The x86 instruction set has a mechanism called "call gates" for system calls. Basically, the OS puts the entry point of the system call handler into a segment d

The x86 instruction set has a mechanism called "call gates" for system calls. Basically, the OS puts the entry point of the system call handler into a segment descriptor with the call gate bits set. The unprivileged user program then performs a far call to an address consisting of a segment selector for that descriptor and an offset which does not matter. Execution resumes at the system call handler, with a privilege level as encoded in the call gate descriptor. That way, you could have thousands of system call entry points and avoid the overhead of an int instruction and the syscall-number dispatch. I believe OS/2 used that mechanism extensively (and all the other elaborate segmentation stuff). And I call x86 segmentation "elaborate" :)