time unit.
– Economical (for large jobs) - no need to make copies of
data and distribute it among several processors.
– Faster (for large jobs) – divide the work on all processors.
– Increased reliability (fault – tolerant) – For example, if we
have 10 processors working together on a job and one
processor failed, then the remaining 9 processors must
pick up a share of the work of the failed processor. Thus,
the entire system is still working but slower by 10%.
Therefore, multiprocessor systems are reliable.
Operating System Concepts
1.13
Parallel Systems (Cont.)
• Symmetric multiprocessing (SMP) model
– Each processor runs an identical copy of the operating
system.
– Most modern operating systems support SMP, such as
UNIX for Multimax computer.
Symmetric Multiprocessing Architecture
Operating System Concepts
1.14
Parallel Systems (Cont.)
•
Asymmetric multiprocessing model
– Each processor is assigned a specific task; master
processor schedules and allocates work to slave
processors.
Master
I/O
Slave
Storage
Processor
Processor
Asymmetric Multiprocessing Architecture
Operating System Concepts
1.15
Parallel Systems (Cont.)
• Asymmetric multiprocessing model (Cont.):
- Master performs I/O and computations.
- Only master may execute the O.S.
- Slave can execute only user programs.
- If master fails the system cannot perform I/O.
- If slave fails some computations are lost but
still the system can function.
- More common in extremely large systems.
Operating System Concepts
1.16
Distributed Systems
• Distribute the computation among several physical processors.
• Loosely coupled system – involves connecting 2 or more
independent computer systems via communication link. So,
each processor has its own O.S. and local memory; processors
communicate with one another through various
communications lines (message passing), such as high-speed
buses or telephone lines.
Storage & O.S
Storage & O.S
Communication Link
Processor
Processor
Message Passing
I/O
I/O
Operating System Concepts
1.17
Distributed Systems (Cont.)
•
Advantages of distributed systems:
– Resources Sharing – You can share files and
printers.
– Computation speed up – A job can be partitioned
so that each processor can do a portion
concurrently (load sharing).
– Reliability – If one processor failed the rest still can
function with no problem.
– Communications – Such as electronic mail, ftp, etc.
Operating System Concepts
1.18
Real-Time Systems
• Real – Time Systems are characterized by supplying immediate
response. For example, sensors bring data to the computer.
• Often used as a control device in a dedicated application such as
controlling scientific experiments, medical imaging systems,
industrial control systems, and some display systems.
• Well-defined fixed-time constraints.
• Hard real-time system.
– Secondary storage limited or absent, data stored in shortterm memory, or read-only memory (ROM)
– Conflicts with time-sharing systems, not supported by
general-purpose operating systems.
• Soft real-time system
– Limited utility in industrial control or robotics
– Useful in applications (multimedia, virtual reality) requiring
advanced operating-system features.
Operating System Concepts
1.19
Storage Structure
• Floppy disks consist of one platter and the head sits
directly on the surface. Its inexpensive, less storage 1.4MB
and slower compare it to the hard disk. Also, it is
removable.
• The load instruction moves a word from main memory to
an internal register, within the CPU for execution.
• The store instruction moves the content of a register to
main memory.
• Can we store the programs and data in main memory
permanently? The answer is no for two reasons:
– Main memory is small to store all programs and data.
– Main memory (RAM) is volatile storage device that
loses its contents when power is turned off or
otherwise lost.
• Therefore, to store all programs and data permanently you
use secondary storage such as hard disk and diskette.
Operating System Concepts
1.20
Storage Hierarchy
• Storage systems organized in hierarchy.
– Speed
– Cost
– Volatility
• Caching – copying information into faster
storage system; main memory can be
viewed as a last cache for secondary
storage.
Operating System Concepts
1.21
Storage-Device Hierarchy
Fast and
Expensive
Size
Small
Size
Large
Operating System Concepts
Slow
and
Cheap
1.22
Storage-Device Hierarchy
• Registers, cache, and main memory are volatile.
• All storage after main memory are non-volatile.
• Caching: Check first cache memory if data not there go to
main memory and copy it into cache under the assumption
that there is a high probability that it will be needed again.
