Computer system architecture

Computer-System Architecture
1- single general-purpose processor
–On a single-processor system, there is one
main CPU capable of executing a general-
purpose instruction set, including
instructions from user processes.
–Most systems have special-purpose
processors as well

2- Multiprocessors systems :
growing in use and importance
– Also known as parallel systems, tightly-coupled systems
– The system have two or more processors in close
communication , sharing the computer bus memory and
peripheral devices.
– Advantages include
1. Increased throughput
2. Economy of scale
3. Increased reliability – graceful degradation or fault
tolerance

– Two types
1. Asymmetric Multiprocessing
2. Symmetric Multiprocessing
1- Asymmetric Multiprocessing
• In which each processor is assigned a specific task.
• A master processor controls the system
• Master-slave relation ship
2- Symmetric Multiprocessing
• All processors are peers
• Many processes can run simultaneous … n processes
can run if n
CPU

Symmetric Multiprocessing Architecture

- Multi-computing core
–Multi processor per chip
–Multi chips with single core
–Which better? And why?
–First
–Because on-chip communication is
faster than between- chips

Clustered Systems
3- Clustered Systems
• Like multiprocessor systems, but multiple systems
working together
• A clustered system uses multiple CPUs to complete a
task.
• It is different from parallel system in that clustered
system consists of two or more individual systems
tied together.
• Definition : The clustered computers share storage
and are closely linked via LAN networking.
10

Clustered Systems
– Provides a high-availability service which survives
failures: A layer of cluster software runs on cluster nodes.
Each node can monitor one or more nodes over the LAN.
– The monitored machine can fail in some cases.
– The monitoring machine can take ownership of its storage.
– The monitoring machine can also restart applications that
were running on the failed machine-
– The failed machine can remain down but the users will see a
brief of the service.
11

Clustered Systems
The clustered system can be of the following forms:
• Asymmetric clustering: In this form, one machine is in hot standby
mode and other machine is running the application. The hot standby
machine performs nothing. It only monitors the server. If faiuler It
becomes the active server if the server fails.
• Symmetric clustering In this mode, two or more machines run the
applications. They also monitor each other at the same time. This
mode is more efficient because it uses all available machines. It can be
used only if multiple applications are available to be executed
• high-performance computing (HPC)
• Applications must be written to use parallelization
12

Operating System Structure
Multiprogramming:
• Multiprogramming is a form of parallel processing in
which several programs are run at the same time on a
uniprocessor.
• Since there is only one processor , there can be no
true simultaneous execution of different programs.
Instead, the operating system executes part of one
program, then part of another, and so on.
• To the user it appears that all programs are executing
at the same time 15

Memory Layout for Multiprogrammed System
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• Multiprogramming needed for efficiency
– Single user cannot keep CPU and I/O devices busy
at all times
– Multiprogramming organizes jobs (code and data)
so CPU always has one to execute
– A subset of total jobs in system is kept in memory
– One job selected and run via job scheduling
– When it has to wait (for I/O for example), OS
switches to another job
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• Multiprogramming means: that several
programs in different stages of execution are
coordinated to run on a single I-stream engine
(CPU).
• Multiprocessing, which is the coordination of
the simultaneous execution of several
programs running on multiple I-stream
engines (CPUs).
18

• Timesharing (multitasking):
is logical extension in which CPU
switches jobs so frequently that
users can interact with each job
while it is running, creating
interactive computing
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• Timesharing:
– Response time should be < 1 second
– Each user has at least one program
executing in memory process
– If several jobs ready to run at the same time
 CPU scheduling
– If processes don’t fit in memory, swapping
moves them in and out to run
– Virtual memory allows execution of
processes not completely in memory
20

Computer-System Operation
• Each device controller is in charge of a particular
device type (disk drive, video displays etc).
• I/O devices and the CPU can execute
concurrently.
• Each device controller 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.
• Device controller informs CPU that it has finished
its operation by causing an interrupt.

Interrupt
• Means : cut off
• Hardware interrupt, e.g. services requests of I/O
devices
• Software interrupt, e.g. signals, invalid memory
access, division by zero, system calls, etc –(trap)
• Procedures: generic handler or interrupt vector
(MS-DOS,UNIX)

Operating-System Operations
• Interrupts are an important part of a
computer architecture.
• Each computer design has its own interrupt
mechanism, but several functions are
common.

