volatile (in comp.lang.c)

[rpw3 at amdcad.UUCP]

In article <1036 at ima.ISC.COM> johnl at ima.UUCP (John R. Levine) writes:
+---------------
| In article <21821 at amdcad.AMD.COM> rpw3 at amdcad.UUCP (Rob Warnock) writes:
| >The "volatile" type is necessary and sufficient for correctly implementing
| >Dekker's Algorithm in "optimizing C".
| Not so fast, there. In large multi-CPU systems, there is often a write-behind
| cache and writes by one processor are not always visible to other processors
| until you do some sort of cache-flush operation...
+---------------

I'm sorry, I was talking about the programming language considerations.
Of course you are correct about the larger systems issue. I was assuming
that any data pages used for inter-processor sychronization would be
flagged as "don't cache" to the MMU/cache. (Most MMU's have some way to
indicate to the cache that given pages are not to be cached, but that
reads/writes are to be made directly to memory.)

Even if you have special semaphore hardware ("magic" memory locations),
you still must flag them as "don't cache"...


Rob Warnock
Systems Architecture Consultant

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