Describe Alpha instruction emulation and instruction subsets?
The Alpha architecture is upward- and downward-compatible, and newer instructions are emulated on older platforms, for those cases where the compiler is explicitly requested to generate the newer Alpha instructions. In particular, OpenVMS Alpha V7.1 and later include the instruction emulation capabilities necessary for the execution of newer Alpha instructions on older Alpha microprocessors. (Instruction emulation capabilities are available for user-mode application code, and are not available to device drivers or other similar kernel-mode code.) Alpha instructions are available in groups (or subsets). Obviously, there is the base instruction set that is available on all Alpha microprocessors.
