x86 CMP Instruction Difference
- by Pindatjuh
Question
What is the (non-trivial) difference between the following two x86 instructions?
39 /r CMP r/m32,r32 Compare r32 with r/m32
3B /r CMP r32,r/m32 Compare r/m32 with r32
Background
I'm building a Java assembler, which will be used by my compiler's intermediate language to produce Windows-32 executables.
Currently I have following code:
final ModelBase mb = new ModelBase(); // create new memory model
mb.addCode(new Compare(Register.ECX, Register.EAX)); // add code
mb.addCode(new Compare(Register.EAX, Register.ECX)); // add code
final FileOutputStream fos = new FileOutputStream(new File("test.exe"));
mb.writeToFile(fos);
fos.close();
To output a valid executable file, which contains two CMP instruction in a TEXT-section. The executable outputted to "text.exe" will do nothing interesting, but that's not the point. The class Compare is a wrapper around the CMP instruction.
The above code produces (inspecting with OllyDbg):
Address Hex dump Command
0040101F |. 3BC8 CMP ECX,EAX
00401021 |. 3BC1 CMP EAX,ECX
The difference is subtle: if I use the 39 byte-opcode:
Address Hex dump Command
0040101F |. 39C1 CMP ECX,EAX
00401021 |. 39C8 CMP EAX,ECX
Which makes me wonder about their synonymity and why this even exists.
