How to convert 8 bit displacement into 16 bit in relative base index addressing mode in 8086?

An 8 bit displacement is a two's complement signed value, with a possible range of -128 to +127. To convert it to an 16 bit displacement, sign extension is done. This means that the implied high byte is FFH if the high bit of the low byte is 1, or 00H if not. The resulting 16 bit displacement will still have a range of -128 to +127.