IPC@CHIP Documentation - SC12 @CHIP-RTOS V1.10

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Critical Sections in Programs
When multitasking is used, the programmer must beware of critical sections which may occur between threads.
Critical sections can be protected with proper design. The important step at program design time is to identify
these code sections, which are not always obvious. Most programmers are well aware of what critical sections
are. However, due to their importance when multitasking, a simple example is provided here for emphasis.
Example Critical Section
Data sharing between tasks leads to critical sections when the shared data object can not be read or
written atomically, within a single non-interruptible machine instruction.
static unsigned long My_Ticker = 1 ;
void huge Task_A(void)
{
if (My_Ticker < 0x7FFFFFFFL)
{
My_Ticker++ ;
// Borland compiler's machine code:
// ADD My_Ticker,1H ; Increment LS 16 bits
// → Sensitive to task switch here!
// ADC My_Ticker+2,0H ; Carry into MS 16 bits
}
}
void huge Task_B(void)
{
if (My_Ticker == 0)
{
Surprised_to_be_Here() ; // How did this happen?
}
}
Figure 3 ) Critical Section Example
After a brief review of the C code in the above example, the C programmer might suspect a hardware
problem if the Surprised_to_be_Here() function was to ever execute. However, with a closer examination of
the resulting machine assembly code and multitasking consideration, we will see that execution of the
Surprised_to_be_Here() function is possible 2 .
2 And "if something bad can happen, it will happen".
All tasks in the @Chip-RTOS system have a unique task priority. So in the above example either Task_A
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