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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Opcode CMP</TITLE></HEAD><BODY><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="CMC.htm"> CMC Complement Carry Flag</A><BR><B>next:</B><A HREF="CMPS.htm"> CMPS/CMPSB/CMPSW/CMPSD Compare String Operands</A><P><HR><P><H1>CMP -- Compare Two Operands</H1><PRE>Opcode Instruction Clocks Description3C ib CMP AL,imm8 2 Compare immediate byte to AL3D iw CMP AX,imm16 2 Compare immediate word to AX3D id CMP EAX,imm32 2 Compare immediate dword to EAX80 /7 ib CMP r/m8,imm8 2/5 Compare immediate byte to r/m byte81 /7 iw CMP r/m16,imm16 2/5 Compare immediate word to r/m word81 /7 id CMP r/m32,imm32 2/5 Compare immediate dword to r/m dword83 /7 ib CMP r/m16,imm8 2/5 Compare sign extended immediate byte to r/m word83 /7 ib CMP r/m32,imm8 2/5 Compare sign extended immediate byte to r/m dword38 /r CMP r/m8,r8 2/5 Compare byte register to r/m byte39 /r CMP r/m16,r16 2/5 Compare word register to r/m word39 /r CMP r/m32,r32 2/5 Compare dword register to r/m dword3A /r CMP r8,r/m8 2/6 Compare r/m byte to byte register3B /r CMP r16,r/m16 2/6 Compare r/m word to word register3B /r CMP r32,r/m32 2/6 Compare r/m dword to dword register</PRE><H2>Operation</H2><PRE>LeftSRC - SignExtend(RightSRC);(* CMP does not store a result; its purpose is to set the flags *)</PRE><H2>Description</H2>CMP subtracts the second operand from the first but, unlike the <A HREF="SUB.htm">SUB</A>instruction, does not store the result; only the flags are changed. CMP istypically used in conjunction with conditional jumps and the <A HREF="SETcc.htm">SETcc</A>instruction. (Refer to Appendix D for the list of signed and unsigned flagtests provided.) If an operand greater than one byte is compared to animmediate byte, the byte value is first sign-extended.<H2>Flags Affected</H2>OF, SF, ZF, AF, PF, and CF as described in <A HREF="appc.htm">Appendix C</A><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;#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 effectiveaddress 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">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B><A HREF="CMC.htm"> CMC Complement Carry Flag</A><BR><B>next:</B><A HREF="CMPS.htm"> CMPS/CMPSB/CMPSW/CMPSD Compare String Operands</A></BODY>⌨️ 快捷键说明
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