There are 16 physical addresses which can be referenced by only one logical address and these 16 are at the very top of memory at addresses 0-0Fh. Since the offset of an address cannot be negative, the segment, after being shifted left by four bits, must be less than the physical address. For physical addresses 0-0Fh, the only segment that meets this criteria is segment 0. (Segment 1, when shifted left by four bits, yields 10h which is too high). Now that we have shown that only one segment is possible, we can refer back to O=P-S<<4. Obviously, only one value for O exists when P and S are known so for these 16 special cases, physical addresses and logical addresses are indeed unique.