A design structure embodied in a machine readable storage medium for designing, manufacturing, and/or testing a design is disclosed for option ROM characterization by establishing an isolating execution environment for an expansion adapter of a computer, the adapter having an option ROM containing initialization code for the adapter, executing the initialization code for the expansion adapter in the isolating execution environment, identifying operating characteristics of the option ROM, including characteristics of the option ROM unavailable prior to execution of the initialization code in the isolating execution environment, and allocating virtual memory address space in a normal execution environment of the computer to the option ROM of the expansion adapter in dependence upon the identified operating characteristics of the option ROM.