computer system structure

Computer-System Structures
• Computer System Operation
• I/O Structure
• Storage Structure
• Storage Hierarchy
• Hardware Protection
• General System Architecture

Computer-System Operation
• A modern computer system consists of a CPU, memory,
system bus and a number of device controllers
• I/O devices and the CPU can execute concurrently.
• Each device controller is in charge of a particular device
type.
• A device controller for each device which contains local
buffer storage and special purpose registers
• A bootstrap program is required to initialize the
computer system
• 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 Handling
• The occurrence of an event is usually signaled by an interrupt from
either the hardware or the software
• System call or monitor call is executed to trigger an interrupt
• When the CPU is interrupted, it stops what it is doing and immediately
transfers execution to a fix location to execute the interrupt service
routine
• On completion of execution of service routine, the CPU resumes the
interrupted computation
• Interrupts are an important part of a modern computer system and
they must be handled quickly
• 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.
• An operating system is interrupt driven.
• In modern systems, priority interrupts have been introduced.

Interrupt Handling
 Each computer design has its own interrupt mechanism
 The interrupt must transfer control to the appropriate interrupt
service routine through a table of pointers which is stored in
LMA
 LMA locations hold the addresses of the interrupt service
routines (interrupt vector) for the various devices
 The operating system preserves the state of the CPU by
storing registers and the program counter.
 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
 The interrupt architecture must also save the address of the
interrupted instruction
 System call is the method used by a process to request action
by the operating system

Interrupt time line for a
single process doing output

I/O Structure
• The computer system has a number of device controllers connected
through a common bus
• A device controller contains local buffer storage and a set of special
purpose registers
• The device driver is responsible for moving the data between the
peripheral devices and it controls its local buffer storage
• I/O interrupts are used by the device controllers for transfer of data
• I/O methods:
 Synchronous
 Asynchronous
• In synchronous method, after I/O starts, control returns to user
program only upon I/O completion.
 Waiting for I/O may be accomplished by either wait instruction or wait
loop
 Wait instruction idles the CPU until the next interrupt
 Wait loop continuous until an interrupt occurs
 At most one I/O request is outstanding at a time, no simultaneous I/O
processing.

I/O Structure
• In asynchronous method, after I/O starts, control returns to
user program without waiting for I/O completion. It requires:-
 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.
 OS will also maintain a wait queue for each I/O device.
 An I/O device interrupts when it needs service, OS determines I/O
device and updates its table entry
 An interrupt signals completion of an I/O request, control then
returns from I/O interrupt to another request or user program
 Interrupt schemes vary from system to system
 This method increases system efficiency

Two I/O Methods
Synchronous Asynchronous

Direct Memory Access Structure
• Involvement of CPU in data transfer is a time-
consuming process. If the CPU needs two
microseconds to respond to each interrupt and
interrupts arrive every four microseconds then less
time is left for process execution.
• DMA is 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 Structure
• DMA controller has its own registers for source and
destination addresses
• A device driver sets the DMA controller registers to
use the appropriate source and destination
addresses, transfer length and it is then instructed to
starts I/O operation
• While the DMA controller is performing the data
transfer, the CPU is free to perform other tasks
• As the DMA controller steals memory cycles from the
CPU so it slows down CPU execution during DMA
operation
• DMA interrupts the CPU when the transfer has been
completed

Check
status
Done?
Issue Read
command to
I/O Module
Read Status
of I/O
Module
Not
ready
Read word
from I/O
Module
Write Word
Into Memory
No
Error
Condition
CPU --> I/O
I/O --> CPU
I/O --> CPU
CPU --> Memory
yes
Next Instruction
(a) Programmed I/O
Check
status
Done?
Issue Read
command to
I/O Module
Read Status
of I/O
Module
ready
Read word
from I/O
Module
Write Word
Into Memory
No
Error
Condition
CPU --> I/O
I/O --> CPU
I/O --> CPU
CPU --> Memory
yes
Next Instruction
(b) Interrupt-driven I/O
Issue Read
Block Command
to I/O Module
Read Status
of DMA
Module
Next Instruction
(c) direct memory access
CPU--> DMA
Do something
else
Interpret
DMA --> CPU
FIGURE : Three techniques for input of a block of data
Do something
else
Interpret

