Firmware Musings 273 reason for doing a test. As a consumer I could care less why the damn phone stopped working if it's dead, I'll take it in for repair or replacement. Is production test--or even engineering test--the real motivation for writing diagnostic code? If so, then define exactly what problems you're looking for and write code that will find those sorts of troubles. Next, inject a dose of reality into your evaluation. Remember that today's hardware is often very highly integrated. In the case of a microcontroller with on-board RAM, the chances of a memory failure that doesn't also kill the CPU is small. Again, if the system is a critical life-support application it may indeed make sense to run a test, as even a minuscule probability of a fault may spell disaster. Does it make sense to ignore RAM failures? If your CPU has an illegal instruction trap, there's a pretty good chance that memory problems will cause a code crash you can capture and process. If the chip includes protection mechanisms (like the x86 protected mode), count on bad stack reads immediately causing protection faults your handlers can process. Perhaps RAM tests are simply not required, given these extra resources.