How to exactly modify the Local Descriptor table on 64 bits Windows?

The aim is to enable 16 bits segments for 16 bits addressing like on 64 bits Linux Kernel with the modify_ldt() system call.

I was unable to find if Cygwin provides a wrapper and I have only barely an idea that it’s about something like NtSetLdtEntries with :

typedef struct
{
    ULONG Start;
    ULONG Length;
    LDT_ENTRY LdtEntries[1];
} PROCESS_LDT_INFORMATION, *PPROCESS_LDT_INFORMATION;

Please note this is not related to the vm86 mode (which is a different method of doing it used by Microsoft on 32 bits systems). And As stated above, this way is used on Linux for running 16 bits code in protected mode without any emulation. See CONFIG_X86_16BIT for more informations.

Of course, if it’s not supported by default it’s Ok to modify system files.