I read post
C volatile variables and Cache Memory
But i am confused.
Question:
whether OS will take care itself OR
programmer has to write program in such a way that variable should not go into cache as mention like declaring variable as _Uncached.
Regards
Learner
Volatile Memory is used to store computer programs and data that CPU needs in real time and is erased once computer is switched off. RAM and Cache memory are volatile memory. Where as Non-volatile memory is static and remains in the computer even if computer is switched off. ROM and HDD are non-volatile memory.
The volatile keyword just prevents the C compiler doing certain optimizations on variables. It does not do anything with the cache. Some compilers might give you the ability to bastardise the meaning of this keyword (the ARC compiler is one) but for most compilers this is not the case.
ANSWER: No, volatile doesn't guarantee no caching for this memory location, and there aren't anything about this in C/C++ Standards or compiler manual.
There is three types of cache: direct-mapped cache; fully associative cache; N-way-set-associative cache.
To clarify:
volatile
is a C concept and tells the compiler to fetch a variable each time from memory rather then use a "compiler-generated" cached version in registers or optimise certain code.
What may be causing confusion here is CPU caches vs software caches (a.k.a variables in registers).
The CPU/Hardware cache is 100% transparent to the program and the hardware makes sure that it is 100% synchronised. There is nothing to worry there, when you issue a load
from memory and the data comes from the CPU cache then it's the same data that is in the addressed memory.
Your compiler may decide though to "cache" frequent use variables in registers which can then go out of sync with memory because the hardware is unaware of those. This is what the volatile
keyword prevents. Common example:
int * lock;
while (*lock) {
// do work
// lock mot modified or accessed here
}
An optimising compiler will see that you are not using lock
in the loop and will convert this to:
if (*lock)
while (true) {
// do work
}
This is obviously not the behaviour you want if lock
is to be modified by e.g. another thread. SO you mark it volatile to prevent this:
volatile int * lock;
while (*lock) {
// do work
}
Hope this makes it a little clearer.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With