Switching between multiple software entities using different operating modes of a processor

A computer program embodied in a non-transitory computer storage medium, the computer program being executable in a computer system, the computer system comprising a processor having a first operating mode and a second, legacy operating mode, the legacy operating mode being suitable for executing a legacy application and a legacy operating system (OS), the computer system further comprising a host OS that is executable on the processor in the first operating mode, the computer program comprising:

• a virtualization software that is executable on the processor in the legacy mode, the virtualization software supporting a virtual machine (VM) having a virtual legacy processor on which a legacy OS and a legacy application can run;
• and
• a switch routine for switching between the host OS that executes on the processor in the first operating mode and the virtualization software that executes on the processor in the legacy operating mode, the processor including a first instruction set for the first operating mode and a second instruction set for the legacy operating mode, the switching includes switching from the first instruction set to the second instruction set and switching paging tables, each of the first operating mode and the legacy operating mode having separate paging tables,
• wherein the switch routine is incorporated in a switch page that is locked in physical memory, and the switch routine includes instructions that are executed with memory paging disabled and employs an identity mapping to map a virtual page having a virtual page number to a physical page having a physical page number that is the same as the virtual page number to locate additional instructions in the physical memory.