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pages/2012-06-26-Memory-Page-Stack.md

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+layout: post
+title: "Memory Page Stack"
+subtitle: "Basic memory management 1/3"
+tags: [osdev]
+
+###Memory management
+One of the most important tasks that the operating system has it to distribute
+the resources of the computer. One of those are the memory.
+
+I like to divide the memory management of the kernel into three main parts:
+
+- Physical memory management - Keeps track of the pages of actual memory
+- Virtual memory management - Keeps track of the virtual address spaces
+- Heap management - Fine grained control of memory for kernel use only
+
+This post is about a part of the physical memory manager. This simple version
+keeps track of free pages of physical memory. When asked for a page, it hands
+it out and forgets about it until it is handed back.
+
+###Memory page stack
+Pointers to the unused memory pages are stored on a stack and just popped off
+when needed.
+
+The stack does of course take up an amount of memory, but through an
+interesting trick, this doesn't matter. You see, the memory where the stack is
+stored is made up of memory pages and if enough pages are handed out to clear
+an entire page in the stack, that page is next to be handed out.
+
+Allow me to illustrate:
+Imagine a computer with really really small memory pages - so small in fact
+that each page only has room for four pointers. The figure below shows eight
+physical pages of a such computer. The two leftmost pages are used by the
+physical memory manager for the free page stack. The stack contains pointers to
+the next five pages, who are free. The rightmost page is handed out.
+![PMM1](/media/img/pmm1b.png){: .noborder .center}
+
+When the _pmm_ receives a request for a memory page it will pop the topmost
+entry from the stack and returns, in this case, the second rightmost page to
+the caller.  ![PMM2](/media/img/pmm2b.png){: .noborder .center}
+
+The next time the pmm receives a request for a memory page it will notice that
+an entire page of the stack is empty and just being wasted, so it will shrink
+its stack by one page-size and return the address of the page that previously
+made up the top of the stack.  ![PMM3](/media/img/pmm3b.png){: .noborder .center}
+
+Likewise, if the stack is full of pointers when a used page is handed back,
+that page is used to increase the stack space. Through the use of virtual
+memory and paging, the stack doesn't have to be contiguous in physical memory,
+so any page can make up a part of the stack.
+
+###Filling the stack
+When the computer is booted up, the bootloader is asked to give the kernel
+a map of the available memory. This is used to initialize the stack. The kernel
+simply goes through the memory map returns each unused page it finds to the
+pmm.
+
+###Improvements
+There's a lot more that can be done by the pmm. One major feature that will be
+added at a later state is keeping track of how many users each page has. This
+is useful if we wish to introduce shared memory or copy-on-write during forks.
+Otherwise we risk that a page is returned and handed out again while someone
+still think they have exclusive access to it. Not a good thing.
+
+###Git
+This page stack has been implemented in Git commit
+[caed8cb8a0](https://github.com/thomasloven/os5/tree/caed8cb8a0e39a1e7d7d2594b86f25b887afab81).
+Also implemented in the same commit is a virtual memory manager with recursive
+page directories described in [this
+post](/blog/2012/06/Recursive-Page-Directory).
+
+__Note__: There's a bug in that commit. In short, when filling the stack, the pmm looks for memory map entries between `assert_higher(mmap_start)` and `mmap_start + mmap_size`. Where the `assert_higher` macro pretty much just adds `0xC0000000` to the adress. Well... I'm sure you see what the problem is.
+The bug has been rectified in Git commit [ebaae73](https://github.com/thomasloven/os5/tree/ebaae7383bbadbfc3de62b1b14aa9a450d8e695c).