x86-64 still requires Task State Segments (TSS), but it doesn't support using them for their original primary purpose of task switching; rather, they are needed

x86-64 still requires Task State Segments (TSS), but it doesn't support using them for their original primary purpose of task switching; rather, they are needed for changing stacks when switching between user and kernel mode. In the original 386 design, the OS kernel would create a separate TSS for each OS process. In x86-64's vestigial use of the concept, there is a single TSS for each CPU, and every process running on that CPU shares the same TSS.