What is the reason behind the byte scission (the next immediate byte following "int 2d" is skipped) behaviour when executing INT 0x2D? I came across this article http://www.drdobbs.com/monitoring-nt-debug-services/184416239 but still cannot understand what is the reason for Windows to skip the byte. Any explanation is welcome. Thanks.

  • 2
    The DrDobbs article does not mention 'byte scission' at all, which seems to be a term invented by Dr Fu in relation to a peculiarity/quirk in Windows (see Peter Ferrie's paper which explains it in detail).
    – DarthGizka
    May 31 '16 at 10:53
  • Thanks! That answer the question. Yeah, I know that the Dr Dobbs article didn't not mention "byte scission". That is the only article I came across after searching for mechanism behind INT 0x2D after reading about it on Dr Fu's blog. The other articles on the net usually only mention that a byte will be skipped without explanation. Hence this question. Thanks for pointing me to Peter Ferrie's paper.
    – xyz
    May 31 '16 at 23:22

The full details of the behavior are described here: http://pferrie.host22.com/papers/antidebug.pdf

Specifically, if the EAX register has the value of 1, 3, or 4 on all versions of Windows, or the value 5 on Windows Vista and later, then Windows will increase by one the exception address. Finally, it issues an EXCEPTION_BREAKPOINT (0x80000003) exception if a debugger is present.

The skipped byte is intended to pass information to the kernel-mode component, but the mechanism is not used by default.

  • Thanks. When you say the skipped byte is intended to pass information to kernel-mode component, specifically what information? Is it an undocumented feature of Windows kernel? I am still puzzle why do INT 0x2D need to behave the way it does. Comparatively the behaviour of INT 3 seem more understandable.
    – xyz
    Jun 6 '16 at 11:50
  • you can pass any single-byte value to the kernel, as a "parameter" to the interrupt, along with register values. This is not possible using int 3 style. Jun 26 '16 at 15:27

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