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copied!<p>It's actually an 8-byte entry in the global descriptor table. It creates a descriptor addressing the entire 4G address space as a selector.</p> <p>The <code>equ $-gdt</code> sets up a value in the assembler equal to the difference between this location (<code>$</code>) and the <code>gdt</code> label. In other words, it's the offset of this entry within the GDT itself.</p> <p>The structure of a GDT entry is as follows:</p> <p>     <img src="https://i.stack.imgur.com/wirfC.png" alt="alt text"></p> <p>where the individual parts are explained below.</p> <p>     <img src="https://i.stack.imgur.com/uExcu.png" alt="alt text"></p> <p>For your specific values:</p> <pre><code>(a) dw FFFFh (b) dw 0 (c) db 0 (d) db 92h ; present, ring 0, data, expand-up, writable (e) db CFh ; page-granular (4 gig limit), 32-bit (f) db 0 </code></pre> <ol> <li><p>The base address is calculated from the <code>f</code>, <code>c</code> and <code>b</code> fields, from most significant to least - because these are all zero, the base is at zero.</p></li> <li><p>The selector limit is calculated from the rightmost 4 bits of <code>e</code> and all of <code>a</code>, to give <code>0xfffff</code> in this case. This has 1 added to it to give <code>0x100000</code>. See point 3 below for what this means.</p></li> <li><p>The top 4 bits of <code>e</code> (the flags) set the granularity (4K rather than 1 byte) and the operand size (32-bit). With a granularity of 4K (12 bits) and page count of <code>0x100000</code> (20 bits), that gives you your full 32-bit (4G) address space.</p></li> <li><p>The <code>d</code> field is the access byte and sets the following properties based on <code>0x92</code>:</p> <ul> <li><code>Pr</code> present (in-memory) bit to true.</li> <li><code>Privl</code> privelege level to 0 (need to be ring 0 to get access).</li> <li><code>Ex</code> executable bit 0 (data selector).</li> <li><code>DC</code>, direction bit is 0, segment grows up.</li> <li><code>RW</code> of 1, memory is writable.</li> <li><code>Ac</code> accessed bit set to 0.</li> </ul></li> </ol>

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