it-ebooks - Real-Time Programming Lecture Notes (Waterloo CS452)

From: CS452 - Real-Time Programming - Spring 2012

1. Due date for assignment 0/1
2. Combination to trains lab
3. Ubuntu 10.10

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Most of the mediation between the internal representations and thereal-world is done by embedded systems

• invisible computing
• sense and control
• billions and billions sold

Two box model

• develop on one box
• execute on a different box.

Development cycle

• edit & compile on one box
• download to second box
• execute

Problem one

• raw code that owns the hardware itself
• hardware-specific libraries provide access to hardware
  • microcontroller/hopper example

Problem two

• OS-like abstraction of hardware
• looks like a bunch of libraries, plus a little more.

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Actually real-world programming, which means

• World is measured in seconds, metres, etc.
• Programs manipulate bits, bytes, words, which must be translated into into real-world measures.
  For example,
  • formatted output: translate from int, which computers manipulate, to decimal, which humans read
    • out in the open: i2a( ), printf( )
    • hidden: print, cout
  • Banking: int translates into, e.g., number of cents
    • program says dispense( 10000 ), which means `put five twemties into the hopper'.
    • microcontrollers start
      • activating motors
      • sensing forces
      • reading digitized video
      • etc.
  • Train control: contents of messages map into change speed, switch turn-out, sensor triggered

1. Throughput
   • e.g., number of frames per second in a game
   • e.g., frequency of sensor sampling in process control
   • no solution except
     1. getting better hardware
     2. getting better algorithms
     3. restructuring the task
2. Response time
   • e.g., time from button press to gun firing in game
   • e.g., time from sensor reading to control code executing in provess control
   • several programming techniques exist
     1. busy-wait
     2. polling loop

     tension between flexibility and performance

In cs452 we take guaranteed response time as the defining quality ofreal-time computation.

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Uses a device called a UART,

• which is really just two shift registers
• with a one byte buffer in front of each one
• plus one bit in a control register for each. They mean
  • read: there's a byte you haven't read yet
  • write: the buffer is empty, you can write

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This is used to synchronize with an external event, minimizing responsetime.

#define FOREVER for( ; ; )FOREVER { while( !ready( ) ) ; do-it( );}

or in another form

FOREVER { if ( ready( ) ) do-it( );}

Sometimes you only want to do the thing once, as you do when putting acharacter on a serial line.

#define UART1_BASE 0x808c0000#define UART_DATA_OFFSET 0x00 // low 8 bits#define UART_FLAG_OFFSET 0x18 // low 8 bits#define TXFF_MASK 0x20 // Transmit buffer full flags = (int *)( UART1_BASE + UART_FLAG_OFFSET ); data = (int *)( UART1_BASE + UART_DATA_OFFSET ); while( ( *flags & TXFF_MASK ) ) ; *data = c;

From the time that the ready bit sets until the first instruction of do-itis executed

• execution time forwhile( !ready ) ;do-it;
• Even a moderately optimizing compiler will produce good machine code. Something likeready: ldb r0, STATUS-ADDRESS and r0, r0, READY-BIT beq _readydo-it: ldb r0, DATA-ADDRESS

  Here do-it is acquiring a single byte from interface hardware when the status register indicates that valid data is available in the dara register.

• Worst case response time is the execution of and, beq, ldb, and and beq
• Normally code like this would be inside a loop, acquiring one byte after another until no more are available.

What if the CPU has to two things at once?

E.g.,

1. collect bytes coming in a serial port
2. maintain a clock

Unless the rate of bytes coming in and rate of clock ticks areidentical

you are guaranteed to lose something sooner or later.

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Return to:

1. Due date for assignment 0
2. Ubuntu 10.10
3. Caches, optimization, clock speed, FIFOs
4. Libraries: memcpy in particular

---

Actually real-world programming, which means

• World is measured in seconds, metres, etc.
• Programs manipulate bits, bytes, words, which must be translated into into real-world measures.
  For example,
  • formatted output: translate from int, which computers manipulate, to decimal, which humans read
    • out in the open: i2a( ), printf( )
    • hidden: print, cout
  • Banking: int translates into, e.g., number of cents
    • program says dispense( 10000 ), which means `put five twenties into the hopper'.
    • microcontrollers start
      • activating motors
      • sensing forces
      • reading digitized video
      • etc.
  • Train control: contents of messages map, sometimes indirectly, into changes in speed of trains, state changes (turn-outs, train locations), sensor reports.

1. Throughput
   • e.g., number of frames per second in a game
   • e.g., frequency of sensor sampling in process control
   • no solution except
     1. getting better hardware
     2. getting better algorithms
     3. restructuring the task
2. Response time
   • e.g., time from button press to gun firing in game
   • e.g., time from sensor reading to control code executing in process control
   • several programming techniques exist
     1. busy-wait
     2. polling loop

     tension between flexibility and performance

In cs452 we take guaranteed response time as the defining quality ofreal-time computation.

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How does one keep time in a computer?

• crystal oscillator in a phase-locked loop
• interrupts from a timer
• ntp

This is used to synchronize with an external event, minimizing responsetime.

#define FOREVER for( ; ; )FOREVER { while( !ready( ) ) ; do-it( );}

or in another form

FOREVER { if ( ready( ) ) do-it( );}

Sometimes you only want to do the thing once, as you do when putting acharacter on a serial line.

#define UART1_BASE 0x808c0000#define UART_DATA_OFFSET 0x00 // low 8 bits#define UART_FLAG_OFFSET 0x18 // low 8 bits#define TXFF_MASK 0x20 // Transmit buffer full flags = (int *)( UART1_BASE + UART_FLAG_OFFSET ); data = (int *)( UART1_BASE + UART_DATA_OFFSET ); while( ( *flags & TXFF_MASK ) ) ; *data = c;

Note. The volatile keyword.

From the time that the ready bit sets until the first instruction of do-itis executed