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<html><!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" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/C17.HTM">Chapter 17 -- 80386 Instruction Set</a><br>
<b>prev:</b><a href="CMC.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/CMC.HTM"> CMC Complement Carry Flag</a><br>
<b>next:</b><a href="CMPS.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/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" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/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" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/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" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/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" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/C17.HTM">Chapter 17 -- 80386 Instruction Set</a><br>
<b>prev:</b><a href="CMC.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/CMC.HTM"> CMC Complement Carry Flag</a><br>
<b>next:</b><a href="CMPS.HTM" tppabs="http://webster.cs.ucr.edu/Page_TechDocs/Doc386/CMPS.HTM"> CMPS/CMPSB/CMPSW/CMPSD Compare String Operands</a>
</body>