Both its 16-bit parts have odd values, it is an illegal address (at least on the systems it was introduced for), and it is recognizable in hex dumps. That makes it a good value for uninitialized memory.
Both its 16-bit parts have odd values, it is an illegal address (at least on the systems it was introduced for), and it is recognizable in hex dumps. That makes it a good value for uninitialized memory.