The Unabridged Pentium 4 IA32 Processor Genealogy

Switch Via a TSS Descriptor

A far CALL or far jump can cause a task switch if the 16-bit segment portion of the target address selects a TSS descriptor in the GDT. However, a GP exception results if the following privilege check isn't passed:

The less-privileged of the RPL (the lower two bits of the 16-bit segment portion of the target address) or CPL must be at least as privileged as the TSS descriptor's DPL. Since TSS descriptors typically have a DPL of zero, this means that only privilege level zero programs can CALL or jump to another task using a TSS descriptor.