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corrections: ES is not overrideable as string destination, and this it is a second exception to " All other references to data use the data segment." |
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In both real and protected modes, the system uses 16-bit ''segment registers'' to derive the actual memory address. {{anchor|Extra segment}}In real mode, the registers CS, DS, SS, and ES point to the currently used program [[code segment]] (CS), the current [[data segment]] (DS), the current [[stack segment]] (SS), and one ''extra'' segment determined by the programmer (ES). The [[Intel 80386]], introduced in 1985, adds two additional segment registers, FS and GS, with no specific uses defined by the hardware. The way in which the segment registers are used differs between the two modes.<ref name=Arch />
The choice of segment is normally defaulted by the processor according to the function being executed. Instructions are always fetched from the code segment. Any stack push or pop or any data reference referring to the stack uses the stack segment. The extra segment is the mandatory destination for string operations (for example MOVS or CMPS); for this one purpose only, the automatically selected segment register cannot be overridden. All other references to data use the data segment. The
== Real mode ==
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