Removal of older features: A number of "system programming" features of the x86 architecture are not used in modern operating systems and are not available on AMD64 in long (64-bit and compatibility) mode. These include segmented addressing (although the FS and GS segments are retained in vestigial form for use as extra base pointers to operating system structures)[1](p70), the task state switch mechanism, and Virtual 8086 mode. These features remain fully implemented in "legacy mode," thus permitting these processors to run 32-bit and 16-bit operating systems without modification. A number of instructions which proved to be rarely useful are not supported in 64-bit mode: saving/restoring of segment registers on the stack, saving/restoring of all registers (PUSHA/POPA), decimal arithmetic, BOUND and INTO instructions, and "far" jumps and calls with immediate operands.