On Thursday 19 January 2006 17:46, David Brown wrote:
> There have already been a dozen answers pointing out the O/P's problem. I
> thought you might like to know, however, that there is another way to do an
> atomic read of such a counter without disabling interrupts, which can be
> useful in some circumstances. If you have complicated and slow interrupt
> routines, or high frequency interrupts, then it's probably not a great idea
> as it is non-deterministic.
>
> unsigned short atomic_read_short(unsigned short *addr) {
> unsigned short a, b;
> a = (volatile)(*addr);
> do {
> b = (volatile)(*addr);
> if (a == b) return a;
> a = b;
> };
> }
Yes, this is a very beautiful idea.
Only one note: to force reading from '*addr'
it is necessary a small correction.
Original:
do {
b = (volatile)(*addr);
if (a == b) return a;
a = b;
} while (1);
produces an infinity loop:
.L2:
cp r24,r18
cpc r25,r19
breq .L7
movw r24,r18
rjmp .L2
and after correction:
do {
b = *(volatile unsigned short *)addr;
if (a == b) return a;
a = b;
} while (1);
works true:
.L2:
ld r18,Z
ldd r19,Z+1
cp r24,r18
cpc r25,r19
breq .L7
movw r24,r18
rjmp .L2
Regards,
Dmitry.
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