• Data must be moved from secondary storage into main
memory before use.
• Data transfer from cache to CPU and registers is usually a
hardware function with no operating system control.
• Data transfer from disk to memory is usually controlled by
the operating system.
• Cache Coherency and Consistency is the state that exists
in a multiprocessor system, when any shared data is held
by 2 or more caches, and no 2 caches hold different values
of such a shared data simultaneously.
Operating System Concepts
1.23
Hardware Protection
• Dual-Mode Operation
• I/O Protection
• Memory Protection
• CPU Protection
Operating System Concepts
1.24
Dual-Mode Operation
• Protection is needed for any shared resource.
• 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 supervisor mode or system
mode) – execution done on behalf of operating
system.
Operating System Concepts
1.25
Dual-Mode Operation (Cont.)
• At system boot time, the hardware starts in monitor
mode. The O.S. is then loaded, and starts user
processes in user mode.
• Whenever, an interrupt occurs, the hardware
switches from user mode to monitor mode.
• Whenever, the O.S. gains control of the computer,
it is in monitor mode.
• If you do not have dual mode then you can wipe or
write over the O.S. Example: MS-DOS for 8088
architecture does not have a dual mode.
• MS-Widows NT and IBM OS/2 take advantage of
dual mode feature and provide greater protection
for the O.S.
Operating System Concepts
1.26
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
1.27
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
1.28
Memory Protection
• We want to protect the O.S. from access by user
programs, and to protect user programs from one
another.
• 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
1.29
A Base And A limit Register Define A Logical Address Space
Limit register = 300040 - 420940
Operating System Concepts
1.30
Protection Hardware
• This protection is accomplished by the CPU hardware
comparing every address generated in user mode with
the registers.
• The base and limit registers can be loaded by only the
O.S.
Operating System Concepts
1.31
Operating-System Structures
• System Components
• Operating System Services
• System Calls
• System Programs
• System Structure
• Virtual Machines
Operating System Concepts
1.32
Common System Components
• Process Management
• Main Memory Management
• File Management
• I/O System Management
• Secondary Storage Management
• Networking
• Protection System
• Command-Interpreter System
Operating System Concepts
1.33
Process Management
• A process is a program in execution.
A process needs
certain resources, including CPU time, memory, files, and
I/O devices, to accomplish its task.
• Processes can create sub-processes to execute
concurrently.
• A program by itself is not a process; a program is a passive
entity, whereas a process is an active entity.
• The execution of a process must progress in a sequential
fashion. The CPU executes one instruction of the process
after another until the process completes.
• Operating System processes: Those execute system code.
• User processes: Those that execute user code.
Operating System Concepts
1.34
Process Management (Cont.)
•
The operating system is responsible for the
following activities in connection with process
management.
– Process creation and deletion.
– Process suspension and resumption.
– Provision of mechanisms for:
1) process synchronization
2) process communication
– Deadlock handling
Operating System Concepts
1.35
Main-Memory Management
• Memory is a large array of words or bytes, each with its
own address. It is a repository (storage) of quickly
accessible data shared by the CPU and I/O devices.
• Main memory is a volatile storage device.
It loses its
contents in the case of system failure.
• The operating system is responsible for the following
activities in connections with memory management:
– Keep track of which parts of memory are currently
being used and by whom.
– Decide which processes to load when memory space
becomes available.
– Allocate and deallocate memory space as needed.
Operating System Concepts
1.36
File Management
• A file is a collection of related information defined by its
creator. Commonly, files represent programs (both source
and object forms) and data.
• A file consists of a sequence of bits, bytes, lines, or records
whose meanings are defined by their creators.
• The operating system is responsible for the following
activities in connections with file management:
– File creation and deletion.
– Directory creation and deletion.
– Support of primitives for manipulating files and
directories.
– Mapping files onto secondary storage.
– File backup on stable (nonvolatile) storage media.
Operating System Concepts
1.37
I/O System Management
• The I/O system consists of:
– A buffer-caching system
– A general device-driver interface
– Drivers for specific hardware devices
• The O.S. hides the peculiarities of
specific hardware devices from the user.