External Interrupts
• An external interrupt is a temporal suspension
of a process caused by an event external to
that process and performed in such a way that
the process can be resumed.
– I/O
– Timer
– Hardware failure

• Interrupt driven by hardware
• Software error or request creates
exception or trap
–Division by zero, request for operating
system service
• Other process problems include infinite
loop, processes modifying each other or
the operating system

Common Functions of Interrupts
• 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
• An operating system is interrupt driven

Instruction Cycle with Interrupts
• CPU checks for interrupts after each instruction.
• If no interrupts, then fetch next instruction of current
program.
• If an interrupt is pending, then suspend execution of
the current program. The processor sends an
acknowledgement signal to the device that issued the
interrupt so that the device can remove its interrupt
signal.
• Interrupt architecture saves the address of the
interrupted instruction (and values of other registers).

Instruction Cycle with Interrupts
• Interrupt transfers control to the interrupt service
routine (Interrupt Handler), generally through the
interrupt vector, which contains the addresses of all the
service routines.
• Separate segments of code determine what action
should be taken for each type of interrupt

Interrupt Driven I/O
• To start an I/O operation, the CPU loads the
appropriate registers within the device controller.
• The device controller examines the contents of
these registers and determines what action to
take and then performs the action.
• Device controller informs CPU that it has finished
its operation by causing an external interrupt

• Interrupt Driven I/O
• A disk block read example:
– For disk reads, the controller reads the block (one or more
sectors) from the derive serially, bit by bit, until the entire
block is in the controller internal buffer.
– It then performs a checksum to verify that no read errors
have occurred.
– The controller sends an interrupt. When the OS starts
running, it can read the disk block from the controller’s
buffer a byte or a word at a time by executing a loop, with
each iteration reading one byte or word from the
controller device register and storing it in the main
memory.

Interrupt Handler
• A program that determines nature of the
interrupt and performs whatever actions are
needed
• Control is transferred to this program
• Generally part of the operating system

Operating-System Operations:
Interrupt Handling Procedure
• Interrupt Handling
Save interrupt information
OS determine the interrupt type (by polling)
Call the corresponding handlers
Return to the interrupted job by the restoring
important information (e.g., saved return
address program counter)

Direct Memory Access (DMA) 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

Direct Memory Access (DMA) Structure
• Execute the device driver to set up the registers
of the DMA controller.
• DMA moves blocks of data between the
memory and its own buffers.
• Transfer from its buffers to its devices.
• Interrupt the CPU when the job is done

• Direct Memory Access (DMA) Structure
•
• Recall that with interrupt driven I/O, the CPU can request data from
an I/O controller one byte/word at a time which wastes the CPU
time.
• DMA is a scheme which allows block of data transfer from the
device to the memory without the intervention of the CPU.
• After setting up buffers, pointers, and counters for the I/O device by
the CPU, the 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.
• The OS can only use DMA if the hardware has a DMA controller.

• Direct Memory Access, or DMA, is an
absolutely essential part of any modern
computing architecture. DMA allows the CPU
to offload intensive memory access tasks to
other components. This then frees the CPU
from these menial chores and provides more
cycles to more complex tasks for which it is
better suited.

OPERATING-SYSTEM OPERATIONS
DUAL MODE

Dual-Mode Operation
• Dual-mode operation allows OS to protect itself and
other system components
• Execution of operating-system code and user defined
code user mode and kernel

• Dual-mode operation allows OS to protect itself and
other system components
– User mode and kernel mode
– Mode bit provided by hardware
• Provides ability to distinguish when system is
running user code or kernel code
• Some instructions designated as privileged,
only executable in kernel mode
• System call changes mode to kernel, return
from call resets it to user

• At system boot time, the hardware starts in
kernel mode.
• The operating system is then loaded and starts
user applications in user mode.
• Whenever a trap or interrupt occurs, the
hardware switches from user mode to kernel
mode.

System Calls
• System calls :
– Interface between processes & OS
• Definition: is how a program requests a service
from an operating system’s kernel . They
provide the interface between a process and
operating system.
• Is an interface between a user-space
application and a service is provided in the
kernel.
43

System Calls
• How to make system calls?
– Assemble-language instructions or
subroutine/functions calls in high-level language
such as C or Perl?
• Generation of in-line instructions or a call to a special
run-time routine.
• Example: read and copy of a file!
– Library Calls vs System Calls
44

System Calls
• Programming interface to the services
provided by the OS
• Typically written in a high-level language
(C or C++)
45

System Calls
• Three most common APIs are Win32 API for
Windows, POSIX API for POSIX-based systems
(including virtually all versions of UNIX, Linux,
and Mac OS X), and Java API for the Java virtual
machine (JVM)
• Why use APIs rather than system calls?
(Note that the system-call names used
throughout this text are generic)
46

