MIDTERM EXAM

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MIDTERM EXAM

• Wednesday, March 12th.
• There will be no makeup exams.

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Timers and Timer Messages

• CPU clock pulses 1 billion times/second
• A piece of an instruction gets executed at ever
  clock.
• System clocks Generate interrupts every 54.9 ms
  (Windows).
• 18.2 times per second.
• NT generates interrupt every 10 ms. Or 100
  times/sec.
• Windows handles these asynchronous interrupts
• Uses hardware interrupts to send WM_TIMER
  messages to application.

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WM_TIMER Messages

• SetTimer(hwnd, id, frequency, NULL)
• frequency in milliseconds.
• Resolution of the timer messages is based on the
  resolution of the underlying hardware clock
• if resolution of clock is 54 milliseconds,
  cannot receive messages at a frequency greater
  than 18.2

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How it Works

• SetTimer(hwnd, 1, X, NULL)
• OS takes X / Clock Resolution Y.
• Every 54.9 ms in Windows. 10 ms in NT.
• Sets counter equal to the number of timer
  interrupts the OS will receive before timer fires
  (Y)
• Maintains counter for each timer. This counter
  initialized to Y.
• On each hardware interrupt, OS decrements
  counter.
• When counter 0, sends WM_TIMER msg to hwnd and
  resets counter to Y.

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Example SetTimer(hwnd, 1, 200, NULL)
Initialize counter to 200/55 3.63 rounded
down to 3
Message Queue
Application
X X X X X X
Counter
3
Windows 2000/NT
Hardware
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Message Queue
Application
X X X X X X
Counter
Interrupt 1
2
Windows 2000/NT
Hardware
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Message Queue
Application
X X X X X X
Counter
Interrupt 2
1
Windows 2000/NT
Hardware
8
Message Queue
Application
X X X X X X
Counter
Interrupt 3
0
Windows 2000/NT
Hardware
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Message Queue
Application
X X X X X X T
Counter
0
Windows 2000/NT
Hardware
10
Message Queue
Application
X X X X X X T
Counter
3
Windows 2000/NT
Hardware
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WM_TIMER Messages

• Application code is not interrupted for a timer
  msg.
• Windows places WM_TIMER messages in message queue
  like any other message.
• If application is busy, the timer messages will
  be received late. Not an exact science.

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Simple Example

• switch (message)
• case WM_CREATE
• SetTimer(hwnd, 1, 1000, NULL)
• SetTimer(hwnd, 2, 60000, NULL)
• return 0
• case WM_VSCROLL ..
• case WM_TIMER
• switch(wParam)
• case1 Do whatever needed at 1 second
  intervals
• case 2 Do whatever needed at 1 minute
  intervals.

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Stopping WM_TIMER messages

• KillTimer(hwnd, 1)
• WM_TIMER messages are low priority and Windows
  will combine these messages in the queue.

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Simple Animtion

• Timer messages can be used to repaint objects in
  the window to simulate movement.
• Three steps
• Erase object.
• Determine new position for object in response to
  each WM_TIMER message.
• Redraw object at new location.

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