REP(N)Z and the EFLAGS
Working on some debugger like automation code for EmsiSoft, I recently discovered a funny property when single stepping REPNZ prefixed SCAS and CMPS instructions using the TF bit set in EFLAGS. As expected, for each single byte / word / doubleword, a debug event occurs. However, the EFLAGS register's status bits (e.g. ZF) are not correct for each single iteration but the last.
I tested this in Windows XP in a VmWare, didn't have the time to reproduce on a physical machine yet. Let me know if you run over this quirk, too.
Labels: assembly
1 Comments:
It happens outside of VMWare too. I found that out too while working on my emulator. Didn't come across this on a p4 I used to test on, but may not have run an appropriate test case to see the necessary behaviour.
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Silvio
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