1. Lecture 26 – Input/output, programmed I/O
I/O, I/O, so off to work I go – bumper sticker seen in Silicon Valley
© 2017 by George B. Adams III
Portions © 2017 Dr. Jeffrey A. Turkstra

2. Announcements Chapters 14-17 Lab 8 this week Homework 6 due Wednesday
Due in lab next week Monday/Tuesday
Final lab sessions next week 7/31 and 8/1
Homework 6 due Wednesday
Homework 7 due Tuesday, 8/1
Final lecture Tuesday 8/1
Final exam Thursday 8/3 8am BRNG 2290
© 2017 by George B. Adams III
Portions © 2017 Dr. Jeffrey A. Turkstra

3. © 2017 by George B. Adams III
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4. Configure then attach requires sys admin knowledge
Wiring approach = sys admin knowledge built in
Automatic is the third option above, very convenient, but
becomes an interface with the Real WorldTM with significant implications for computer security
© 2017 by George B. Adams III
Portions © 2017 Dr. Jeffrey A. Turkstra

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7. © 2017 by George B. Adams III
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8. LED example requires op to be specified, could just have separate addresses
© 2017 by George B. Adams III
Portions © 2017 Dr. Jeffrey A. Turkstra

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19. Connections created and deleted dynamically by action
of “Input” devices
© 2017 by George B. Adams III
Portions © 2017 Dr. Jeffrey A. Turkstra

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23. – Software services the I/O needs, but hardware initiates the service: the hardware dynamically inserts a “jsr” into the CPU instruction fetch stream
– Perhaps “interrupt-triggered I/O” would be a better name, but we will use interrupt-driven I/O
© 2017 by George B. Adams III
Portions © 2017 Dr. Jeffrey A. Turkstra

24. – How do we make a processor “wait”?
Cheap cheap controller
circuit for device and bus
– How do we make a processor “wait”?
– We code a while loop for it to execute.
– Drawback: processor kept 100% busy just waiting; CPU cannot do anything else!
(Quelle nightmare.)
© 2017 by George B. Adams III
Portions © 2017 Dr. Jeffrey A. Turkstra

25. © 2017 by George B. Adams III
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26. © 2017 by George B. Adams III
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27. Array Elements 1 2 3 4 © 2017 by George B. Adams III1 2 3 4
© 2017 by George B. Adams III
Portions © 2017 Dr. Jeffrey A. Turkstra

28. Example of polling to control printer
© 2017 by George B. Adams III
Portions © 2017 Dr. Jeffrey A. Turkstra

29. Example bus interface for ink-jet printer
List of the Control and Status “registers” for the ink-jet printer
For code on next slide, assume that Address 0 above (printer-relative) has been assigned physical address 0x
© 2017 by George B. Adams III
Portions © 2017 Dr. Jeffrey A. Turkstra

30. C code to poll // Non-zero if printer is on
// Device is busy if *(p+6) != 0; proxy for “Load is complete”
// Busy now a proxy for “Data is loaded”
// This is not an infinite loop if the printer operates correctly
// Printer failure requires a system reboot
© 2017 by George B. Adams III
Portions © 2017 Dr. Jeffrey A. Turkstra

31. Polling code using struct
© 2017 by George B. Adams III
Portions © 2017 Dr. Jeffrey A. Turkstra

32. Programmed I/O: What’s to Like?
Cheap, bare-bones hardware can do it
Cheap, don’t even have need for an O/S
Con
Programmer must include I/O busy wait loop in app code
While waiting on one I/O need, cannot monitor any other I/O
© 2017 by George B. Adams III
Portions © 2017 Dr. Jeffrey A. Turkstra

33. – Makes good sense now that hardware is cheaper so that building smarter devices is feasible and since a CPU is wasted with polling (busy wait)
© 2017 by George B. Adams III

34. © 2017 by George B. Adams III

35. Synchronous with the CPU
within the app itself ^
“Asynch” with the CPU
System programmer, not every app programmer
© 2017 by George B. Adams III