Chapters 8

1
Chapters 8

• (partial coverage)

2
Interfacing Processors and Peripherals

• Processors Memory focus on performance and cost
• I/O focus on dependability and cost
• Design affected by many factors (expandability,
  resilience to failure, performance)
• Performance, smaller role, but more complex
  access latency throughput connection
  between devices and the system the memory
  hierarchy the operating system

3
I/O

• Important but neglected The difficulties in
  assessing and designing I/O systems have often
  relegated I/O to second class status courses
  in every aspect of computing, from programming
  to computer architecture often ignore I/O or
  give it scanty coverage textbooks leave the
  subject to near the end, making it easier for
  students and instructors to skip it!
• GUILTY! we wont be looking at I/O in much
  detail be sure and read Chapter 8 in its
  entirety. you should probably take a
  networking class!

4
I/O Devices

• Very diverse devices behavior (i.e., input vs.
  output, storage) partner (who is at the other
  end? Human or machine) data rate (peak rate
  data is transferred between I/O and main
  memory or processor)

• Ex Keyboard
• Input device
• Used by human
• Peak Data Rate 10 bytes/sec

5
I/O Example Disk Drives

• To access data seek position head over the
  proper track (3 to 14 ms. avg.) rotational
  latency wait for desired sector (.5 / RPM)
  transfer grab the data (one or more sectors)
  30 to 80 MB/sec

In 2004
6
I/O Example Buses

• Shared communication link
• One or more wires, used to connect multiple
  subsystems
• Advantages
• Versatility (Easy to add new devices)
• Low cost (Single set of wires shared in multiple
  ways)
• Disadvantages
• Can be major bottleneck
• Limiting the max I/O throughput
• Difficult to design because of physical limits
• Length of the bus
• Number of devices
• Different device latencies and data transfer rates

7
I/O Example Buses

• Consist of
• Set of control lines
• Send request and acks (indicate type of info on
  data lines)
• Set of data lines
• Between the source and the destination (data,
  complex commands, addresses)
• Buses are shared resource
• Need protocol to decide who uses it next
• Bus transaction
• 2 parts sending address and receiving or sending
  data
• Input and output transactions
• Input inputting data from the device to memory
  where the processor can read it
• Output outputting data to a device from memory
  where the processor wrote it

8
I/O Example Buses

• Types of buses
• processor-memory (short high speed, custom design
  to maximize mem-processor bandwidth)
• I/O (lengthy, different devices, e.g., USB,
  Firewire, wide range bandwidth of devices)
• Backplane interface connection between the I/O
  and memory buses (high speed, often standardized,
  e.g., PCI)
• Others buses do exist, such as graphics buses
• Types of Communication protocol
• Synchronous
• Clock in control lines
• Fixed protocol for communication relative to the
  clock
• Fast and small protocol (implement in FSM)
• But every device must operate at same rate and
  clock skew requires the bus to be short
• EX processor-memory bus
• Asynchronous
• Dont use a clock
• Instead use handshaking
• Accommodates wide range of devices
• No clock skew or synchronization problems
• Bus can be lengthy

9
I/O Bus Standards

• Today we have two dominant bus standards
  

10
Pentium 4

• I/O Options

11
Other important issues

• Bus Arbitration daisy chain arbitration (not
  very fair) centralized arbitration (requires
  an arbiter), e.g., PCI collision detection,
  e.g., Ethernet
• Operating system polling interrupts
  direct memory access (DMA)
• Performance Analysis techniques queuing
  theory simulation analysis, i.e., find the
  weakest link (see I/O System Design)

12
Fallacies and Pitfalls

• Fallacy the rated mean time to failure of disks
  is 1,200,000 hours (140 years), so disks
  practically never fail.
• Marketing numbers, measured by running disk
  simultaneously and counting failed ones. Really,
  3.6 of disks will fail over 5-year period
• Fallacy magnetic disk storage is on its last
  legs, will be replaced.
• Reliability, no volatility, low cost, reasonable
  access time
• Might become true with solid state
• Fallacy A 100 MB/sec bus can transfer 100
  MB/sec.
• Cant use 100 of resource, at best 70-80
• 1MB of storage ? 1 MB/second bus speed (base 2
  vs. base 10)