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copied!<p>It means the instruction is referencing memory in the Data Segment - and can pretty much be ignored on modern OSes, since they run with a flat address space model (code, data and stack segments all refer to the same memory range, and memory protection is handled with paging).</p> <h2>EDIT:</h2> <p>A little elaboration - note that, to keep things simple, this is in the context of 32bit protected mode running Windows.</p> <p>A <em>segment register</em> (CS,DS,SS,ES,FS,GS) holds a <code>selector</code> pointing to a <code>descriptor</code>. There's two descriptor tables: global (<a href="http://en.wikipedia.org/wiki/Global_Descriptor_Table" rel="noreferrer">GDT</a>) and local (<a href="http://en.wikipedia.org/wiki/Local_Descriptor_Table" rel="noreferrer">LDT</a>), and the selector has a bit indicating which to use. Windows (almost?) exclusively uses the global table.</p> <p>A <em>descriptor</em> is <strong>basically</strong> a <em>{beginning-address, size}</em> pair - there's more to it, but that's outside the scope of this post.</p> <p>Windows uses a <a href="http://en.wikipedia.org/wiki/Flat_memory_model" rel="noreferrer">Flat Memory Model</a>: each process has a 4GB address space starting at memory address 0, and uses <a href="http://en.wikipedia.org/wiki/Virtual_memory#Paged_virtual_memory" rel="noreferrer">paging</a> to isolate processes from eachother.</p> <p>Since processes have this flat view of the world, they run with all segments using {0, 4GB} descriptors - and thus, instead of allocating per-process descriptors, Windows can use only a few global descriptors and have all processes use those.</p> <h2>EDIT 2:</h2> <p>The <a href="http://en.wikipedia.org/wiki/Portable_Executable" rel="noreferrer">Portable Executable</a> format defines <code>sections</code>, which are unrelated to the x86 <code>segments</code> - even if there's some conceptual overlap. The PE EXEs can have pretty much any section layout you wish, but the normal is to split into (at least) code (read/execute), data (read/write), resources (readonly?). Splitting the executable into sections makes it possible to apply x86 page-level memory protection to the memory ranges.</p> <h2>EDIT 3:</h2> <p>While the normal segments don't change per-process, Windows uses the <code>FS</code> register to point to the per-<em>thread</em> <a href="http://en.wikipedia.org/wiki/Win32_Thread_Information_Block" rel="noreferrer">TIB</a> structure.</p> <h2>EDIT 4:</h2> <p>See <a href="http://pdos.csail.mit.edu/6.828/2005/readings/i386/s05_01.htm" rel="noreferrer">this</a> for an overview. This is from an old document on the 80386, but the information still applies.</p>

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