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📁 Memory Management—Algorithms and implementation in C/C++ Introduction Chapter 1 - Memory Manag
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<title>Chapter 2: Memory Management Policies</title>
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<div class="chapter">
<a name="ch02"></a>
<h1 class="chapter-title"><span class="chapter-titlelabel">Chapter 2: </span>Memory Management Policies</h1><div class="section">
<h2 class="sect2-title">
<a name="132"></a><a></a>Overview</h2>
<a name="133"></a><a name="IDX-45"></a>
<blockquote class="blockquote">
<p class="first-para">"If I could remember the names of all these particles, I'd be a botanist."</p>
<p class="last-para">&mdash; Enrico Fermi</p>
</blockquote>
<p class="para">In the <a href="LiB0012.html#49" target="_parent" class="chapterjump">previous chapter</a>, I discussed the basic mechanisms that processors provide to allow memory regions to be read, modified, isolated, and simulated. Now you are ready to examine the ways in which operating systems construct policies that make use of these mechanisms. The processor presents the means to do things with memory through a series of dedicated data structures, system instructions, and special registers. It offers a set of primitives that can be combined to form a number of different protocols. It is entirely up to the operating system to decide how to use the processor's fundamental constructs, or even to use them at all.</p>
<div class="highlights">
<p class="first-para">There are dozens of operating systems in production. Each one has its own design goals and its own way of deciding how to use memory resources. In this chapter I will take an in-depth look at the memory subsystems of several kernels, ranging from the simple to the sophisticated. I will scrutinize source code when I can and hopefully give you a better feel for what is going on inside the LeMarchand cube.</p>
<p class="para">In this chapter, I am going to gradually ramp up the level of complexity. I will start with DOS, which is possibly the most straightforward and simple operating system that runs on a PC. DOS is really nothing more than a thin layer of code between you and the hardware. Next, I will kick the difficulty up a notch with MMURTL. MMURTL, unlike DOS, is a 32-bit operating system that runs in protected mode. Finally, this chapter will culminate with a discussion of two production-quality systems: Linux and Windows.</p>
<p class="last-para">
<a name="134"></a><a name="IDX-46"></a>After having looked at all four operating systems, I think that Windows is the most complicated system. Anyone who disagrees with me should compare implementing a loadable kernel module for Linux with writing a kernel mode PnP driver for Windows. There are people who make a living off of telling people how to write Windows kernel mode drivers. Don't get me wrong, the documentation for writing kernel mode drivers is accessible and copious; it is just that the process is so involved. After literally wading through Windows, I gained an appreciation for the relatively straightforward nature of the Linux kernel.</p>
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