How can you fill the garbage value.Any value from 00 to FF is opcode.

A single byte isn't garbage. 2+ bytes that don't make up any meaningful program - that is garbage. Any attacker first has to figure out which memory ranges contain code that actually makes sense and is executed by the processor. This is much more difficult if the "empty" memory ranges don't contain 0xFF.

Of course, as Jan said, this approach becomes even more effective if the garbage isn't obvious - i.e. the "empty" memory ranges are actually filled with instructions that would make sense, but are never executed.