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dec r8d assembles wrong (bug?)

 
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jgoemat
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PostPosted: Fri Nov 16, 2012 7:37 pm    Post subject: dec r8d assembles wrong (bug?) Reply with quote
I'm pretty new to 64-bit assembler, but this instruction assembles differently and doesn't seem to work:

Code:
02CD0D5B - 41 FF C8              - dec r8d // original instruction

0060008A - 41 48                 - dec r8d // assembled by CE
0060008C - 49 48                 - dec r8  // another try
0060008E - E9 CB0C6D02           - jmp 02CD0D5E // jump back to code


That's just a snippet. Neither "dec r8d" nor "dec r8" assembled by CE seems to work, but "db 41 FF C8" does. If I step at 0060008A in the CE debugger, or NOP it and step at 0060008C, the debugger skips back to the return point, almost like the CPU skips over those instructions and executes the jmp all in one step. If I replace those with "db 41 FF C8", the CE debugger steps over it properly and I can see the value in R8 decrease. If I replace those with "db 49 FF C8" then it also decrements R8 and I can step over it fine.
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Dark Byte
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PostPosted: Sat Nov 17, 2012 3:20 am    Post subject: Reply with quote
Thanks, it's fixed in the svn
the FF C* variant was set as only encodable for addresses

the 4* opcodes can't be encoded in 64-bit as those are prefixes there
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