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Editorial Team
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Asked: 2026-05-27T01:17:17+00:00

On Microsoft’s site can be found some details of the __addgsbyte ( offset, data

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On Microsoft’s site can be found some details of the

__addgsbyte  ( offset, data )
__addgsword  ( offset, data )
__addgsdword ( offset, data )
__addgsqword ( offset, data )

intrinsic functions. It is stated that offset is

the offset from the beginning of GS. I presume that GS refers to the processor register.

How does GS relate to the stack, if at all? Alternatively, how can I calculate an offset with respect to GS?

(And, are there any ‘gotchas’ relating to this and particular calling conventions, such as __fastcall?)

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1 Answer

  1. Editorial Team

    The GS register does not relate to the stack at all and therefore no relation to callign convensions. On x64 versions of Windows it is used to point to operating system data:

    From wikipedia:

    Instead of FS segment descriptor on x86 versions of the Windows NT
    family, GS segment descriptor is used to point to two operating system
    defined structures: Thread Information Block (NT_TIB) in user mode and
    Processor Control Region (KPCR) in kernel mode. Thus, for example, in
    user mode GS:0 is the address of the first member of the Thread
    Information Block. Maintaining this convention made the x86-64 port
    easier, but required AMD to retain the function of the FS and GS
    segments in long mode — even though segmented addressing per se is not
    really used by any modern operating system.

    Note that those intrinsics are only available in kernel mode (e.g. device drivers). To calculate an offset, you would need to know what segment of memory GS is pointing to. So in kernel mode you would need to know the layout of the Processor Control Region.

    Personally I don’t know what the use of these would be.

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