Operating System Concepts
1.38
Secondary-Storage Management
• Since main memory (primary storage) is volatile and too
small to accommodate all data and programs permanently,
the computer system must provide secondary storage to
back up main memory.
• Most modern computer systems use disks as the principle
on-line storage medium, for both programs and data.
• The operating system is responsible for the following
activities in connection with disk management:
– Free space management
– Storage allocation
– Disk scheduling
Operating System Concepts
1.39
Networking (Distributed Systems)
• A distributed system is a collection of processors
that do not share memory or a clock. Each
processor has its own local memory and clock.
• The processors in the system are connected
through a communication network.
• A distributed system provides user access to
various system resources.
• Access to a shared resource allows:
– Computation speed-up
– Increased data availability
– Enhanced reliability
Operating System Concepts
1.40
Protection System
• Protection refers to a mechanism for
controlling access by programs,
processes, or users to both system and
user resources.
• The protection mechanism must:
– distinguish between authorized and
unauthorized usage.
– specify the controls to be imposed.
– provide a means of enforcement.
Operating System Concepts
1.41
Command-Interpreter System
• Command-Interpreter system is a system program,
which is the interface between the user and the
operating system.
• Command-Interpreter system is known as the
shell.
• Some operating systems provide a user-friendly
interface (mouse-based window) such as,
Macintosh and Microsoft Windows.
• Some operating systems provide text interface
(commands are typed on keyboard) such as MSDOS and Unix shells.
Operating System Concepts
1.42
Command-Interpreter System (Cont.)
•
•
•
Many commands are given to the operating system by control
statements which deal with:
– I/O handling
– process creation and management
– secondary-storage management
– main-memory management
– file-system access
– protection
– networking
The program that reads and interprets control statements is called
variously:
– control-card interpreter
– command-line interpreter
– shell (in UNIX)
Its function is to get and execute the next command statement.
Operating System Concepts
1.43
Operating System Services
•
Program execution – system capability to load a program into memory
and to run it.
•
I/O operations – since user programs cannot execute I/O operations
directly, the operating system must provide some means to perform I/O.
•
File-system manipulation – program capability to read, write, create, and
delete files.
•
Communications – exchange of information between processes
executing either on the same computer or on different systems tied
together by a network. Implemented via shared memory or message
passing.
•
Error detection – ensure correct computing by detecting errors in the
CPU (such as power failure) and memory hardware, in I/O devices (such
as connection failure), or in user programs.
Operating System Concepts
1.44
Additional Operating System
Functions
Additional functions exist not for helping the user, but
rather for ensuring efficient system operations.
• Resource allocation – allocating resources, such
as CPU cycles, main memory, file storage, I/O
devices, to multiple users or multiple jobs
running at the same time.
• Accounting – keep track of and record which
users use how much and what kinds of
computer resources for account billing or for
accumulating usage statistics.
• Protection – ensuring that all access to system
resources is controlled.
Operating System Concepts
1.45
System Calls
• System calls provide the interface between a running
program and the operating system.
– Generally available as assembly-language instructions.
– Languages defined to replace assembly language for
systems programming allow system calls to be made
directly (e.g., C. Bliss, PL/360, PERL)
• Three general methods are used to pass parameters
between a running program and the operating system.
– Pass parameters in registers.
– Store the parameters in a table in memory, and the
table address is passed as a parameter in a register.
– Push (store) the parameters onto the stack by the
program, and pop off the stack by operating system.
Operating System Concepts
1.46
Passing of Parameters As A Table
X
Register
X:
Parameter
for call
Load
address x
Use parameters
from table x
System call
13
User Program
Operating System
Operating System Concepts
1.47
Code
for
system
call 13
MS-DOS Execution
Running a Program
At System Start-up
Free Memory
Free Memory
Process
Command
Interpreter
Operating System Concepts
Command
Interpreter
Kernel
Kernel
(a)
(b)
1.48
UNIX Running Multiple Programs
Process D
Free Memory
Process C
Interpreter
Process B
Kernel
Operating System Concepts
1.49
Communication Models
Message Passing
Shared Memory
Process A
Process A
Shared Memory
Process B
Process B
Kernel
Kernel
(a)
Operating System Concepts
(b)
1.50
System Calls Categories
•
System calls can be grouped into 5 categories:
1. Process Control: end, abort, load, execute,
create process, terminate process, allocate and
free memory.
