OK, so _my_ memory is failing! That code does in fact test the memory. (Although, looking at it, I can't understand how it works; after writing the contents of R3 into the memory section it it asked to test, it complements the test value in R3, before comparing it with the memory it just wrote with R3, to make sure they are the same. Maybe there's an error in the dis-assembly?) Anyway, it should have left the memory mostly containing all 0's. Noel