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CMP.htm
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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 STYLE="width:80ch">
<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 Description
3C ib CMP AL,imm8 2 Compare immediate byte to AL
3D iw CMP AX,imm16 2 Compare immediate word to AX
3D id CMP EAX,imm32 2 Compare immediate dword to EAX
80 /7 ib CMP r/m8,imm8 2/5 Compare immediate byte to r/m
byte
81 /7 iw CMP r/m16,imm16 2/5 Compare immediate word to r/m
word
81 /7 id CMP r/m32,imm32 2/5 Compare immediate dword to r/m
dword
83 /7 ib CMP r/m16,imm8 2/5 Compare sign extended immediate
byte to r/m word
83 /7 ib CMP r/m32,imm8 2/5 Compare sign extended immediate
byte to r/m dword
38 /r CMP r/m8,r8 2/5 Compare byte register to r/m
byte
39 /r CMP r/m16,r16 2/5 Compare word register to r/m
word
39 /r CMP r/m32,r32 2/5 Compare dword register to r/m
dword
3A /r CMP r8,r/m8 2/6 Compare r/m byte to byte
register
3B /r CMP r16,r/m16 2/6 Compare r/m word to word
register
3B /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 is
typically 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 flag
tests provided.) If an operand greater than one byte is compared to an
immediate 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 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">
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>