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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Opcode LGS</TITLE></HEAD><BODY><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="LGDT.htm"> LGDT/LIDT Load Global/Interrupt Descriptor Table Register</A><BR><B>next:</B><A HREF="LLDT.htm"> LLDT Load Local Descriptor Table Register</A><P><HR><P><H1>LGS/LSS/LDS/LES/LFS -- Load Full Pointer</H1><PRE>Opcode Instruction Clocks DescriptionC5 /r LDS r16,m16:16 7,p=22 Load DS:r16 with pointer from memoryC5 /r LDS r32,m16:32 7,p=22 Load DS:r32 with pointer from memory0F B2 /r LSS r16,m16:16 7,p=22 Load SS:r16 with pointer from memory0F B2 /r LSS r32,m16:32 7,p=22 Load SS:r32 with pointer from memoryC4 /r LES r16,m16:16 7,p=22 Load ES:r16 with pointer from memoryC4 /r LES r32,m16:32 7,p=22 Load ES:r32 with pointer from memory0F B4 /r LFS r16,m16:16 7,p=25 Load FS:r16 with pointer from memory0F B4 /r LFS r32,m16:32 7,p=25 Load FS:r32 with pointer from memory0F B5 /r LGS r16,m16:16 7,p=25 Load GS:r16 with pointer from memory0F B5 /r LGS r32,m16:32 7,p=25 Load GS:r32 with pointer from memory</PRE><H2>Operation</H2><PRE>CASE instruction OF LSS: Sreg is SS; (* Load SS register *) LDS: Sreg is DS; (* Load DS register *) LES: Sreg is ES; (* Load ES register *) LFS: Sreg is FS; (* Load FS register *) LGS: Sreg is DS; (* Load GS register *)ESAC;IF (OperandSize = 16)THEN r16 := [Effective Address]; (* 16-bit transfer *) Sreg := [Effective Address + 2]; (* 16-bit transfer *) (* In Protected Mode, load the descriptor into the segment register *)ELSE (* OperandSize = 32 *) r32 := [Effective Address]; (* 32-bit transfer *) Sreg := [Effective Address + 4]; (* 16-bit transfer *) (* In Protected Mode, load the descriptor into the segment register *)FI;</PRE><H2>Description</H2>These instructions read a full pointer from memory and store it in theselected segment register:register pair. The full pointer loads 16 bitsinto the segment register SS, DS, ES, FS, or GS. The other register loads 32bits if the operand-size attribute is 32 bits, or loads 16 bits if theoperand-size attribute is 16 bits. The other 16- or 32-bit register to beloaded is determined by the r16 or r32 register operand specified.<P>When an assignment is made to one of the segment registers, thedescriptor is also loaded into the segment register. The data for theregister is obtained from the descriptor table entry for the selectorgiven.<P>A null selector (values 0000-0003) can be loaded into DS, ES, FS, orGS registers without causing a protection exception. (Any subsequentreference to a segment whose corresponding segment register is loadedwith a null selector to address memory causes a #GP(0) exception. Nomemory reference to the segment occurs.)<P>The following is a listing of the Protected Mode checks and actions taken inthe loading of a segment register:<PRE>IF SS is loaded: IF selector is null THEN #GP(0); FI; Selector index must be within its descriptor table limits ELSE #GP(selector); Selector's RPL must equal CPL ELSE #GP(selector); AR byte must indicate a writable data segment ELSE #GP(selector); DPL in the AR byte must equal CPL ELSE #GP(selector); Segment must be marked present ELSE #SS(selector); Load SS with selector; Load SS with descriptor;IF DS, ES, FS, or GS is loaded with non-null selector: Selector index must be within its descriptor table limits ELSE #GP(selector); AR byte must indicate data or readable code segment ELSE #GP(selector); IF data or nonconforming code THEN both the RPL and the CPL must be less than or equal to DPL in AR byte; ELSE #GP(selector); Segment must be marked present ELSE #NP(selector);Load segment register with selector and RPL bits;Load segment register with descriptor;IF DS, ES, FS or GS is loaded with a null selector: Clear descriptor valid bit;</PRE><H2>Flags Affected</H2>None<H2>Protected Mode Exceptions</H2>#GP(0) for an illegal memory operand effective address in the CS, DS,ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;the second operand must be a memory operand, not a register; #GP(0)if a null selector is loaded into SS; #PF(fault-code) for a page fault<H2>Real Address Mode Exceptions</H2>The second operand must be a memory operand, not a register; Interrupt13 if any part of the operand would lie outside of the effective addressspace from 0 to 0FFFFH<H2>Virtual 8086 Mode Exceptions</H2>Same exceptions as in Real Address Mode; #PF(fault-code) for a pagefault<P><HR><P><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="LGDT.htm"> LGDT/LIDT Load Global/Interrupt Descriptor Table Register</A><BR><B>next:</B><A HREF="LLDT.htm"> LLDT Load Local Descriptor Table Register</A></BODY>?? 快捷鍵說明
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