API – System Call – OS Relationship
47

System Call Implementation
• The system call interface invokes intended
system call in OS kernel and returns status of
the system call and any return values
• The caller need know nothing about how the
system call is implemented
– Just needs to obey API and understand what OS will do as
a result call
– Most details of OS interface hidden from programmer by
API
• Managed by run-time support library (set of functions
built into libraries included with compiler)
48

Example of Standard API
• Consider the ReadFile() function in the
• Win32 API—a function for reading from a file
• A description of the parameters passed to ReadFile()
– HANDLE file—the file to be read
– LPVOID buffer—a buffer where the data will be read into and written from
– DWORD bytesToRead—the number of bytes to be read into the buffer
– LPDWORD bytesRead—the number of bytes read during the last read
– LPOVERLAPPED ovl—indicates if overlapped I/O is being used 49

Standard C Library Example
• C program invoking printf() library call, which calls write() system call
50

System Call Parameter Passing
• Three general methods used to pass parameters to the OS
– Simplest: pass the parameters in registers
• In some cases, may be more parameters than registers
– Parameters stored in a block, or table, in memory, and address
of block passed as a parameter in a register
• This approach taken by Linux and Solaris
– Parameters placed, or pushed, onto the stack by the program
and popped off the stack by the operating system
– Block and stack methods do not limit the number or length of
parameters being passed
52

Parameter Passing via Table
53

Types of System Calls
• Process control
• File management
• Device management
• Information maintenance
• Communications
• Protection
54

Examples of Windows and Unix System Calls
55

MS-DOS execution
(a) At system startup (b) running a program
56

FreeBSD Running Multiple Programs
57

System Programs
• Provide a convenient environment for program
development and execution
– Some of them are simply user interfaces to
system calls; others are considerably more
complex
• File management - Create, delete, copy,
rename, print, dump, list, and generally
manipulate files and directories
59

System Programs
• Status information
– Some ask the system for info - date, time, amount of
available memory, disk space, number of users
– Others provide detailed performance, logging, and
debugging information
– Typically, these programs format and print the
output to the terminal or other output devices
– Some systems implement a registry - used to store
and retrieve configuration information
60

System Programs (cont’d)
• File modification
–Text editors to create and modify files
–Special commands to search contents
of files or perform transformations of
the text
• Programming-language support -
Compilers, assemblers, debuggers and
interpreters sometimes provided
61

• Program loading and execution- Absolute
loaders, reloadable loaders, linkage
editors, and overlay-loaders, debugging
systems for higher-level and machine
language
62

• Communications - Provide the
mechanism for creating virtual
connections among processes, users, and
computer systems
–Allow users to send messages to one
another’s screens, browse web pages,
send electronic-mail messages, log in
remotely, transfer files from one
machine to another
63

User Operating System Interface
64

User Operating System Interface - CLI
1- Command Line Interface (CLI)
or command interpreter allows direct command
entry
• Sometimes implemented in kernel, sometimes by
systems program
• Sometimes multiple flavors implemented – shells
65

User Operating System Interface - CLI
• Primarily fetches a command from user and
executes it
–Sometimes commands built-in, sometimes
just names of programs
»If the latter, adding new features doesn’t
require shell modification
66

User Operating System Interface - GUI
2- Graphical User Interface (GUI )
– User-friendly desktop metaphor interface
– Usually mouse, keyboard, and monitor
– Icons represent files, programs, actions, etc
– Various mouse buttons over objects in the interface
cause various actions (provide information, options,
execute function, open directory (known as a folder)
– Invented at Xerox PARC
67

User Operating System Interface - GUI
• Many systems now include both CLI and GUI
interfaces
– Microsoft Windows is GUI with CLI
“command” shell
– Apple Mac OS X as “Aqua” GUI interface with
UNIX kernel underneath and shells available
– Solaris is CLI with optional GUI interfaces
(Java Desktop, KDE)
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Simple Structure
• Called “no structure”
• The operating system is a collection of
procedures, each of which call any of the
other whenever it requires their needs.
71

Simple Structure
• 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
72

Traditional UNIX System Structure
74

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.
• With modularity, layers are selected such that
each uses functions (operations) and services of
only lower-level layers
75

Microkernel System Structure
• Philosophy of microkernel is to have the bare
essentials in the kernel.
• Removing all non-essential components from
kernel and implement them in system-and -
user level. Program thst results a smaller
kernel called microkernal.
77

Microkernel System Structure
• Moves as much from the kernel into
“user” space.
• Communication takes place between user
modules using message passing.
• Benefits:
– Easier to extend a microkernel
– Easier to port the operating system to new
architectures
– More reliable (less code is running in kernel mode)
– More secure
79

Computer system architecture

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