Storage Structure
• Registers
• Cache
• Main Memory
• Electronic Disk
• Magnetic Disk
• Optical Disk
• Hard Disk
• Magnetic Tape

Registers
• Registers are available in the CPU and are
accessible within one cycle of the CPU clock.
• Faster operations are carried out on contents of
CPU registers due to faster accesses.
• Processor does not stall while performing
operations on registers.
• Size of the registers is very small.
• Registers are volatile.

Cache
• CPU needs to stall as RAM is slower than CPU speed for
providing data required to complete the instruction.
• Cache is a faster memory between the CPU and main
memory and is a remedial measure to reduce idling time of
CPU.
• Cache is a memory buffer which stores information required
by the CPU using register-allocation and register-
replacement algorithms.
• Instruction cache holds the next instruction expected to be
executed whereas data cache keeps required data for the
instruction t. They are known as hardware caches.
• Cache have limited size so cache management is a problem
for designers.
• Careful selection of the cache size and of a replacement
algorithm can provide 80 – 99% of all accesses within the
cache – maximizing system performance.
• Caches are volatile.

Main Memory
• Main memory can be viewed as a fast cache for secondary storage.
• Programs must be loaded in the RAM for execution and main memory
is a large storage media that the CPU can access directly.
• Main memory is implemented in a semiconductor technology (DRAM).
• Load and store instructions specify memory addresses for
interaction.
• A typical instruction is executed using fetch-decode-execute cycle.
• All programs and data can not be stored in a RAM because of its
small size and volatility.
• Special I/O instructions allow data transfers between the device
controller registers and main memory.
• In memory-mapped I/O, ranges of memory addresses are set aside
and are mapped to the device registers for providing more convenient
access to I/O devices.
• In programmed I/O, CPU uses polling to watch the bit in the control
register to see whether the device is ready for transfer of data
between device and main memory.
• In an interrupt-driven I/O, CPU receives an interrupt when the device
is ready for the data transfer.

Magnetic Disks
• Magnetic disks provide a large space for storing programs and
data on permanent basis.
• Disks are relatively simple and consist of:
• Platters covered with the magnetic material
• Read-write head
• Disk arm
• Each surface of platter has tracks, sectors and cylinders
• Disk speed depends upon Transfer rate and positioning time (seek
time & rotational latency)
• Head crash damages the magnetic surface and the entire disk is
replaced for safety of data and programs.
• The storage size of a HD is in GBs.
• FDD rotates slowly than HDD which reduces wear on the disk
surface. Its storage capacity is very small compare to HD or CD.
• Buses attached to a disk drive are EIDE, ATA and SCSI.
• Data transfer through a bus is carried out between the host
controller and disk controller.
• Magnetic disks are non-volatile.

Magnetic Tapes
• Magnetic tapes are used to backup the data and
programs in order to protect any loss due to HD failure.
• Magnetic tapes can hold large quantities of data /
programs.
• Access time is too slow compared to HD, CD, Main
Memory etc.
• Magnetic tapes are non-volatile.
• Storage / handling of magnetic tapes requires special
care.
• Recording / reading of information is very slow due to
winding / rewinding of tapes.
• Random access is not available on tapes.

Storage Hierarchy
• Storage systems can be organized in a hierarchy
according to:
 Speed
 Cost
 Capacity
 Volatility
• Registers, cache and memory are constructed using
semiconductor memory and are volatile.
• Electronic disks can be volatile or nonvolatile.
• All secondary storage devices (magnetic disk, optical
disk, floppy disk, magnetic tapes, magnetic drums
etc) are nonvolatile.

