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Ref-docs/manual Intel386/I386Manual/DEC.HTM Normal file
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<html><!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html>
<head>
<title>80386 Programmer's Reference Manual -- Opcode DEC</title>
</head>
<body>
<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>
<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>
<b>next:</b><a href="DIV.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/DIV.HTM"> DIV Unsigned Divide</a>
<p>
<hr>
<p>
<h1>DEC -- Decrement by 1</h1>
<pre>
Opcode Instruction Clocks Description
FE /1 DEC r/m8 2/6 Decrement r/m byte by 1
FF /1 DEC r/m16 2/6 Decrement r/m word by 1
DEC r/m32 2/6 Decrement r/m dword by 1
48+rw DEC r16 2 Decrement word register by 1
48+rw DEC r32 2 Decrement dword register by 1
</pre>
<h2>Operation</h2>
<pre>
DEST := DEST - 1;
</pre>
<h2>Description</h2>
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.
<h2>Flags Affected</h2>
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>.
<h2>Protected Mode Exceptions</h2>
#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
<h2>Real Address Mode Exceptions</h2>
Interrupt 13 if any part of the operand would lie outside of the effective address space from 0 to 0FFFFH
<h2>Virtual 8086 Mode Exceptions</h2>
Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault
<p>
<hr>
<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>
<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>
<b>next:</b><a href="DIV.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/DIV.HTM"> DIV Unsigned Divide</a>
</body>