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.NET Forum / Languages / Managed C++ / January 2007What is a *Page*?
Hi, I'm viewing this stack overflow help page ( http://support.microsoft.com/kb/315937 ) at the ms support site. I cant say I understand the concept of a "page" the way its used in this example/explanation. I mean I only am aware of the *simple*, *general*, paging concept, the one that is discussed often when explaning how an operating system can support more memory than actually is available by doing paging on the hard drive (and that also only at a higher-level theoratical concept). Although the "page"/"page guard"/etc thats referenced here *feels* something like that but nothing makes sense to me. For example take this sentence: "The stack then grows on demand to meet the needs of the thread. This is implemented by placing a page with PAGE_GUARD access at the end of the current stack." here I'm confused. - What is a "page" here? Is it some data structure? What do they contain? Can we see it? Is it documented? I need the links to its docs. - is "stack" some linked list kind of dynamically growing LIFO data structure? Are the contents of this stack these (unknown for me) "pages", in which case if the stack size is 5 than there will be 5 pages? plz clarify... Regards, ..ab > here I'm confused. > - What is a "page" here? Is it some data structure? What do they contain? > Can we see it? Is it documented? I need the links to its docs. On an operating system that uses virtual memory, a page is the smallest amount of memory which is moved in one go from the page file to physical memory. If you need to know how much memory is in a page, you call GetSystemInfo(). > - is "stack" some linked list kind of dynamically growing LIFO data > structure? No. The stack is a contiguous region of memory. By default, it is 1MB large. But like every other region of memory in Windows it is virtual memory. Address ranges of memory in the stack region are backed ("committed") by physical memory on an as needed basis. > Are the contents of this stack these (unknown for me) "pages", in which > case if the stack size is 5 than there will be 5 pages? It's a rare application which measures the size of anything in pages. The stack is no different, it is measured in bytes. Why do you need to know? Regards, Will Hi, thanks for the nice explanation. > Why do you need to know? Well, I was going through the kb site on microsoft support and it showed how I could catch the stack overflow exception and handle it and the talk was all about "pages". I have heard about pages before but I thought that finally I need to remove my confusion and build up some knowledge about this concept so I wont have any confusions in the future when somebody mentions "stack" or "pages" or "heap" etc. If u find/know any links related to these concepts, do share here. Regards, ..ab >> here I'm confused. >> - What is a "page" here? Is it some data structure? What do they contain? [quoted text clipped - 23 lines] > Regards, > Will > thanks for the nice explanation. You are welcome. > Well, I was going through the kb site on microsoft support and it showed > how I could catch the stack overflow exception and handle it and the talk > was all about "pages". I have heard about pages before but I thought that > finally I need to remove my confusion and build up some knowledge about > this concept so I wont have any confusions in the future when somebody > mentions "stack" or "pages" or "heap" etc. Well, first you have to remember that the stack by deafult is 1MB large. Your application shouldn't have to worry in general about overflowing the stack. If it does overflow it, that's generally due to one of two main causes: 1) a recursive algorithm recursing too much <g> 2) the allocation of a very large "automatic" array on the stack One deals with the first problem by recasting an algorithm, and with the second by moving the allocation to the heap. Regards, Will >Hi, > [quoted text clipped - 16 lines] > - What is a "page" here? Is it some data structure? What do they contain? >Can we see it? Is it documented? I need the links to its docs. A page is the smallest unit of memory the Windows virtual memory system deals with. Look up VirtualAlloc, and you'll find a lot of info on pages. > - is "stack" some linked list kind of dynamically growing LIFO data >structure? No, a thread's stack is a contiguous region of memory that grows dynamically up to its maximum configured size as the thread pushes data into the guard page, which causes a structured exception, which further causes the OS to commit the page and create a new guard page just below it. >Are the contents of this stack these (unknown for me) "pages", in >which case if the stack size is 5 than there will be 5 pages? It's a lot more common to measure in bytes; when there's doubt, you should specify the units, as I'm sure the Mars Climate Orbiter people wish they had done. You should search MSDN for PAGE_GUARD. This article talks about it, but its example doesn't illustrate using it to grow a stack: http://msdn2.microsoft.com/en-us/library/aa366549.aspx This one talks about reserving and committing memory and shows how to use structured exceptions to handle page faults to grow an array: http://msdn2.microsoft.com/en-us/library/aa366803.aspx What the system does with the thread stack lies somewhere in between...  SignatureDoug Harrison Visual C++ MVP Hey thanks for the useful links and the explanation. I'll have to read a lot before being able to ask more on this topic. Regards, ..ab >>Hi, >> [quoted text clipped - 51 lines] > > What the system does with the thread stack lies somewhere in between... > Hi, > [quoted text clipped - 8 lines] > referenced here *feels* something like that but nothing makes sense to me. > For example take this sentence: While a emulating additional memory using a swap file uses paging as its implementation (in most OSes, including Windows), it is not the definition of paging/virtual memory. It's extremely useful for isolating user-mode processes from each other for security and reliability, and dynamic memory commitment. A good place to start is by reading up on virtual memory and the Translation Look-aside Buffer (TLB). http://en.wikipedia.org/wiki/Virtual_memory http://en.wikipedia.org/wiki/Page_fault http://en.wikipedia.org/wiki/Page_table http://en.wikipedia.org/wiki/Translation_Lookaside_Buffer > "The stack then grows on demand to meet the needs of the thread. This is > implemented by placing a page with PAGE_GUARD access at the end of the [quoted text clipped - 12 lines] > > ..ab
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