Chapter 6
System Integration and
Performance
Chapter goals
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Describe the implementation of the system
bus and bus protocol.
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Describe how the CPU and bus interact with
peripheral devices.
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Describe the purpose and function of device
controllers.
Chapter goals cont.
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Describe how interrupts coordinate actions of
the CPU with secondary storage and I/O
devices
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Describe how buffers, caches, and data
compression improve computer system
performance
Role of the system bus
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Bus is mechanism that allows computer
components to work together
Is made up of parallel communication
lines connecting computer components
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CPU, hard drive, parallel port, modem, etc.
Can connect two or more devices
Information can travel in both directions
System bus (cont.)
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Can connect both internal (hard drive) and
external (printer) devices
System bus has three parts
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Data bus – carries data
Address bus – used if RAM is involved
Control bus – commands and status information
Each bus line carries 1 bit of information
System bus
Bus Clock
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Like the CPU, the bus has a clock that
acts as a timing device
For CPU, each tick is trigger to execute
an instruction
For system bus, each tick is an
opportunity to transmit data or a control
message
Bus clock cont.
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Bus clock is MUCH slower than CPU
clock
Think of CPU as the highway and the
system bus as the local streets
Why slower?
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Data on bus must travel a longer physical
distance than data in CPU
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Even though data is traveling at the speed of light
it still needs more time to travel over a greater
distance
Need to allows time to factor out noise,
interference
Also allows time to operate controller logic in
peripheral devices
Bus data transfer rate
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Called the bus data capacity
Expresses how much data can travel
across bus over time
Is a combination of
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Bus clock speed
Data transfer unit (usually a word)
Is used to calculate things like
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Time required to load large files (i.e. video)
Bus protocol
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Data transportation rules that ensure the
smooth transfer of information without error
Dictates the format, content, and timing of
data, memory addresses, and messages
Every peripheral device (no matter the
manufacturer) must follow the bus protocol
rules
Bus protocol cont.
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Protocol can impact (reduce) data
transfer rates
Protocols often require exchanges of
control signals
Control signals consume bus cycles that
could otherwise send data
Sample protocol
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Example: if a disk drive transfers data
to RAM as the result of an explicit CPU
instruction, the following steps are
followed:
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CPU sends command to the drive
Drive send acknowledgement to CPU
Drive carries out transfer
Drive sends confirmation to CPU that
transfer is complete
Why use protocols?
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Protocol regulates bus access
Stops devices from interfering with each
other
I/O data transfer is the largest cause of
errors in computers
I/O commands need to be
acknowledged and confirmed
What if two devices need the
bus?
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When two (or more) peripheral devices
need access to the bus at the same
time that is called a collision
Three solutions are in place to deal with
this
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Master-slave
Multiple master
Peer to peer
Master-slave
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CPU is bus master
Traditional computer architecture
No device can access the bus unless in
response to explicit command from CPU
Allows a very simple protocol
No collision is possible as long as CPU
waits for response from device before
proceeding to the next bus request
Master-slave cont.
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Overall system performance is severely
degraded
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If devices can only communicate through
the CPU, then transfers between devices,
i.e. memory to disk, must pass through the
CPU
Every transfer takes at least 2 bus cycles
CPU cannot execute software while it is
managing the bus
A better solution
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System performance is improved if storage
and I/O devices can transmit data among
themselves without explicit CPU involvement
Direct Memory Access (DMA) controller is
attached to the bus and main memory
DMA assumes the role of bus master for all
transfers between memory and other storage
or I/O devices
CPU is free to do whatever
Multiple master bus
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Any device can assume control of the bus, or
act as bus master for transfer to any other
device (not just memory)
Still only a single device can be master at one
time
Bus arbitration unit is a simple processor
attached to a multiple master bus
It decides which devices must wait when
multiple devices want to become a bus
master
Logical vs. Physical Access
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I/O port is a communication pathway
from the CPU to a peripheral device
I/O port is often implemented as a
memory address that can be accessed
(read or written to) by
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The CPU
Or a single peripheral device
Logical and Physical Access
I/O Ports
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Each peripheral device may have several I/O
ports and use them for different purposes
Dedicated bus hardware controls data
movement between I/O ports and peripheral
devices
CPU reads and writes to I/O ports using
ordinary data movement instructions or
dedicated I/O instructions
The CPU and I/O Ports
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I/O port is more than a memory
address, it is a data conduit
It is a logical abstraction used by the
CPU and the bus to interact with each
peripheral device in a similar way
Logical access
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CPU and the bus both interact with each
peripheral device as if it was a storage device
containing one or more bytes of contiguous
memory
CPU and the bus deals with each device the
same way, but devices are different
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Storage capacity
Internal data coding methods
If storage or I/O device
Linear address space
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A read/write operation to/from this
hypothetical device is called a logical
access
The set of sequentially numbered
storage locations is called a linear
address space
How logical becomes physical
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Logical access assumes device is similar
to memory (RAM)
Bus address lines carry the position
within the linear address space being
read or written
Device controller makes the conversion
via a conversion table or a simple
algorithm
Conversion table for disk
Device controllers
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Storage devices have intermediaries
that connect them to the system bus
Translate logical access to physical
access
Handles bus protocol (receiving and
acknowledging commands)
Permits several devices to share a bus
connection
Device controllers
Device controllers cont.
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Device controllers monitor the bus
control lines for signals to peripheral
devices
Translates those signals into appropriate
commands for its device
Interrupts
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Secondary storage and I/O device
transfer rates are much slower than the
CPU
Why?
