A system comprising: a plurality of machines comprising a first generation machine, a second generation machine, and a third generation machine, each of the plurality of machines comprising a machine version; the first, second and third generation machines comprising different machine versions, the first generation machine executing a first virtual machine and a virtual architecture level, the second generation machine executing a second virtual machine and the virtual architecture level, and the third generation machine executing a third virtual machine and the virtual architecture level, wherein the system is characterised by: the virtual architecture level being configured to provide a compatibility level for a complex interruptible instruction to the first, second and third virtual machines, the compatibility level being architected for a lowest common denominator machine version across the plurality of machines, the compatibility level comprising a lowest common denominator indicator identifying the lowest common denominator machine version, the complex interruptible instruction having a state that is required to be saved out and meets a parameter block format, the compatibility level comprises a parameter block format associated with the lowest common denominator indicator, the compatibility level comprises a local parameter block format for the complex interruptible instruction for each of the plurality of machines, the local parameter block format being architected for the machine version local to each of the plurality of machines, the lowest common denominator indicator is generated by a machine of the plurality of machines to first execute the complex interruptible instruction, and the lowest common denominator indicator is propagated from the machine of the plurality of machines to first execute the complex interruptible instruction to a remaining number of the plurality of machines. the virtual architecture level being configured to provide a compatibility level for a complex interruptible instruction to the first, second and third virtual machines, the compatibility level being architected for a lowest common denominator machine version across the plurality of machines, the compatibility level comprising a lowest common denominator indicator identifying the lowest common denominator machine version, the complex interruptible instruction having a state that is required to be saved out and meets a parameter block format, the compatibility level comprises a parameter block format associated with the lowest common denominator indicator, the compatibility level comprises a local parameter block format for the complex interruptible instruction for each of the plurality of machines, the local parameter block format being architected for the machine version local to each of the plurality of machines, the lowest common denominator indicator is generated by a machine of the plurality of machines to first execute the complex interruptible instruction, and the lowest common denominator indicator is propagated from the machine of the plurality of machines to first execute the complex interruptible instruction to a remaining number of the plurality of machines.