RE: [avr-gcc-list] memcpy() : problem when passing destination pointer

[Dave Hansen]

 From: address@hidden

[...re: volatile use cases...]

> fourth case: nasty situations where 'volatile' is only a part of the
> solution but does not insure a correct result:
>
> For instance if ISR1 and ISR2 are *nested* ISRs, IsrCounter does not
> correctly hold the count of interrupts:
>
> volatile uint8_t IsrCounter;
>
> ISR1()
> {
> IsrCounter++;
> }
>
> ISR2()
> {
> IsrCounter++;
> }
>
> Consider also memory locations larger than the MCU data processing unit
> size (8 bits for AVR).
>
> The following code gives bad results on an AVR but works on a i386:
>
> volatile uint32_t MilliSeconds;
>
> ISR()
> {
> MilliSeconds++;
> }
>
> main()
> {
> while(MilliSeconds < SOME_LIMIT) ...
> }
>

In both of these cases, "volatile" is insufficient because it does not imply "atomic".  In the first case, you need an atomic read-modify-write, and in the second you need an atomic read of a wide value.  In both cases you can achieve that by enclosing the the variable access in a cli-sei pair.  For example, the latter case can be fixed by changing the routine to something like
 
main()  // or "int main(void)", yada yada
{
   uint32_t ms;   // does not need to be volatile
 
   do{ cli(); ms = Milliseconds; sei(); } while (ms < SOME_LIMIT);
}
 
Regards,
 
   -=Dave
 
 

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