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Slower bus clock
Peripheral devices have mechanical
elements (access arm, spin mechanism)
that are slower than speed of electricity
Interrupts cont.
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When the CPU issues a read/write
instruction it ALWAYS has to wait
This waiting time can translate into
thousands, millions, or even billions of
CPU cycles
To allow CPU to be used more
efficiently, interrupts are used
How interrupts work
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When a program (task, process, thread)
needs I/O, CPU makes I/O request over
the system bus
Then puts your task aside (asleep)
Does something else for the time being
Interrupts cont.
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When I/O is complete, interrupt signal
is sent to the CPU
CPU can now restart your task with I/O
task being complete
CPU and Interrupts
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Portion of the CPU (separate from the
fetch execute cycle) continuously
monitors the bus for interrupt signals
The signal is an interrupt code that
indicates the bus port number of the
device sending the interrupt
CPU copies any interrupt signals it
encounters into an interrupt register
The CPU and Interrupts cont.
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As an extra step in the fetch execute
cycle, the CPU checks the interrupt
register after completing an instruction
but before fetching another one
If interrupt register has a non-zero
value CPU must respond to the
interrupt
CPU and Interrupts
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If CPU is to process an interrupt it does
the following:
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Puts aside (suspends) current task
Resets interrupt register to 0 (zero)
Processes interrupt by calling interrupt handler
After interrupt processing is complete, resumes
suspended program
Interrupt handlers
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Interrupts are a mechanism for calling
(invoking) system software processes
and programs
Operating system (OS) provides lowlevel processing routines (service calls)
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Examples: reading data in from the
keyboard
Writing to a file
Interrupt handlers cont.
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There is a unique individual interrupt
handler (i.e. program) to process each
possible interrupt
Each handler is a separate program
stored in a separate part of main
memory
Interrupt table
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A conversion table in main memory that
has a list of all interrupt codes
Interrupt code is used as an index into
interrupt table
For each interrupt code, interrupt table
has the memory address of each
interrupt handler
Interrupt handlers
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Supervisor (OS) examines the interrupt
code, uses it as an index into the
interrupt table
Looks up memory location of needed
interrupt handler
Loads that memory location into the PC
(program counter)
Interrupt handler begins executing
Multiple interrupts
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It is possible (even likely) that
interrupts will interrupt each other
OS has an algorithm to determine what
goes first
Assigns priorities to different interrupts
based on
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Error conditions
Critical hardware failures
Suspending a process
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Whenever a process is suspended or
interrupted the system must save
whatever information is necessary to
allow the process to restart again
Typically that involved saving
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PC and IR
Any other specialized or general purpose
registers that were in use
Saving a process
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The collection of information needed to
restart a process is called the “machine
state”
It is saved in a special storage location
called the stack
The Stack
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The stack is a specialize storage
location in RAM
It is a data structure where you add
and delete information from the same
end
Therefore the last process saved by the
CPU is the first one it will pick up
Interrupt process
Buffers
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Buffers are a mechanism that uses RAM
to overcome slow data transfer rate to
peripheral devices
Small storage area (in RAM) used to
hold data in transit from one device to
another
Buffers and printing
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Printed version of document with
formatting information is copied to RAM
When full page is ready it is released
from the buffer
Document is written from RAM to
printer
Also have input buffers – keyboard,
modem, etc.
Buffers
Cache
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Pronounced “cash”
Separate high speed storage area
specifically managed to improve overall
system performance
Idea is most often needed data is kept
in the cache
Must be managed intelligently
Cache cont.
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Data content is not automatically
removed (unlike buffer)
Used for bi-directional data transfer
Used only for storage device access
Larger than buffer
Cache cont.
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Basic idea is that access to high speed
cache is faster than hard drive
During a write operation cache acts as a
buffer
Data written to cache then to drive
Cache controller
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Manages the content of the cache
It must “guess” which files should be in the
cache, i.e. what data the CPU will ask for
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Cache hit – when data is found in the cache
Cache miss – when data is not found
Cache swap – old data removed and new data
inserted
Cache cont.
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Even a small cache can significantly
improve performance
Ratio of primary storage to cache of
10,000 to 1 can result in cache hit rate
of 90%
Compression
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Technique to reduce the number of bits
used to encode a set of related data
items, i.e. a file or stream of video
Some formats (MP3, GIF) are
intentionally compressed data formats
Compression cont.
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Compression is accomplished by the
application of a compression algorithm
(specific mathematical technique)
Also need corresponding decompression algorithms to restore data
to its original state
Compression cont.
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Compression algorithms vary
What types of data are appropriate
Whether any data is lost
Amount by which data is compressed
Lossless compression (zip) – no loss of
data
Lossy – some loss of data (audio or
video)
Compression cont.
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Compression rate – size before and
after compression
Used to reduce secondary storage
requirements
Transmit over Internet
Package files together
Data compression
MP3 encoding elements
MP3
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How MP3 works
http://www.howstuffworks.com/mp31.h
tm
Summary
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The system bus is the communication
pathway that connects the CPU with memory
and other devices
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The CPU communicates with peripheral
devices through I/O ports
Summary cont.
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Application programs use interrupt processing
to coordinate data transfers to or from
peripheral devices, notify the CPU of errors,
and call operating system service programs
A buffer is a region of memory that holds a
single unit of data for transfer to or from a
device
Compression reduces the number of bits
required to encode a data set or stream,
effectively increasing the capacity of a
communication channel or storage device