How to catch if buffer memory is full

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Originally Posted by zerver

Well, I can't realize something if I have no idea about how it's implemented. But you are right that I strongly suspect there is a penalty - otherwise it would be in widespread use among performance freaks such as me :cool:

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Check out Creating Guard Pages

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Originally Posted by zerver

I have another question related to this:

Are there any facilities for generating an interrupt when a specific memory address is written?

For instance, if I have a vector class in which I add one element at a time and want to enter an interrupt routine when I write outside the allocated array. In this interrupt routine I need to reallocate the array to avoid a crash.

The purpose would be to avoid having to check the bounds on each write, thus increasing performance slightly.

I know a debugger can "break on memory write", but I suppose there is quite an overhead in using this technique. How does the debugger accomplish this?

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The way a debugger may implement 'break on memory write' is as follows

1. Set the memory protection on the (virtual memory) page of interest.
2. Trap the interrupt that is generated.
3. Check if the specified memory address has been written; if yes break; else continue.

I do not know if any CPU mechanism that can break on a specific memory address write. Breaking on a page basis is possible. This I suspect is the reason why 'break on this memory write' is not practically feasible - the thing would cause an interrupt on write to any of the memory addresses on that page and some other piece has to then check if the accesses memory is what someone wanted to be interrupted about; even with small page size (say 2K) that may be costly.

Ah, excellent. Many thanks!

This means that if you always make sure to allocate at least two pages more than you currently need and then set up the memory protection accordingly, this technique could actually be used to reallocate a growing array.

And if I understand correctly, performance will not be negatively affected in this case since no other code will write to the protected page. When the interrupt is triggered you disable the protection, reallocate and finally protect the new page.

At least the technique is interesting from a technical standpoint.