There is an x86 instruction
sysenter. This seems to be the Intel fast-system-call method.
SYSENTER seems to be valid in LONG mode (64-bit) on Intel processors. This seems to have fallen out of favor because of the AMD-provided
syscall. From my understanding, this instruction is like a
syscall but the onus is on the user to set it with EIP/ESP, etc.
You can see the argument list for
sysenter 32-bit. With absolute minimal focus to
int 0x80, what does the
sysenter instruction do in Linux when the chip is in Long mode? Can it be of any use as-is?