Overwriting executable machine code in memory is a rare case (only JITs need it, only for emitting code). Some CPUs do wire the memory write operation to the instruction cache to make sure the instruction cache knows some code changed, but ARM chose a simpler design where the instruction cache assumes the code never changes in memory and if this assumption is wrong, the programmer has to use a special instruction to tell the instruction cache to throw something out of cache. The cache clearing instruction throws out either 64 bytes aligned to multiple of 64 or 128 bytes aligned to multiple of 128, depending on the core.