Conventional software-based cache self-tests typically ignore system related testing issues, such as physical memory layout, virtual memory mapping, and isolating faulty effects, especially for on-line testing. We propose an architectural support for data cache software-based self-testing (SBST): Processor Shield, which can tackle difficult-to-test issues during on-line SBST. The proposed processor shield includes a software framework and design for testing (DFT) hardware, which enables SBST program to run without influencing other processes and on-bus devices even if a cache test fails. The proposed SBST process can be iteratively executed and cooperate with dynamic voltage frequency scaling (DVFS) system to calibrate the required guardbands to accommodate transistor aging effects. Finally, we present a case study that performs SBST programs under Linux kernel on an ARMv5-compatible processor system. Our method can successfully switch between the SBST process and the kernel process and achieve the expected high fault coverages for cache control logic and RAM module testing.