52 lines
2.4 KiB
HTML
52 lines
2.4 KiB
HTML
<html><!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
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<html>
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<head>
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<title>80386 Programmer's Reference Manual -- Opcode DEC</title>
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</head>
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<body>
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<b>up:</b> <a href="C17.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/C17.HTM">Chapter 17 -- 80386 Instruction Set</a><br>
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<b>prev:</b><a href="DAS.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/DAS.HTM"> DAS Decimal Adjust AL after Subtraction</a><br>
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<b>next:</b><a href="DIV.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/DIV.HTM"> DIV Unsigned Divide</a>
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<p>
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<hr>
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<p>
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<h1>DEC -- Decrement by 1</h1>
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<pre>
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Opcode Instruction Clocks Description
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FE /1 DEC r/m8 2/6 Decrement r/m byte by 1
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FF /1 DEC r/m16 2/6 Decrement r/m word by 1
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DEC r/m32 2/6 Decrement r/m dword by 1
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48+rw DEC r16 2 Decrement word register by 1
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48+rw DEC r32 2 Decrement dword register by 1
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</pre>
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<h2>Operation</h2>
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<pre>
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DEST := DEST - 1;
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</pre>
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<h2>Description</h2>
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DEC subtracts 1 from the operand. DEC does not change the carry flag. To affect the carry flag, use the <a href="SUB.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/SUB.HTM">SUB</a> instruction with an immediate operand of 1.
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<h2>Flags Affected</h2>
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OF, SF, ZF, AF, and PF as described in <a href="APPC.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/APPC.HTM">Appendix C</a>.
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<h2>Protected Mode Exceptions</h2>
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#GP(0) if the result is a nonwritable segment; #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; #PF(fault-code) for a page fault
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<h2>Real Address Mode Exceptions</h2>
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Interrupt 13 if any part of the operand would lie outside of the effective address space from 0 to 0FFFFH
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<h2>Virtual 8086 Mode Exceptions</h2>
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Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault
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<p>
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<hr>
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<p><b>up:</b> <a href="C17.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/C17.HTM">Chapter 17 -- 80386 Instruction Set</a><br>
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<b>prev:</b><a href="DAS.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/DAS.HTM"> DAS Decimal Adjust AL after Subtraction</a><br>
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<b>next:</b><a href="DIV.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/DIV.HTM"> DIV Unsigned Divide</a>
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</body>
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