i386/i386.reference/DEC.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode DEC</TITLE>
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   7 <B>up:</B> <A HREF="c17.htm">
   8 Chapter 17 -- 80386 Instruction Set</A><BR>
   9 <B>prev:</B><A HREF="DAS.htm"> DAS Decimal Adjust AL after Subtraction</A><BR>
  10 <B>next:</B><A HREF="DIV.htm"> DIV Unsigned Divide</A>
  11 <P>
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  13 <P>
  14 <H1>DEC -- Decrement by 1</H1>
  15
  16 <PRE>
  17 Opcode    Instruction        Clocks   Description
  18
  19 FE /1     DEC r/m8           2/6      Decrement r/m byte by 1
  20 FF /1     DEC r/m16          2/6      Decrement r/m word by 1
  21           DEC r/m32          2/6      Decrement r/m dword by 1
  22 48+rw     DEC r16            2        Decrement word register by 1
  23 48+rw     DEC r32            2        Decrement dword register by 1
  24 </PRE>
  25
  26 <H2>Operation</H2>
  27
  28 <PRE>
  29 DEST := DEST - 1;
  30 </PRE>
  31
  32 <H2>Description</H2>
  33
  34 DEC subtracts 1 from the operand. DEC does not change the carry flag.
  35 To affect the carry flag, use the
  36 <A HREF="SUB.htm">SUB</A> instruction with an immediate
  37 operand of 1.
  38
  39 <H2>Flags Affected</H2>
  40
  41 OF, SF, ZF, AF, and PF as described in <A HREF="appc.htm">Appendix C</A>.
  42
  43 <H2>Protected Mode Exceptions</H2>
  44
  45 #GP(0) if the result is a nonwritable segment; #GP(0) for an illegal
  46 memory operand effective address in the CS, DS, ES, FS, or GS
  47 segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
  48 for a page fault
  49
  50 <H2>Real Address Mode Exceptions</H2>
  51
  52 Interrupt 13 if any part of the operand would lie outside of the effective
  53 address space from 0 to 0FFFFH
  54
  55 <H2>Virtual 8086 Mode Exceptions</H2>
  56
  57 Same exceptions as in Real Address Mode; #PF(fault-code) for a page
  58 fault
  59
  60
  61 <P>
  62 <HR>
  63 <P>
  64 <B>up:</B> <A HREF="c17.htm">
  65 Chapter 17 -- 80386 Instruction Set</A><BR>
  66 <B>prev:</B><A HREF="DAS.htm"> DAS Decimal Adjust AL after Subtraction</A><BR>
  67 <B>next:</B><A HREF="DIV.htm"> DIV Unsigned Divide</A>
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