Systems, apparatuses, and methods related to a virtual machine register in a computer processor are described. For example, a memory coupled to the computer processor can store instructions of routines of predefined, non-hierarchical domains. The computer processor can store, in the virtual machine register, an identifier of a virtual machine for which the processor is currently executing instructions in a current domain in the set of domains. For example, the processor can implement resource restriction/mapping and/or perform address translation for the virtual machine based on the identifier stored in the virtual machine register.