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Same opcodes,different Bytes

 
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TurboKid
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Joined: 01 Aug 2018
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PostPosted: Tue Aug 18, 2020 2:59 pm    Post subject: Same opcodes,different Bytes Reply with quote
Hi,while I was trying to make a cheat for unlimited ammo in Enter the Gungeon I've encountered something strange as you can see on the screenshot,there are two identical opcodes(sub ecx,r15d)
but the bytes are different
If I choose to assemble the instruction and I put the same opcode again,then the bytes change to (44 29 F9) instead of (41 2B CF)

How is this possible? does that means different bytes can be interpreted as the same oppcode?

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Dark Byte
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Joined: 09 May 2003
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Location: The netherlands
PostPosted: Tue Aug 18, 2020 3:30 pm    Post subject: Reply with quote
yes, there are multiple versions that do the same thing

in this case there are 2 similar instructions
Code:

29 xx sub reg/memlocation,reg
2b xx sub reg,reg/memlocation


since no memorylocation is needed it's going to be
sub reg,reg

and both versions can be picked for that.
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TurboKid
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Joined: 01 Aug 2018
Posts: 14
PostPosted: Wed Aug 19, 2020 12:41 pm    Post subject: Reply with quote
Ok thanks for your answer,that means next time I modify an instruction,I better copy both the opcode and the bytes in order to restore it
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