Abstract: A program loader for virtual machine (VM) operating systems (OS) for computer and data processing systems permits the loading and execution of programs in different virtual machine environments, depending on addressing and residency requirements of the programs. An improved subsystem to the VM operating system contains command language and file management capability that allows programs to be invoked via a direct branch linkage in the addressing mode of the target program, allows the program's addressing mode and residency mode to be dynamically and interactively overridden at any point in the load process, controls whether previously loaded programs remain in storage along with the program that is currently being loaded, and allows the loading and execution of programs that are architecturally dependent as well as programs that have no architecture dependencies.