In addition to the standard Alpha EV4 instruction set, the Avanti CPU also supports the BWX, CIX, FIX, and MVI instruction.  These instructions were not natively supported on the EV4 CPU.  They were introduced on the EV56, EV6, and EV68 series Alpha CPUs.  Native EV4 systems running code generated for EV5 and EV6 processors would trap to the operating system and emulate these instructions.  Adding the instructions to Avanti's emulated EV4 CPU allows the instructions to be processed "natively", resulting in improved performance.

For example, Python is compiled by default for the EV56 processor.  Most of what the Python interpreter does is string manipulation and it uses the byte/word-extension instructions to do so. These instructions do not exist on an EV4 system, so each of these instructions takes a TRAP, firmware passes the trap to VMS, and VMS emulates the instructions. This is very slow, both on real and virtual EV4 hardware.

The addition of the EV56 instruction set to Avanti eliminates the EV4 performance issue.