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1 <html><head><title>Homework: x86 paging</title> <link rel="stylesheet" href="homework.css" type="text/css" /></head><body><h1>Homework: x86 paging</h1>Read chapters 5 and 6 of the <a href="../readings/i386/toc.htm">Intel 80386 Reference Manual</a>. These chapters explain the x86 Memory Management Unit (MMU), which we will cover in lecture today and which you need to understand in order to do lab 2.<h2>Hand-In Procedure</h2>You are to turn in this homework during lecture. Please write up your answers to the exercises below and hand them in to a 6.828 staff member at the beginning of lecture.<h2>Assignment</h2>Get the source code for <a href="../labs/lab2/">lab 2</a>, and read through the <code>mem_init()</code> function in <tt>kern/pmap.c</tt>. Try to understand from the code what page mappings are being constructed. You may find the discussion in part 3 of the <a href="../labs/lab2/">lab assignment</a> helpful.<div class="question"><p><b>Submit</b>: Draw a diagram of the virtual address space constructed by <tt>mem_init()</tt>, and identify any instances where multiple regions of virtual memory might be pointing to the same physical memory. Explain why the kernel might want to have different virtual addresses pointing to the same physical memory.</p></div>At the end of <tt>mem_init()</tt>, the kernel finally switches from the <code>entry_pgdir</code> set up in <tt>kern/entry.S</tt> to the newly created <code>kern_pgdir</code>.<div class="question"><p><b>Submit</b>: Draw a diagram of the virtual address space represented by <code>entry_pgdir</code> (the definition is in <tt>kern/entrypgdir.c</tt>). What about the layouts of the <code>entry_pgdir</code> and the <code>kern_pgdir</code> make it safe for the kernel to switch from the former the latter and continue executing?</p></div></body></html>