Intel archs have support for call gates that allow jumps to privileged code (ring 0/1/2). It maps nicely to the notion of syscalls, but I wonder why it hasn't been used.
Not sure about the times before the sysenter mechanism (maybe int 0x80 was more portable?), but now that sysenter is in place it is faster than call gates, because most of its behaviour is hardcoded in the CPU itself.
