i386/i386.reference/CMP.htm

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   4 <TITLE>80386 Programmer's Reference Manual -- Opcode CMP</TITLE>
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   8 Chapter 17 -- 80386 Instruction Set</A><BR>
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  13 <P>
  14 <H1>CMP -- Compare Two Operands</H1>
  15
  16
  17 <PRE>
  18 Opcode          Instruction        Clocks   Description
  19
  20 3C  ib          CMP AL,imm8        2        Compare immediate byte to AL
  21 3D  iw          CMP AX,imm16       2        Compare immediate word to AX
  22 3D  id          CMP EAX,imm32      2        Compare immediate dword to EAX
  23 80  /7 ib       CMP r/m8,imm8      2/5      Compare immediate byte to r/m
  24                                             byte
  25 81  /7 iw       CMP r/m16,imm16    2/5      Compare immediate word to r/m
  26                                             word
  27 81  /7 id       CMP r/m32,imm32    2/5      Compare immediate dword to r/m
  28                                             dword
  29 83  /7 ib       CMP r/m16,imm8     2/5      Compare sign extended immediate
  30                                             byte to r/m word
  31 83  /7 ib       CMP r/m32,imm8     2/5      Compare sign extended immediate
  32                                             byte to r/m dword
  33 38  /r          CMP r/m8,r8        2/5      Compare byte register to r/m
  34                                             byte
  35 39  /r          CMP r/m16,r16      2/5      Compare word register to r/m
  36                                             word
  37 39  /r          CMP r/m32,r32      2/5      Compare dword register to r/m
  38                                             dword
  39 3A  /r          CMP r8,r/m8        2/6      Compare r/m byte to byte
  40                                             register
  41 3B  /r          CMP r16,r/m16      2/6      Compare r/m word to word
  42                                             register
  43 3B  /r          CMP r32,r/m32      2/6      Compare r/m dword to dword
  44                                             register
  45 </PRE>
  46
  47 <H2>Operation</H2>
  48
  49 <PRE>
  50 LeftSRC - SignExtend(RightSRC);
  51 (* CMP does not store a result; its purpose is to set the flags *)
  52 </PRE>
  53
  54 <H2>Description</H2>
  55
  56 CMP subtracts the second operand from the first but, unlike the
  57 <A HREF="SUB.htm">SUB</A>
  58 instruction, does not store the result; only the flags are changed. CMP is
  59 typically used in conjunction with conditional jumps and the
  60 <A HREF="SETcc.htm">SETcc</A>
  61 instruction. (Refer to Appendix D for the list of signed and unsigned flag
  62 tests provided.) If an operand greater than one byte is compared to an
  63 immediate byte, the byte value is first sign-extended.
  64
  65 <H2>Flags Affected</H2>
  66
  67 OF, SF, ZF, AF, PF, and CF as described in <A HREF="appc.htm">Appendix C</A>
  68
  69 <H2>Protected Mode Exceptions</H2>
  70
  71 #GP(0) for an illegal memory operand effective address in the CS, DS, ES,
  72 FS, or GS segments; #SS(0) for an illegal address in the SS segment;
  73 #PF(fault-code) for a page fault
  74
  75 <H2>Real Address Mode Exceptions</H2>
  76
  77 Interrupt 13 if any part of the operand would lie outside of the effective
  78 address space from 0 to 0FFFFH
  79
  80 <H2>Virtual 8086 Mode Exceptions</H2>
  81
  82 Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault
  83
  84
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  87 <P>
  88 <B>up:</B> <A HREF="c17.htm">
  89 Chapter 17 -- 80386 Instruction Set</A><BR>
  90 <B>prev:</B><A HREF="CMC.htm"> CMC Complement Carry Flag</A><BR>
  91 <B>next:</B><A HREF="CMPS.htm"> CMPS/CMPSB/CMPSW/CMPSD Compare String Operands</A>
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