OpenVMS Version 7.1 and subsequent releases will provide an operating system kernel that includes an instruction emulator. This emulator allows new instructions, not implemented on the host processor chip, to execute and produce correct results. Applications using emulated instructions will run correctly, but may incur significant software emulation overhead at runtime.
All Alpha processors implement a core set of instructions.
Certain Alpha processor versions include additional instruction extensions. This is the default. HOST Generates code for the processor generation in use on the system being used for compilation. Running programs compiled with this option on other implementations of the Alpha architecture may encounter instruction emulation overhead. Running programs compiled with the EV4 option will run without instruction emulation overhead on all Alpha processors.