2. File Manipulation: create file, delete file, open
file, close file, read file, and write file.
3. Device Manipulation: request device, release
device, read, write.
4. Information Maintenance: get time or date, set
time or date, get process or file or device.
5. Communications: create or delete
communication connection, send and receive
messages.
Operating System Concepts
1.51
System Programs
•
System programs provide a convenient environment for program
development and execution. They can be divided into several categories:
– File manipulation: create, delete, copy, rename, print files.
– Status information: Some programs ask the system for date and time,
disk space, number of users.
– File modification: Text editors to create and modify the content of files
stored on disk.
– Programming language support: Compilers and assemblers are
provided to the user with the O.S.
– Program loading and execution: After a program is assembled or
compiled, it must be loaded into memory to be executed. The system
may provide loaders, linkage editors and debuggers.
– Communications: Programs provide mechanism for creating virtual
connections among processes, users, and computer systems, such as
sending messages and transferring files.
Operating System Concepts
1.52
System Structure – Simple Approach
• MS-DOS – written to provide the most
functionality in the least space
– Not divided into modules
– Although MS-DOS has some
structure, its interfaces and levels of
functionality are not well separated.
– No dual mode and no hardware
protection (Intel 8088) in MS-DOS.
Operating System Concepts
1.53
MS-DOS Layer Structure
Application Program
Resident System Program
MS-DOS Device Drivers
ROM BIOS Device Drivers
Operating System Concepts
1.54
System Structure – Simple Approach (Cont.)
• UNIX – limited by hardware functionality, the
original UNIX operating system had limited
structuring. The UNIX OS consists of two
separable parts.
– Systems programs
– The kernel
1. Consists of everything below the system-call
interface and above the physical hardware
2. Provides the file system, CPU scheduling,
memory management, and other operatingsystem functions; a large number of
functions for one level.
Operating System Concepts
1.55
UNIX System Structure
Users
Shells and commands
Compilers and interpreters
System Libraries
System – call interface to the kernel
Kernel
Signals terminal
handling character
I/O system
terminal drivers
File system
swapping I/O
system disk and
tape drivers
CPU scheduling
page replacement
demand paging
virtual memory
Kernel interface to the hardware
Hardware
Terminal controllers
terminals
Operating System Concepts
Device controllers
disks and tapes
1.56
Memory controllers
physical memory
System Structure – Layered Approach
•
•
The operating system is divided into a number of layers (levels), each
built on top of lower layers. The bottom layer (layer 0), is the hardware;
the highest (layer N) is the user interface.
Main advantage of layered approach:
– First layer can be debugged without any concern, because it uses
only basic hardware.
– Once the first layer is debugged, its correct functioning while second
layer is worked on and so on.
– If an error occur we know in which layer.
– Each layer is implemented using those operations provided by
lower-level layers.
– A layer does not need to know how the low-level operations are
implemented, it needs to know what these operations do.
– Each layer hides the existence of data structures, operations, and
hardware from higher-level layer.
Operating System Concepts
1.57
An Operating System Layer
It consists of data structures and a set of routines that can be
invoked by higher-level layers.
Layer M
new
operations
Hidden
operations
existing
operations
Operating System Concepts
1.58
Layer M - 1
Layered Structure of the THE OS
• A layered design was first used in THE operating
system. Its six layers are as follows:
--------------------------------------------------------------------------Layer 5: user programs
--------------------------------------------------------------------------Layer 4: buffering for I/O devices
--------------------------------------------------------------------------Layer 3: operator-console device deriver
--------------------------------------------------------------------------Layer 2: memory management (virtual memory)
--------------------------------------------------------------------------Layer 1: CPU scheduling
--------------------------------------------------------------------------Layer 0: hardware
--------------------------------------------------------------------------Operating System Concepts
1.59
Venus Layer Structure
• It consists of 7 layers as follows:
--------------------------------------------------------------------------Layer 6: user programs
--------------------------------------------------------------------------Layer 5: device drivers and schedulers
--------------------------------------------------------------------------Layer 4: virtual memory
--------------------------------------------------------------------------Layer 3: I/O channel
--------------------------------------------------------------------------Layer 2: CPU scheduling
--------------------------------------------------------------------------Layer 1: instruction interpreter
--------------------------------------------------------------------------Layer 0: hardware
--------------------------------------------------------------------------Operating System Concepts
1.60
Venus Layer Structure (Cont.)
