Intel 64 and IA-32 Architectures Software Developers Manual Volume 3A, System Programming Guide, Part 1

Vol. 3A 17-45
ARCHITECTURE COMPATIBILITY
in a 32-bit software system should have 32-bit TSSs. It is not necessary to
change the 16-bit object modules themselves; TSSs are usually constructed by
the operating system, by the loader, or by the system builder. See Chapter 16,
“Mixing 16-Bit and 32-Bit Code,” for more detailed information about mixing
16-bit and 32-bit code.
Because the 32-bit processors use the contents of the reserved word of 16-bit
segment descriptors, 16-bit programs that place values in this word may not run
correctly on the 32-bit processors.