Kernel Memory and User Memory in .NET

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Kernel Memory and User Memory
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Probably the most important concept in memory management is the distinctions between kernel memory and user memory. It is well known that in order to create a robust operating system, applications must not be able to access the operating system s internal data structures. That s because we don t want a single programmer s bug to overwrite some important data structure and destabilize the entire system. Additionally, we want to make sure malicious software can t take control of the system or harm it by accessing critical operating system data structures. Windows uses a 32-bit (4 gigabytes) memory address that is typically divided into two 2-GB portions: a 2-GB application memory portion, and a 2-GB shared kernel-memory portion. There are several cases where 32-bit systems use a different memory layout, but these are not common. The general idea is that the upper 2 GB contain all kernel-related memory in the system and are shared among all address spaces. This is convenient because it means
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that the kernel memory is always available, regardless of which process is currently running. The upper 2 GB are, of course, protected from any user-mode access. One side effect of this design is that applications only have a 31-bit address space the most significant bit is always clear in every address. This provides a tiny reversing hint: A 32-bit number whose first hexadecimal digit is 8 or above is not a valid user-mode pointer.
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The Kernel Memory Space
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So what goes on inside those 2 GB reserved for the kernel Those 2 GB are divided between the various kernel components. Primarily, the kernel space contains all of the system s kernel code, including the kernel itself and any other kernel components in the system such as device drivers and the like. Most of the 2 GB are divided among several significant system components. The division is generally static, but there are several registry keys that can somewhat affect the size of some of these areas. Figure 3.1 shows a typical layout of the Windows kernel address space. Keep in mind that most of the components have a dynamic size that can be determined in runtime based on the available physical memory and on several user-configurable registry keys. Paged and Nonpaged Pools The paged pool and nonpaged pool are essentially kernel-mode heaps that are used by all the kernel components. Because they are stored in kernel memory, the pools are inherently available in all address spaces, but are only accessible from kernel mode code. The paged pool is a (fairly large) heap that is made up of conventional paged memory. The paged pool is the default allocation heap for most kernel components.The nonpaged pool is a heap that is made up of nonpageable memory. Nonpagable memory means that the data can never be flushed to the hard drive and is always kept in physical memory. This is beneficial because significant areas of the system are not allowed to use pagable memory. System Cache The system cache space is where the Windows cache manager maps all currently cached files. Caching is implemented in Windows by mapping files into memory and allowing the memory manager to manage the amount of physical memory allocated to each mapped file. When a program opens a file, a section object (see below) is created for it, and it is mapped into the system cache area. When the program later accesses the file using the ReadFile or WriteFile APIs, the file system internally accesses the mapped copy of the file using cache manager APIs such as CcCopyRead and CcCopyWrite.
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