A data processing system incorporating watchpoint registers is provided. The memory accesses to be detected may be unaligned memory accesses. The watchpoint may operate in a normal mode and also in a guard mode. In the guard mode of operation a watchpoint comparator generates a match signal if the upper N bits of the memory address match the upper end bits of the watchpoint address and the length of the memory access L is such that the memory access extends to include a memory address having a different upper N bits but located at a predetermined address offset P from the watchpoint address W.