Coherency and Consistency
• The same data may appear in different levels of the storage
system. For example, value of variable (X) of file G may reside
on magnetic disk, main memory, cache or CPU registers.
• In a multitasking environment, each process wishing to use the
value of variable (X) must obtain the most recently updated
value.
• A copy of variable (X) may exist simultaneously in several
caches having different value in a multiprocessor environment.
For cache coherency, system hardware must make sure that an
update to value of X in one cache is immediately reflected in all
other caches where X resides for concurrent execution of file G.
• For cache consistency in a distributed environment, the various
replicas of the file G may be accessed and updated concurrently
so system must ensure that when a replica is updated in one
computer, all other replicas are brought up-to-date quickly
through client or server initiated approach.

Migration of X From Disk to Register
X X X

Hardware Protection
 Dual-Mode Operation
 I/O Protection
 Memory Protection
 CPU Protection

DUAL MODE OPERATION
• Sharing of system resources improved CPU utilization
but increased problems. Many jobs could be affected by
a bug in one program.
• A good OS must ensure that a faulty program cannot
cause other programs to execute incorrectly.
• If a user program fails, the hardware will trap to OS, the
OS dumps the memory of the program for debugging and
terminates it.
• The hardware-supported dual-mode operation protects
the OS, all other programs and their data from any
malfunctioning program.
 User-mode of operation (mode-bit is 1).
 Monitor/supervisor/system mode of operation (mode-bit is 0).
• Whenever an interrupt or trap occurs, the hardware
switches from user-mode to monitor-mode. OS is in the
monitor-mode.

Dual-Mode Operation
• The dual-mode of operation provides us with
the means for protecting the OS from errant
users and errant users from one another.
• The hardware allows privileged instructions
(e.g. system call) to be executed only in
monitor mode.
• When an interrupt or fault occurs hardware
switches to monitor mode.
monitor user
Interrupt/fault
set user mode

I/O Protection
 All I/O instructions are defined as privileged
instructions so users cannot issue I/O instructions
from user mode.
 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).
 To do I/O, a user programme executes a system call to
request that the OS perform I/O on its behalf and
returns the control to the users after completion of I/O
operation.

Use of a System Call to Perform I/O

Memory Protection
 Must provide memory protection for the interrupt
vector, the interrupt service routines and user
programs from one another.
 In order to have memory protection, two registers are
used to 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.
 A trap is generated if any user’s program attempts to
access unauthorized memory area.
 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.

Use of a Base and Limit Register

CPU Protection
 A user program may:
be stuck in an infinite loop
fail to call system services
fail to 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 and
control is automatically transferred to the OS.
 Timer is also commonly used to implement time
sharing mechanism.
 Time can be used to compute the current time.
 Load-timer is a privileged instruction.

Network Structure
 Local Area Networks (LAN)
LANs were introduced in 1970 for economical use of a
number of small computers and sharing of computer
resources.
LANs cover a small geographical area and are
generally used in an office environment.
Communication links of LANs have a higher speed
and lower error rate.
High-quality cables (TP, Fiber Optic etc) are used for
establishment of LANs.
Common topologies are bus, ring and star.
Communications speed range from Mbps to Gbps.
A typical LAN may consist of PCs/Laptops/PDAs,
shared peripheral devices and one or more gateways.

Network Structure
 Wide Area Networks (WAN)
 WANs emerged in the late 1960s to provide efficient communication among sites.
 WANs are physically distributed over a large geographical area.
 Hardware and software resources are shared conveniently and economically by a wide
community of users.
 ARPANet grew from four sites to millions of sites using internet.
 The communication links (telephone lines, leased lines, microwave links, satellite
channels etc) are relatively slow and less reliable.
 Communication processors control the communication links for transferring information
among the various sites.
 The internet WAN provide the ability for hosts at geographically separated sites to
communicate with one another.
 The host computers differ from each other in type, speed, word length, operating system
etc.
 Connections between networks use a telephone-system service to provide
communication:
 T1 service provides a transfer rate of 1.544 Mbps

Wide Area Network Structure
The router control the path each message takes through the net. Dynamic routing
enhances communication efficiency whereas static routing reduces security risks.
Modems convert digital data to analog signals and vice versa for communication.
WANs are slower than LANs (1200 bps to 1 Mbps) and uses PPP for connecting
computers to the internet.

computer system structure

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computer system structure