• The low layers (layer 4 to layer 0) put into
micro-code.
• Advantage: Additional speed of execution and
clarity between micro-coded layers and
higher layers.
• Order is important in layers.
• Each layer adds overhead to the system call,
and the result is a system call takes longer
than one does on a non-layered system.
Operating System Concepts
1.61
OS/2 Layer Structure
• OS/2 is a descendant of MS-DOS that adds
multitasking and dual mode operation.
• Access to low-level facilities directly by user
is not allowed.
• OS/2 provides more control over the
hardware and more knowledge of which
resources each user program is using than
the MS-DOS.
Operating System Concepts
1.62
OS/2 Layer Structure (Cont.)
application
application
application
Application – programming interface
subsystem
s
subsystem
s
System
kernel
Device
driver
Operating System Concepts
Device
driver
subsystem
s
• memory
management
• task dispatching
• device management
Device
driver
1.63
Device
driver
Virtual Machines (VM)
• A virtual machine takes the layered approach to its logical
conclusion. It treats hardware and the operating system
kernel as though they were all hardware.
• A virtual machine provides an interface identical to the
underlying bare hardware.
• Example of disk systems in virtual machines: suppose you
have 3 disks drives in physical machine and you want 7
disks drives in virtual machine. The solution is to provide
virtual disks, which are identical in all respects except size,
called minidisks in IBM’s VM. The sum of the sizes of all
minidisks must be less than the actual amount of physical
disk space available.
Operating System Concepts
1.64
Virtual Machines (Cont.)
• The resources of the physical computer are
shared to create the virtual machines.
• Implementation:
– Two modes for protection.
– In virtual machine, we must have a
virtual user mode and a virtual monitor
mode.
– Both modes run in a physical user mode.
Operating System Concepts
1.65
System Models
Non-virtual Machine
Virtual Machine
processe
s
processes
processe
s
processes
Programmin
g interface
kernel
kernel
kernel
Virtual machine
hardware
hardware
(a)
(b)
Operating System Concepts
1.66
kernel
Advantages/Disadvantages of Virtual Machines
•
•
•
•
•
The virtual-machine concept provides complete protection of system
resources since each virtual machine is isolated from all other virtual
machines. This isolation, however, permits no direct sharing of
resources.
The virtual machine concept is difficult to implement due to the effort
required to provide an exact duplicate to the underlying machine.
Sharing minidisk: Files can be shared.
Sharing using a network of virtual machines, each of which can send
information over the virtual communication networks. Note that virtual
minidisks and communication networks are modeled after physical disks
and communication networks. The virtual minidisks and communication
networks are implemented in software.
So, two ways for sharing in virtual: minidisks and communication
networks.
Operating System Concepts
1.67
Advantages/Disadvantages of Virtual Machines (Cont.)
• Modifying or changing an O.S. is difficult. Do
it at night; the machine should be stopped,
since the O.S. runs on and controls the entire
machine.
• Virtual machines solves system compatibility
problems: For example, MS-DOS programs
on Intel CPU-based systems. Users like to
use Sun Microsystems and DEC (faster
processors). Solution is to create a virtual
Intel machine on top of the Sun processors.
Operating System Concepts
1.68
System Design Goals
• User goals – operating system should
be convenient to use, easy to learn,
reliable, safe, and fast.
• System goals – operating system
should be easy to design, implement,
and maintain, as well as flexible,
reliable, error-free, and efficient.
Operating System Concepts
1.69
System Implementation
• Traditionally written in assembly language,
operating systems can now be written in
higher-level languages.
• Code written in a high-level language:
– can be written faster.
– is more compact.
– is easier to understand and debug.
• An operating system is far easier to port
(move to some other hardware) if it is
written in a high-level language.
Operating System Concepts
1.70