System Integration and Performance

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Chapter 6

• System Integration and Performance

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Chapter goals

• Describe the implementation of the system bus and
  bus protocol.
• Describe how the CPU and bus interact with
  peripheral devices.
• Describe the purpose and function of device
  controllers.

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Chapter goals cont.

• Describe how interrupts coordinate actions of the
  CPU with secondary storage and I/O devices
• Describe how buffers, caches, and data
  compression improve computer system performance

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Role of the system bus

• Bus is mechanism that allows computer components
  to work together
• Is made up of parallel communication lines
  connecting computer components
• CPU, hard drive, parallel port, modem, etc.
• Can connect two or more devices
• Information can travel in both directions

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System bus (cont.)

• Can connect both internal (hard drive) and
  external (printer) devices
• System bus has three parts
• 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

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System bus
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Bus Clock

• 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

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Bus clock cont.

• Bus clock is MUCH slower than CPU clock
• Think of CPU as the highway and the system bus as
  the local streets

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Why slower?

• Data on bus must travel a longer physical
  distance than data in CPU
• 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

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Bus data transfer rate

• Called the bus data capacity
• Expresses how much data can travel across bus
  over time
• Is a combination of
• Bus clock speed
• Data transfer unit (usually a word)
• Is used to calculate things like
• Time required to load large files (i.e. video)

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Bus protocol

• 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

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Bus protocol cont.

• Protocol can impact (reduce) data transfer rates
• Protocols often require exchanges of control
  signals
• Control signals consume bus cycles that could
  otherwise send data

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Sample protocol

• Example if a disk drive transfers data to RAM as
  the result of an explicit CPU instruction, the
  following steps are followed
• CPU sends command to the drive
• Drive send acknowledgement to CPU
• Drive carries out transfer
• Drive sends confirmation to CPU that transfer is
  complete

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Why use protocols?

• 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

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What if two devices need the bus?

• 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
• Master-slave
• Multiple master
• Peer to peer

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Master-slave

• 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

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Master-slave cont.

• Overall system performance is severely degraded
• 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

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A better solution

• 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

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Multiple master bus

• 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

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Logical vs. Physical Access

• 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
• The CPU
• Or a single peripheral device

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Logical and Physical Access
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I/O Ports

• 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

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The CPU and I/O Ports

• 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

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Logical access

• 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
• Storage capacity
• Internal data coding methods
• If storage or I/O device

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Linear address space

• 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

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How logical becomes physical

• 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

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Conversion table for disk
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Device controllers

• 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

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Device controllers
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Device controllers cont.

• Device controllers monitor the bus control lines
  for signals to peripheral devices
• Translates those signals into appropriate
  commands for its device

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Interrupts

• Secondary storage and I/O device transfer rates
  are much slower than the CPU
• Why?
• Slower bus clock
• Peripheral devices have mechanical elements
  (access arm, spin mechanism) that are slower than
  speed of electricity

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Interrupts cont.

• 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

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How interrupts work

• 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

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Interrupts cont.

• When I/O is complete, interrupt signal is sent to
  the CPU
• CPU can now restart your task with I/O task being
  complete

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CPU and Interrupts

• 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

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The CPU and Interrupts cont.

• 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

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CPU and Interrupts

• If CPU is to process an interrupt it does the
  following
• 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

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Interrupt handlers

• Interrupts are a mechanism for calling (invoking)
  system software processes and programs
• Operating system (OS) provides low-level
  processing routines (service calls)
• Examples reading data in from the keyboard
• Writing to a file

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Interrupt handlers cont.

• 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

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Interrupt table

• 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

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Interrupt handlers

• 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

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Multiple interrupts

• 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
• Error conditions
• Critical hardware failures

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Suspending a process

• 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
• PC and IR
• Any other specialized or general purpose
  registers that were in use

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Saving a process

• 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

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The Stack

• 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

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Interrupt process
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Buffers

• 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

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Buffers and printing

• 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.

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Buffers
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Cache

• 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

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Cache cont.

• Data content is not automatically removed (unlike
  buffer)
• Used for bi-directional data transfer
• Used only for storage device access
• Larger than buffer

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Cache cont.

• 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

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Cache controller

• 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
• 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

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Cache cont.

• 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

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Compression

• 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

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Compression cont.

• Compression is accomplished by the application of
  a compression algorithm (specific mathematical
  technique)
• Also need corresponding de-compression algorithms
  to restore data to its original state

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Compression cont.

• 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)

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Compression cont.

• Compression rate size before and after
  compression
• Used to reduce secondary storage requirements
• Transmit over Internet
• Package files together

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Data compression
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MP3 encoding elements
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MP3

• How MP3 workshttp//www.howstuffworks.com/mp31.ht
  m

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Summary

• The system bus is the communication pathway that
  connects the CPU with memory and other devices
• The CPU communicates with peripheral devices
  through I/O ports

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Summary cont.

• 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