The use of micro-processors and commercial operating systems in real-time applications demands a good understanding of factors which influence software performance. Advances in micro-processor design (e.g. pipelining) make performance prediction based on instruction cycle counts difficult. In addition, the increasing complexity of operating systems raises doubts about our ability to ensure that their performance will meet system requirements. Performance measurement is more important than ever. This paper describes an ongoing project intended to use performance measurements to characterize the performance of real-time systems software. To date the project has conducted extensive experiments on an in-house operating system running on Intel's 286/10 micro-computer in order to test the feasibility of accurate and repeatable measurement of O/S performance. The measurement approach, which views the software from a resource-consumption standpoint, can be applied to both O/S and application level software. Some of the measurement results are presented here and are used to test the manufacturer's assumptions about the hardware's performance.